Apparatus and method

Abstract

A fluid purification apparatus is detailed, comprising: a plasma generating vessel 1; a fluid supply system 70 configured to supply a fluid to be purified to the plasma generating vessel 1. The plasma

Description

Technical Field The present disclosure relates to the field of plasma generating vessels. In particular, the present disclosure relates to the field of apparatuses and methods for controlling operation of plasma generating vessel. Background GB 2604853 discloses a heating system including a cell which applies electrical energy to liquid in that cell to generate bubbles of plasma therein. In turn, this causes energy to be released into the cell, both into the fluid contained within the cell and also into a housing of the cell. The result of this energy release is to generate a heated fluid within the cell. The heated fluid can then be output from the cell and used by a work extraction system to extract useable work from this heated fluid. This arrangement disclosed in GB 2604853 provides for a highly efficient generation of heated fluid. Summary Aspects of the disclosure are set out in the independent claims and optional features are set out in the dependent claims. Aspects of the disclosure may be provided in conjunction with each other, and features of one aspect may be applied to other aspects. In an aspect, there is provided an apparatus comprising: a plasma generating vessel; a fluid supply system configured to supply a liquid to the plasma generating vessel, and wherein the plasma generating vessel comprises an electrode configured to apply electrical energy to said liquid to generate one or more bubbles of plasma therein; and a fault detection controller configured to: obtain an indication of an electrical resistance to the electrical energy applied to the electrode; and detect that a liquid supply fault has occurred in the event that the indication of electrical resistance is above an upper threshold value. Embodiments may enable improved control and operation of the plasma generating vessel. For instance, embodiments may inhibit excess voltage being applied to the electrode while there is insufficient liquid in the vessel. This may facilitate greater longevity of the components involved, as well as reducing the likelihood of damage to one or more components associated with applying electrical energy without liquid present. The fault detection controller may be configured to output a liquid supply fault signal indicating that the liquid supply fault has been detected. For example, the liquid supply fault signal may comprise an alert, such as an audible or visual alert, that indicates that the liquid supply fault has been detected. Outputting the liquid supply fault signal may comprise outputting a command signal to control operation of one or more components of the apparatus. The fault detection controller may be configured to control operation of the apparatus in response to detecting that the liquid fault has occurred. Controlling operation of the apparatus may comprise stopping or reducing the application of electrical energy to the electrode. Controlling operation of the apparatus may comprise controlling operation of the fluid supply system to try to increase delivery of liquid to the plasma generating vessel. The fault detection controller may be configured to repeatedly monitor the electrical resistance, e.g. the fault detection controller may provide continuous monitoring of the electrical resistance. The fault detection controller may be configured to repeatedly monitor the electrical resistance after trying to increase the delivery of liquid from the fluid supply system to the plasma generating vessel. The fault detection controller may be configured to detect that the liquid supply fault has been resolved in the event that an obtained indication of electrical resistance is below a threshold value. Said threshold value may comprise the upper threshold value. The fault detection controller may be configured to increase the electrical energy applied to the electrode in the event that it detects that the liquid supply fault has been resolved. The fault detection controller may be configured to control the application of electrical energy to the electrode with at least one of: (i) a maximum permitted current, (ii) a maximum permitted voltage, and (iii) a maximum permitted power. The fault detection controller may be configured to inhibit any of: (i) an applied current exceeding the maximum permitted current, (ii) an applied voltage exceeding the maximum permitted voltage, and (iii) an applied power exceeding the maximum permitted power. The fault detection controller may be configured to determine the indication of resistance based on obtained values for: (i) a voltage applied to the electrode, and (ii) a current resulting from that applied voltage. In an aspect, there is provided a method comprising: operating a fluid supply system to supply a liquid to a plasma generating vessel; applying electrical energy to the liquid using an electrode of the plasma generating vessel to generate one or more bubbles of plasma therein; obtaining an indication of an electrical resistance to the electrical energy applied to the electrode; and detecting that a liquid supply fault has occurred when the indication of electrical resistance is above an upper threshold value. In an aspect, there is provided an apparatus comprising: a plasma generating vessel; a fluid supply system configured to supply a liquid to the plasma generating vessel, and wherein the plasma generating vessel comprises an electrode configured to apply electrical energy to said liquid to generate one or more bubbles of plasma therein; and a fault detection controller configured to: obtain an indication of an electrical resistance to the electrical energy applied to the electrode; and detect that a fault has occurred in the event that the indication of electrical resistance is below at least one lower threshold value. Embodiments may enable the detection of electrical shorting faults. This may facilitate greater longevity for the apparatus, while also avoiding damage associated with applying voltages, e.g. high voltages, which are not applied to the liquid in the vessel, as intended. The at least one lower threshold value may comprise a first lower threshold value, and wherein the fault detection controller is configured to detect that an electrical shorting fault has occurred in the event that the indication of electrical resistance is below the first lower threshold value. The fault detection controller may be configured to output an electrical shorting fault signal indicating that the electrical shorting fault has been detected. The fault detection controller may be configured to control operation of the apparatus in response to detecting that the electrical shorting fault has occurred. Controlling operation of the apparatus may comprise stopping or reducing the application of electrical energy to the electrode. The fault detection controller may be configured to confirm that the electrical shorting fault has occurred based on an obtained indication of at least one other parameter for the vessel. Said obtained indication of at least one other parameter may comprise an indication of an output parameter for the vessel. The fault detection controller may be configured to confirm that the electrical shorting fault has occurred in the event that the output parameter has dropped below a threshold value and / or by at least a threshold amount. The at least one lower threshold value may comprises a second lower threshold value, and wherein the fault detection controller is configured to detect a potential arcing event fault in the event that the indication of electrical resistance is below the second lower threshold value. The fault detection controller may be configured to output a potential arcing event fault signal indicating that the potential arcing event fault has been detected. The fault detection controller may be configured to control operation of the apparatus in response to detecting that the potential arcing event fault has occurred. Controlling operation of the apparatus may comprise stopping or reducing the application of electrical energy to the electrode. The fault detection controller may be configured to confirm that the potential arcing event fault has occurred based on an obtained indication of at least one other parameter for the vessel. Said obtained indication of at least one other parameter may comprise an indication of a voltage and / or a current for the electrical energy applied to the electrode. The fault detection controller may be configured to confirm that the potential arcing event fault has occurred in the event that the value for current and / or voltage is above a threshold value and / or has increased by at least a threshold amount. In an aspect, there is provided a method comprising: operating a fluid supply system to supply a liquid to a plasma generating vessel; applying electrical energy to the liquid using an electrode of the plasma generating vessel to generate one or more bubbles of plasma therein; obtaining an indication of an electrical resistance to the electrical energy applied to the electrode; and detecting that a fault has occurred when the indication of electrical resistance is below at least one lower threshold value. In an aspect, there is provided an apparatus comprising: a plasma generating vessel; a fluid supply system configured to supply a liquid to the plasma generating vessel, and wherein the plasma generating vessel comprises an electrode configured to apply electrical energy to said liquid to generate one or more bubbles of plasma therein; and a controller configured to: obtain an indication of an electrical resistivity associated with the liquid within the plasma generating vessel; and control operation of the apparatus based on the obtained indication of resistivity. Embodiments may provide greater control over operation of the vessel, as well as greater prediction of operating conditions for the vessel. For instance, the apparatus may be able to control operation based on the insight obtained through monitoring of electrical resistivity. Such control may provide more efficient operation of the apparatus, and greater predictability for how the vessel will behave when subject to different operating conditions. Controlling operation of the apparatus may comprise controlling operation of the fluid supply system. Controlling operation of the fluid supply system may comprise adjusting at least one property of the fluid supplied to the vessel in the event that the obtained indication of resistivity is outside a selected range. The fluid supply system may be configured to replace at least some of the liquid in the vessel in the event that the obtained indication of resistivity is above an upper threshold value. The fluid supply system may be configured to replace at least some of the liquid in the vessel in the event that the obtained indication of resistivity is below a lower threshold value. The fluid supply system may be configured to flush out the existing liquid in the vessel to replace it with liquid having at least one property thereof altered. The fluid supply system may be configured to replace the liquid in the vessel until the liquid in the vessel has a resistivity within the selected range. The resistivity may be below the upper threshold value and / or greater than the lower threshold value. The fluid supply system may be configured to add at least one substance to the fluid to be supplied thereby to adjust the property of the fluid. For example, the substance may comprise a mineral, such as a transition metal. The mineral may be added in suspension, solution and / or as a standalone solid. Controlling operation of the apparatus may comprise controlling the application of electrical energy to the electrode. Controlling the application of electrical energy to the electrode may comprise selecting one or more values for the electrical energy to be applied based on the obtained indication of resistivity. The one or more values may comprise at least one of: (i) an upper threshold voltage to be applied to the electrode, (ii) an upper threshold current flow for the electrode, and / or (iii) an upper threshold power to be applied to the electrode. The controller may be configured to predict when plasma generation will occur within the vessel based on the obtained indication of resistivity. Predicting when plasma generation will occur may comprise predicting an applied voltage above which plasma will be generated. The controller may be configured to output an alert signal in the event that plasma generation does not occur as predicted in the vessel. The controller may be configured to output an alert signal in the event that the obtained indication of resistivity is outside a selected range and / or in the event that the resistivity value changes by more than a threshold amount. The controller may be configured to detect that a fault condition is present in the event that the resistivity changes by more than the threshold amount. The controller may be configured to stop or reduce the application of electrical energy to the electrode in the event that the fault condition is detected. In an aspect, there is provided a method comprising: operating a fluid supply system to supply a liquid to a plasma generating vessel; applying electrical energy to the liquid using an electrode of the plasma generating vessel to generate one or more bubbles of plasma therein; obtaining an indication of an electrical resistivity associated with liquid within the vessel; and controlling operation based on the obtained indication of electrical resistivity. In an aspect, there is provided an apparatus comprising: a plasma generating vessel; a fluid supply system configured to supply a liquid to the plasma generating vessel, and wherein the plasma generating vessel comprises an electrode configured to apply electrical energy to said liquid to generate one or more bubbles of plasma therein; and a controller configured to: obtain an indication of at least one wavelength of light being emitted within the vessel; and control operation of the apparatus based on a comparison of said at least one wavelength against one or more selected wavelengths. Embodiments may enable greater control of operation of the apparatus to prevent against undesirable substances being present in the vessel. For example, substances may be detected based on the particular wavelengths of light emitted within the vessel, and the controller may identify when undesirable, e.g. potentially dangerous, substances are present in the vessel, and the apparatus may be controlled accordingly. Similarly, the apparatus may be configured to identify that desired reactions are occurring if certain wavelengths (e.g. associated with known and wanted substances) are present. The one or more selected wavelengths may comprise one or more wavelengths associated with at least one unwanted substance. Comparing the at least one wavelength against the one or more selected wavelengths may comprise determining whether a said unwanted substance is present in the vessel based on the at least one wavelength of light being emitted within the vessel. The controller may be configured to stop or reduce the application of power to the electrode in the event that it is determined that an unwanted substance is present in the vessel. The controller may be configured to increase the flow of fluid through the vessel in the event that it is determined that an unwanted substance is present in the vessel. The controller may be configured to flush the vessel with liquid, e.g. until the wavelength associated with the unwanted substance is no longer identified within the vessel. Comparing the at least one wavelength of light being emitted within the vessel against the one or more selected wavelengths may comprise determining whether an intensity of said one or more selected wavelengths within the vessel is above a threshold intensity. The controller may be configured to stop or reduce the application of electrical energy to the vessel in the event that the intensity is above the threshold intensity. The controller may be configured to adjust at least one property of the fluid supplied to the vessel in the event that the intensity is below the threshold intensity. The one or more selected wavelengths may comprise one or more wavelengths associated with at least one wanted substance. Comparing the at least one wavelength against the one or more selected wavelengths may comprise determining whether a said wanted substance is present in the vessel based on the at least one wavelength of light being emitted within the vessel. The controller may be configured to continue operation of the apparatus in the event that the wanted substance is present. The controller may be configured to increase the power applied to the vessel and / or adjust at least one property of the fluid supplied to the vessel in the event that an intensity of the wavelength of light associated with the wanted substance is below a threshold intensity. The controller may be configured to continue with existing operational parameters for controlling operation of the apparatus in the event that the intensity of the wavelength of light associated with the wanted substance is above the threshold intensity. The one or more selected wavelengths may comprise one or more wavelengths associated with at least one informative substance or energy level transition. The controller may be configured to control operation of the apparatus based on whether or not any informative substances are present. Controlling operation of the apparatus based on whether any informative substances are present may comprise adjusting the operational parameters for the apparatus based on the informative substances present in the vessel. The controller may be configured to adjust the flow of liquid and / or increase the amount of electrical energy applied to the electrode in the event that the intensity of wavelengths detected and / or the intensity of selected wavelengths detected is below a threshold value. The apparatus may be operable to re-use fluid which has already passed through the vessel. The controller may be configured to selectively re-use fluid based on the obtained indication of one wavelength being emitted. The controller may be configured to increase the use of recycled fluid in the event that it is determined that one or more unwanted substances are present in the vessel. In an aspect, there is provided a method comprising: operating a fluid supply system to supply a liquid to a plasma generating vessel; applying electrical energy to the liquid using an electrode of the plasma generating vessel to generate one or more bubbles of plasma therein; obtaining an indication of at least one wavelength of light being emitted within the vessel; and controlling operation based on a comparison of said at least one wavelength against one or more selected wavelengths. In an aspect, there is provided a fluid purification apparatus comprising: a plasma generating vessel; a fluid supply system configured to supply a fluid to be purified to the plasma generating vessel, and wherein the plasma generating vessel comprises an electrode configured to apply electrical energy to liquid within the vessel to generate one or more bubbles of plasma therein; and a controller configured to control operation of the apparatus based on an obtained indication of at least one wavelength of light being emitted within the vessel. Embodiments may enable improve and more efficient fluid purification. For example, operation of the vessel may be controlled and monitored to ensure that the intended substances are being removed (e.g. changed into less undesirable form) from the fluid. The controller may be configured to control operation of the apparatus based on a comparison of the obtained indication of at least one wavelength of light being emitted against one or more selected wavelengths. The one or more selected wavelengths may comprise one or more wavelengths associated with at least one unwanted substance. Comparing the at least one wavelength against the one or more selected wavelengths may comprise determining whether a said unwanted substance is present in the vessel based on the at least one wavelength of light being emitted within the vessel. The controller may be configured to stop or reduce the application of power to the electrode in the event that it is determined that an unwanted substance is present in the vessel. The controller may be configured to increase the flow of fluid through the vessel in the event that it is determined that an unwanted substance is present in the vessel. The controller may be configured to flush the vessel with liquid, e.g. until the wavelength associated with the unwanted substance is no longer identified within the vessel. Comparing the at least one wavelength of light being emitted within the vessel against the one or more selected wavelengths may comprise determining whether an intensity of said one or more selected wavelengths within the vessel is above a threshold intensity. The controller may be configured to stop or reduce the application of electrical energy to the vessel in the event that the intensity is above the threshold intensity. The controller may be configured to adjust at least one property of the fluid supplied to the vessel in the event that the intensity is below the threshold intensity. The one or more selected wavelengths may comprise one or more wavelengths associated with at least one wanted substance. Comparing the at least one wavelength against the one or more selected wavelengths may comprise determining whether a said wanted substance is present in the vessel based on the at least one wavelength of light being emitted within the vessel. The controller may be configured to continue operation of the apparatus in the event that the wanted substance is present. The controller may be configured to increase the power applied to the vessel and / or adjust at least one property of the fluid supplied to the vessel in the event that an intensity of the wavelength of light associated with the wanted substance is below a threshold intensity. The controller may be configured to continue with existing operational parameters for controlling operation of the apparatus in the event that the intensity of the wavelength of light associated with the wanted substance is above the threshold intensity. The one or more selected wavelengths may comprise one or more wavelengths associated with at least one informative substance or energy level transition. The controller may be configured to control operation of the apparatus based on whether or not any informative substances are present. Controlling operation of the apparatus based on whether any informative substances are present may comprise adjusting the operational parameters for the apparatus based on the informative substances present in the vessel. The controller may be configured to adjust the flow of liquid and / or increase the amount of electrical energy applied to the electrode in the event that the intensity of wavelengths detected and / or the intensity of selected wavelengths detected is below a threshold value. The apparatus may be operable to re-use fluid which has already passed through the vessel. The controller may be configured to selectively re-use fluid based on the obtained indication of one wavelength being emitted. The controller may be configured to increase the use of recycled fluid in the event that it is determined that one or more unwanted substances are present in the vessel. In an aspect, there is provided a fluid purification method comprising: operating a fluid supply system to supply a fluid to be purified to a plasma generating vessel; applying electrical energy to liquid within the vessel using an electrode of the plasma generating vessel to generate one or more bubbles of plasma therein; obtaining an indication of at least one wavelength of light being emitted within the vessel; and controlling operation based on the obtained indication of at least one wavelength of light being emitted within the vessel. Figures Some examples of the present disclosure will now be described, by way of example only, with reference to the figures, in which: Fig. 1 is a schematic diagram of an apparatus comprising a plasma generating vessel. Fig. 2a is a graph illustrating example operating conditions for a plasma generating vessel. Fig. 2b is a graph illustrating example parameters for controlling operation of a plasma generating vessel. Fig. 3 is a graph illustrating different example operating conditions for a plasma generating vessel. Figs. 4 to 6 illustrate example flow charts for different operating controls of the apparatus. In the drawings like reference numerals are used to indicate like elements. Specific Description Embodiments of the present disclosure relate to methods and apparatuses for controlling operation of a plasma generating vessel. In particular, fault detection apparatuses and methods are disclosed to detect faults with the apparatus and its operation. The apparatus may then be controlled accordingly based on any detected fault(s) it may have. This fault detection may be implemented by monitoring electrical properties of the vessel, such as applied voltage / current, electrical resistance and / or electrical conductivity / resistivity. Also, fault detection may be implemented by monitoring wavelengths of thermoelectric emissions within the vessel to detect the presence (or absence) of selected substances of interest. By detecting and addressing such faults, operation of the apparatus may be controlled to inhibit malfunction of the apparatus occurring, and / or to control the apparatus so that desired operating conditions may be achieved. An example apparatus with a plasma generating vessel will first be described with reference to Fig. 1. After this, different operating conditions forthat vessel will be described with reference to Fig. 2a, and parameters for controlling that vessel will be described with reference to Fig. 2b. The influence of resistivity / conductivity of supplied fluid will then be described with reference to Fig. 3. Finally, different approaches for fault detection will be described, as well as approaches for addressing the detected faults, with reference to Figs. 4 to 6. Plasma generating vessel Fig. 1 shows a schematic diagram of an apparatus 100. The apparatus 100 includes an energy vessel 1, as well as an electrical supply system 60, a fluid supply system 70 and a work extraction system 80. The vessel 1 includes a fluid inlet 54 and a fluid outlet 58. The vessel 1 has a housing 50. The housing 50 defines an internal volume 56 of the vessel 1. The vessel 1 also includes a plurality of electrodes. As shown, this includes a first electrode 10, a second electrode 20 and a third electrode 30. The vessel 1 may also include a resistive element 40. A mount 32 for the third electrode 30 is shown in Fig. 1. The vessel 1 may also include an earth connector 22. The earth connector 22 is coupled to the second electrode 20 and an electrical earth 24. The electrical supply system 60 is coupled to the first electrode 10. The fluid supply system 70 is coupled to the fluid inlet 54 of the vessel 1. The work extraction system 80 is coupled to the fluid outlet 58 of the vessel 1. The electrical supply system 60 is configured to supply electrical energy to the first electrode 10. The fluid supply system 70 is configured to supply fluid to the internal volume 56 of the vessel 1. The work extraction system 80 is configured to extract useable work from heated fluid received from the fluid outlet 58 of the vessel 1. The housing 50 of the vessel 1 encapsulates the internal volume 56. The fluid inlet 54 provides a flow path for fluid into the internal volume 56 of the vessel 1. The fluid outlet 58 provides a flow path for fluid out from the internal volume 56 of the vessel 1. Fluid may flow along any suitable path between the fluid inlet 54 and the fluid outlet 58. For example, it may flow along a very indirect (e.g. tortuous) path. The internal volume 56 of the vessel 1 may otherwise be sealed by the housing 50. The first electrode 10 is at least partially disposed within the internal volume 56 of the vessel 1. The second electrode 20 may also be disposed at least partially within the internal volume 56 of the vessel 1. The first and second electrode 20 are arranged concentrically. The first electrode 10 extends within a central region of the internal volume 56 of the vessel 1. The second electrode 20 is arranged radially outward from the first electrode 10. The second electrode 20 may be cylindrical, as may the first electrode 10. The first and second electrode 20 are arranged co-axially in the example shown in Fig. 1. The second electrode 20 is located adjacent to an internal surface of the housing 50 (however in some examples, the second electrode 20 may be integrated with the housing 50, e.g. to form a part thereof, and / or a portion of the housing 50 may provide the second electrode 20, e.g. if said portion of the housing 50 is electrically conductive). A first end of the first electrode 10 is located outside the internal volume 56 of the housing 50. A second end of the first electrode 10, distal to the first end, is located within the internal volume 56 of the housing 50. The second electrode 20 may extend along some, or all, of the length of the internal volume 56 of the housing 50. At least one end of the second electrode 20 may extend out of the internal volume 56 of the vessel 1. Although not shown in Fig. 1 the first and / or second electrode 20 may each be coupled to a power supply. For example, each electrode may have one end which extends outside the internal volume 56 (e.g. into the housing 50), and this end may be coupled to the power supply. In some examples, the housing 50 may provide a ground (e.g. via the earth connector 22 to the electrical ground), and the first electrode 10 may be connected to a positive terminal of the power supply. In Fig. 1, the second electrode 20 is shown as being a separate component to the housing 50, but this need not be the case, as the second electrode 20 may be provided by the housing 50 (e.g. the housing 50 may be made of an electrically conductive material which may function to provide the second electrode 20). The third electrode 30 is also provided in the internal volume 56 of the vessel 1. As shown in Fig. 1, the third electrode 30 may be located entirely within the internal volume 56. For example, a mount 32 is shown for attaching the third electrode 30 to the housing 50. The third electrode 30 may extend from a first end (e.g. where it is held by the mount 32) to a second end located proximal to the second end of the first electrode 10 within the internal volume 56. The first and third electrodes 10, 30 may be parallel (e.g. they may be co-axial). The second and third electrodes 20, 30 may be parallel (e.g. coaxial). The first electrode 10 may extend from outside a first end of the housing 50 into the internal volume 56 towards an opposite end of the housing 50. The third electrode 30 may extend towards the first electrode 10 within the housing 50. The first and third electrodes 10, 30 may extend into the internal volume 56 so that there is no spatial overlap between these electrodes 10, 30 (e.g. their respective second ends do nottouch / overlap). The second electrode 20 may extend along the length of the internal volume 56 from at or outside the first end to at or outside the opposite end. The distance between the second end of the first electrode 10 and the second end of the third electrode 30 may be less than the smallest distance between the first electrode 10 and the second electrode 20. The third electrode 30 may be located away from an expected current path between the first and second electrode 20. The third electrode 30 may be arranged to electrically float within the internal volume 56. While the third electrode 30 and first electrode 10 are shown as being coaxial, parallel and central within the vessel 1, this need not be the case. One or both electrodes 10, 30 may be laterally offset from the centre of the vessel. For example, the third electrode 30 may be laterally offset from the first electrode, e.g. the third electrode 30 may not be central (e.g. but the first electrode 10 may still be central). Additionally, or alternatively, the first electrode 10 may not be central. A resistive element 40 may also be included in the internal volume 56. The resistive element 40 may be cylindrical. The resistive element 40 may be arranged to increase the electrical resistance of the conductive path between the first electrode 10 (anode) and the second electrode 20 (cathode). The resistive element 40 may be provided by a single (e.g. contiguous) piece of material or it may be provided by multiple pieces of material. For example, different portions of the resistive element 40 could be provided by different components, wherein each potion may contribute to providing an electrical resistance for the resistive element 40 as a whole. Different portions of the resistive element 40 could be electrically connected and provided by different materials / components. For example, the resistive element 40 could include circuitry, such as a sensor (e.g. a photovoltaic sensor). The different portions of the resistive element 40 need not be physically and / or electrically connected. The resistive element 40 may increase the electrical resistance between the first and second electrodes 10, 20. The resistive element 40 may extend around a majority of the internal volume 56 (e.g. along a length and width of the internal volume 56 to impede the majority of possible conductive paths from anode to cathode). The resistive element 40 may be located between the first / third and second electrodes 10, 20. For example, the resistive element 40 may be located radially outward from the first / third electrodes 10, 30, but not as far radially outward than the second electrode 20. The resistive element 40 may extend along some or all of the length of the internal volume 56. The housing 50 may be cylindrical. That is, a cross-sectional shape (i.e. when viewed in plan) of the housing 50 may be circular. Alternatively, the housing 50 may be polygon shaped. The housing 50 may be provided by a shape which is tessellatable (i.e. which is capable of being tessellated with other copies of that same shape). For example, multiple vessels 1 may be provided together, e.g. to increase output as compared to that provided by a single vessel. In which case, the vessels 1 may be stacked together. The vessels 1 may be designed to facilitate more space efficient stacking. For example, the vessels 1 may be arranged so that, when stacked together, they tessellate with each other (or at least substantially tessellate to provide more space efficient stacking). As will be appreciated, any suitable tessellatable shape may be used forthis purpose. For example, the shape may be any suitable polygon, such as a hexagon or an octagon. The shape may be imparted by an outer surface of the housing 50, e.g. with everything thereinside being circular (including the inner surface of the housing 50), or the shape may be impaired by the inner surface of the housing 50. The fluid inlet 54 is arranged at an opposite end of the housing 50 to the fluid outlet 58. The first and second electrode 20 extend along an axis extending from the fluid inlet 54 to the fluid outlet 58 (e.g. a longitudinal axis of the vessel 1). The fluid outlet 58 may be arranged higherthan (e.g. above, such as directly above or above and laterally offset from) the fluid inlet 54. For example, the fluid outlet 58 may extend vertically out of the internal volume 56 of the vessel or it may extend with a lateral component, such as diagonally or horizontally from an upper portion of the vessel 1. The housing 50 is configured to encapsulate the internal volume 56. The housing 50 is arranged to define the internal volume 56 to provide a region in which liquid may be heated. An internal surface of the housing 50 (e.g. which faces / defines the internal volume 56) may be configured to generate heat in response to incident photons (for example, the housing 50 may be conductive). The internal surface may comprise the region of the housing 50 which lies adjacent to the internal volume 56. This may comprise part of the housing 50 and / or it may comprise an additional component, such as a layer / film provided there to absorb incident photons, and in response, to generate heat. For example, the internal surface may be configured to absorb electromagnetic energy, such as in the form of visible light. The internal surface is configured to heat up as it receives incident photons. The internal surface is configured to provide heating of fluid within the internal volume 56, e.g. as it heats up from incident photons. The housing 50 may be made of a metal, such as steel, or other materials may be used, such as a ceramic. For example, a glass with e.g. boron or lead, or Quartz may be used. The housing 50 may be formed of multiple different materials. The different materials may be selected based on their photon absorption and / or transmissive characteristics (e.g. based on application needs). For example, materials may be selected which absorb photons in different wavelength range(s) for which photons are expected within the internal portion 56, e.g. for visible, infrared, ultraviolet. The housing 50 may comprise a plurality of layers, e.g. with an outer housing layer, and an inner layer, such as a sleeve, inside the outer layer. The different layers may be made of different materials. The housing 50 is configured to retain fluid in the internal volume 56 under pressure. The fluid inlet 54, the internal volume 56, and the fluid outlet 58 are arranged to define a flow path for fluid to flow through the internal volume 56 of the housing 50 (e.g. from the fluid supply system 70 through the vessel 1 and to the work extraction system 80). The internal volume 56 is arranged to receive liquid to be heated through the fluid inlet 54. The vessel 1 is arranged to heat this liquid in the internal volume 56 to provide a heated fluid. The fluid outlet 58 is arranged to provide a flow path for this heated fluid away from the internal volume 56. The first and second electrodes 10, 20 are configured to provide a current flow path through the internal volume 56 of the vessel 1. One of the electrodes 10, 20 may provide an anode, and the other may provide a cathode. For instance, the first electrode 10 may provide the anode for bringing current into the internal volume 56 of the vessel 1 (e.g. from the electrical supply system 60). The second electrode 20 may then provide the cathode for carrying current away from the internal volume 56 of the vessel 1. The first and second electrode 20 are spaced apart from each other. The first electrode 10 is arranged to receive a voltage so that a potential difference exists between the first and second electrodes 10, 20. The first and second electrodes 10, 20 are arranged capacitively. The fluid may provide an electrical resistance between the two electrodes 10, 20 (e.g. so that they are arranged capacitively across this fluid). As will be appreciated, the fluid may not have an infinite resistance, and so some electrical conduction may occur through the fluid from the first electrode 10 to the second electrode 20. In this sense, the electrodes 10, 20 and the fluid in the vessel 1 may act as a leaky capacitor. For instance, the first and second electrodes 10, 20 with fluid in the vessel 1 may effectively provide a circuit having a capacitance and a resistance. This resistance provided by this fluid may be sufficiently large that only a relatively small current will flow across the fluid, even in response to high voltages being applied. The first and second electrodes 10, 20 are configured to provide a voltage stress to fluid and / or plasma within the internal volume 56. The third electrode 30 may be active or passive. When active, a voltage is applied to the third electrode 30. When passive, the third electrode 30 may be conductive for receiving current within the internal volume 56, but without receiving power from the power supply 30. The third electrode 30 is shown passive in Fig. 1. The third electrode 30 may be configured to provide a balancing electrode (e.g. it may be arranged to balance electric field / current generated within the internal volume 56). The third electrode 30 may comprise a tip of electrically conductive material (i.e. which is arranged within the internal volume 56 of the vessel 1). The tip need not be electrically connected to a component outside of the vessel 1. For example, where the third electrode 30 is passive, the provision of an electrical conductor within the housing 50 may provide passive balancing. For example, such a tip could be capable of charging and discharging by itself. For example, the first electrode 10 may be active, the second electrode 20 may be passive and third electrode 30 could be active or passive. A distal tip of the first electrode 10 (i.e. the exposed tip within the vessel 1 would be electrically connected to a voltage source (e.g. external to the vessel 1). The second electrode 20 may be electrically grounded (e.g. so that current may flow from the second electrode 20 to ground). The third electrode 30, when passive, may provide an exposed portion of electrically conductive material within the internal volume 56 of the vessel 1. That passive exposed portion of electrically conductive material may be arranged to be charged and / or discharged within the vessel 1 (e.g. due to internal electrical conditions of the vessel 1). The third electrode 30, when active, may be connected to a voltage source. An exposed portion of the third electrode 30 within the vessel 1 may then be connected to the voltage source. The resistive element 40 may be arranged on a current flow path between the first electrode 10 and the second electrode 20, e.g. so that current would need to flow through the resistive element 40 to get from the first electrode 10 to the second electrode 20. The resistive element 40 may extend along one or both of the ends of the internal volume 56 (e.g. to reduce the likelihood of a conductive path from anode to cathode not via the resistive element 40 being possible). The resistive element 40 may be configured to be of relatively high resistance (e.g. as compared to the resistance of the electrodes and / or fluid within the internal volume 56). In other words, the resistive element 40 may increase the electrical resistance for current flow from one electrode to the other (e.g. across the fluid). In operation, a fluid is supplied from the fluid supply system 70 through the fluid inlet 54 and into the internal volume 56 of the vessel 1. In this example, the fluid will be water, but other liquids may be used. For example, the fluid may be any aqueous solution, such as tap water, sea water, ionised water etc. The fluid may comprise one or more minerals therein, e.g. in either solution or suspension within the liquid. The minerals(s) may comprise one or more transition metals. The fluid may be any non-Newtonian liquid. The liquid may be a non-electrically insulating liquid. The liquid may be at least partially electrically resistive (but not fully resistive). The vessel 1 will fill up with fluid (e.g. water). Any gas previously in the vessel 1 may be forced out through the fluid outlet 58 of the vessel 1. The vessel 1 may then be substantially filled with water. It will be appreciated that the fluid supplied to the vessel 1, while predominantly liquid (e.g. water) may contain trapped gasses and / or solid substances (e.g. which are otherwise trapped within the fluid. A voltage is applied to the first electrode 10 (anode). This will cause some current flow into the water. Due to the electrical resistance of water, this current flow and resistance will cause some heating of the water (e.g. I2R heating). This process of resistive heating continues as a voltage is applied to the first electrode 10. As the temperature of the water within the internal volume 56 rises, microbubbles of gas will start to form within the water in the internal volume 56. These may be steam bubbles forming or bubbles of air being released which were trapped in the water supplied to the internal volume 56 of the vessel 1. As a result, some pockets of gas will develop within the liquid in the internal volume 56 of the vessel 1. With continued application of the voltage to the first electrode 10, bubbles of plasma will be generated within the internal volume 56 of the housing 50. These bubbles will release energy into the surrounding fluid and the internal surface of the housing 50. In turn this provides heating of the fluid within the internal volume 56. By applying the voltage to the first electrode 10, this may charge up the capacitor provided by the first and second electrode 20. As the fluid within the internal volume 56 heats up, its permittivity may change, and this may change a capacitance of the vessel 1 (e.g. between the first and second electrodes 10, 20). For example, when water is used, its permittivity will decrease as it heats up (and then also when it becomes steam). In particular, where microbubbles of gas (e.g. steam) begin to form within the liquid in the internal volume 56, these will provide localised regions of lower permittivity. This process may effectively provide a permittivity collapse in localised regions. For example, where water is used, this difference in permittivity between bubbles forming in the water and the surrounding water may be a factor of approximately 40 (e.g. the capacitance per unit volume in those bubbles may be 1 / 40th of that of the surrounding water). During this process, the volumetric energy density for fluid and / or plasma within the internal volume 56 will remain constant. Due to the permittivity collapse within the bubbles of gas, capacitance will decrease in this region. As the volumetric energy density remains constant and the capacitance decreases, the voltage per meter will rise accordingly (e.g. to conserve energy as per E=1 / 2 CV2). For examples where water is used, the voltage per meter will rise by a factor of approximately ^40. With electrical energy still being applied to the first electrode 10, these microbubbles of gas (at lower density than surrounding liquid) will try to rapidly expand into their surroundings. However, the surrounding liquid will resist this expansion, e.g. due to the non-Newtonian nature of the liquid in these conditions. This will cause the microbubbles to rapidly increase in temperature and pressure. In turn, their capacitance will further decrease (e.g. causing an increased dV / dr), thereby further increasing the voltage stress across the bubble. With sufficient voltage stress across the bubble, ionization may occur leading to the formation of plasma within the bubble. Thus, one or more plasma bubbles may form in the liquid in the internal volume 56. The plasma may be at an even lower density than the gas, and so with a voltage still applied to the first electrode 10, the plasma bubble will further try to rapidly expand. In particular, this process of plasma bubble generation will occur rapidly, and so each bubble of plasma will drive for rapid expansion. In turn, this will bring about non-Newtonian fluid responses in the liquid in the internal volume 56 of the vessel 1. For instance, where water is used, the water does not immediately yield before the pressure wave brought about by the bubble of plasma trying to expand. The bubble of plasma is therefore held in a relatively fixed volume (e.g. it may only expand relatively slowly). While the volume of the plasma remains relatively constant, the temperature and pressure within this bubble rise rapidly in response to the voltage stress brought about by the voltage applied to the first electrode 10. As mentioned above, the breakdown of gas may occur such that a low impedance bridge forms (e.g. the gas resistivity drops), but not as far as a full breakdown in which electrical arcing occurs. In addition to this, thermionic emission may occur within the vessel 1. Electron spraying may occur with electrons moving between different electrodes of the vessel 1. In particular, electrons may pass from the first electrode 10 to the second electrode 20 and / or from the first electrode 10 to the third electrode 30. In turn, this may also cause electrons to pass from the third electrode 30 to the second electrode 20. In other words, the third electrode 30 may act to draw in electrons (i.e. from the first electrode 10) before then sending them out (i.e. to the second electrode 20). This may act to stretch out the plasma generating region, which in turn may increase the stability thereof. The electrons may accelerate through the gas bubbles which have formed. The electrodes may be designed to provide a preferential flow for the electron movement. For example, the material of each electrode (and in particular its valence) may be selected to impart this preferential flow of electrons. For example, tungsten may be used for the first electrode as it has a high valence and high melting point. The electrodes may be arranged to provide a preferential flow from the first electrode 10 to the third electrode 30 (as compared to a flow from the first electrode 10 to the second electrode 20). This may act to stretch out the plasma generating region, which in turn may provide greater stability and / or a greater amount of work output. Energy may be absorbed by atoms (and molecules) within the bubble. The energy levels (e.g. states) of these particles may therefore rise. Within the plasma, atoms may have their electrons move to higher electron energy levels, and / or spin states for these particles may change. For example, Hydrogen atom spin states may change from their lower energy para-state to their higher energy ortho-state. Molecules may also move to higher rotational and / or vibrational energy levels, and / or further splitting up of these molecules may occur. As a result, the atoms within each bubble will be at disproportionately high energy levels (e.g. as compared to conventional fluids / the fluid within the internal volume 56). Photon emission from the plasma may occur to accommodate for the high energy within the plasma. Electrons may move to lower energy electron states, and / or changes to lower energy vibrational / rotational / spin states may occur for atoms / molecules. It is this returning to lower energy configurations which gives rise to the emission of photons (e.g. to accommodate for the drop in energy levels as per the Bohr model). This emission of photons may occur on a relatively large scale. Where water is used, a large proportion of this photon emission may occur in the visible light spectrum. The photons emitted from each plasma bubble will then be absorbed by either fluid in the internal volume 56 or the housing 50 of the vessel 1. In response to receiving such incident photons, the fluid and / or housing 50 will heat up as it absorbs said photons. The inner surface of the housing 50 in particular may absorb a large number of these photons and thus increase in temperature. As the inner surface of the housing 50 heats up, it will in turn provide conductive heating of the fluid within the internal volume 56. This may give rise to convection currents occurring and thus increased turbulence for fluid within the internal volume 56 of the vessel 1. As a result of this process, the fluid within the internal volume 56 will heat up. The majority of the liquid provided to the internal volume 56 of the vessel 1 may then evaporate to provide a gas (e.g. steam). It is to be appreciated in the context of the present disclosure that some of the fluid which exits the vessel 1 may have somewhat unconventional, or at least lower energy configurations, as compared to the liquid that was provided to the vessel 1. This is as a consequence of the plasma generation and subsequent energy release which occurred within the vessel 1. In this sense, the vessel 1 may operate as a heat pump. That is, the vessel 1 is receiving a liquid, such as water (e.g. cold water) and turning this into steam. Although not shown, the vessel 1 may also include one or more filters. The filters may be for filtering solid contaminants, such as Manganese, Iron compounds or other material deposits which may accumulate within the vessel 1. For example, this may comprise a gravity filter or another suitable type of filter arranged to prevent excess build up of such material deposits within the vessel 1. This heated fluid then passes through the fluid outlet 58. Typically, the heated fluid is in the form of steam, which is generated within the internal volume 56, and which rises up and out through the fluid outlet 58. This heated fluid output from the vessel 1 may then used in the work extraction system 80 to extract useable work from that heated fluid. Operating conditions for the vessel 1 will now be described with reference to Fig. 2a. Operating conditions for the vessel Fig. 2a shows a graph with a curve illustrating an example relationship between voltage and current for the vessel 1 of Fig. 1. Three regions of operation are shown in the graph of Fig. 2a. The first is a resistive region, the second is a plasma region, and the third is an arcing region. These regions are labelled in Fig. 2a. The process for supplying electrical energy to the vessel 1, as well as how this may be controlled, will be described in more detail below with reference to Figs 2b to 6. In short, electrical energy is applied to the first electrode 10 of the vessel 1 in the form of an applied voltage signal. For this, the electrical supply system 60 provides a variable voltage source configured to deliver a voltage signal to the first electrode 10. The electrical supply system 60 may be configured to monitor both a voltage and a current as applied to the first electrode 10. The voltage and current values shown in Fig. 2a may be representative of these corresponding values, as measured by the electrical supply system 60. Fig. 2a shows the relationship between the voltage as applied to the first electrode 10 and the resulting current associated with that applied voltage. The process starts on the left-hand end of the curve, with a low voltage being applied and a low current resulting from that applied voltage. This is in the resistive region of the process. The voltage is increased overtime, and this gives rise to a corresponding increase in current. The relationship between voltage and current may be linear within this region. This linear relationship may have a relatively steep gradient, in that each additional increment of applied voltage will cause a relatively large additional increment in resulting current (as compared to the plasma region). In this resistive region, the electrical resistance may be constant or at least approximately constant. This electrical resistance may be provided at least in part by the liquid within the internal volume 56 of the vessel 1, e.g. which resists the flow of current from the first electrode 10 to the second electrode 20. While in the resistive region, this application of voltage to the first electrode 10 will act predominantly to provide resistive (e.g. I2R) heating of the liquid within the internal volume 56. Within this region, bubbles of plasma are unlikely to form within that liquid, and certainly no substantial plasma generation (and associated heat release) will be occurring. As the applied voltage gets greater, the vessel 1 may transition into its plasma region. In Fig. 2a, this is shown once the voltage gets above a first threshold voltage Vthi. Within the plasma region, the vessel 1 will operate in the manner described above, e.g. with bubbles of plasma forming and heat release being obtained therefrom. Within this region, each additional increment of voltage applied will typically cause a much smaller additional increment in current, as compared to the resistive region. As shown in Fig. 2a, the curve is much shallowerwithin the plasma region than in the resistive region. For the majority of the plasma region, the curve is again approximately linear (with a shallower gradient). For instance, the curve may only deviate from this linear regime towards the higher voltage end of the plasma region. At the higher voltage end of the plasma region, the gradient of the curve may begin to increase (e.g. with current values accelerating upwards for each additional increment of voltage applied). While in the plasma region, the vessel 1 may generate heated fluid in the manner described above in relation to Fig. 1. In particular, the conditions within the vessel 1 may give rise to a number of different mechanisms for releasing heat (in addition to the resistive heating which is the predominant source of heat release in the resistive region). For instance, thermionic emissions from the first electrode 10 may cause heating, as may photoelectric emissions from substances in the internal volume 56. One particular source of heat release may arise due to thermionically emitted electrodes being rapidly accelerated across plasma bubbles and inducing very high-speed collisions within the fluid in the internal volume 56. In turn, this may result in heavily exothermic chemical reactions occurring within the vessel 1 which further generate heat therein. For example, the conditions within the vessel 1 may cause one or more Fenton reactions to occur, one or more single-atom catalyst (‘SAC’) reactions to occur, oxidation of minerals and / or hydration of protons to occur. As a result, within the plasma region of operation, the vessel 1 is capable of providing much more heating of fluid than in the resistive region. In particular, towards the higher voltage end of the plasma region, the vessel 1 may output a great deal more heated fluid, and the processes which give rise to this heated fluid may have a far superior coefficient of performance (‘COP’) to the COP associated with resistive heating. As will be described in more detail below, the control of flow rate for fluid through the vessel 1 will also have a significant impact on performance of the vessel 1 (in addition to the voltage applied). Due to the beneficial increases in performance associated with operating in the plasma region, the vessel 1 will be controlled to operate within this region as much as possible. The arcing region is shown in Fig. 2a to illustrate deleterious effects associated with applying too great a voltage to the first electrode 10. In Fig. 2a, this is shown as the region beyond a second threshold voltage Vth2, which is greater than Vth-i. As the applied voltage gets too high, e.g. above Vth2, the conditions within the vessel 1 may begin to approach those which permit formation of an electrical arc (from the first electrode 10 across to the second electrode 20). For apparatuses of the present disclosure, electrical arcing is not wanted. Once the voltage gets too great, the effective resistance between the first and second electrodes 10, 20 will be insufficient to prevent an electrical arc occurring therebetween, e.g. due to a complete column of ionised gas being generated therebetween. As a result, and as shown by the curve in Fig. 2a, a large increase in current may then result. This may place unwanted strains on the vessel 1 due to the need to dissipate such pulses of very current electricity, and it may also cause a reduction in the plasma generation / heat release occurring within the vessel 1. As a result, apparatuses of the present disclosure are designed to inhibit electrical arcing occurring within the vessel 1. As mentioned above, the efficiency of the vessel 1 may be improved by operating towards the higher voltage end of the plasma region. To illustrate this, a ‘Main Region’ of operation is shown in Fig. 2a. The Main Region may be selected to be a region of operation which provides a relatively high COP (i.e. within a selected range) while also inhibiting the likelihood of any arc formation occurring within the vessel 1. To facilitate control of the vessel 1 to remain in or close to the Main Region while inhibiting any electrical arcing, the electrical supply system 60 may be configured to apply one or more limits to the electrical energy supplied. In Fig. 2a, three lines are shown (in dashed) which indicate a maximum current value (lmax), a maximum voltage value (Vmax), and a maximum power value (Pmax). As will be appreciated, the applied power may be derived as per P = IV, and so this represents a combined voltage and current metric. This approach will now be described in more detail with reference to Figs. 2a and 2b. Control of power supply Fig. 2b also shows a graph with a voltage-current curve. The values for voltage and current may be the same as those described above with reference to Fig. 2a, e.g. for the signal as applied to the first electrode 10 from the electrical supply system 60. The curve shown in Fig. 2b illustrates how the electrical supply system 60 may be controlled. As mentioned above in relation to Fig. 2a, it may be desirable to operate in or close to the Main Region shown in Fig. 2a, while also avoiding any electrical arcing occurring within the vessel 1. To provide this functionality, the electrical supply system 60 may be controlled to have a maximum current limit lmax, a maximum voltage limit Vmax, and a maximum power limit Pmax. Within the boundaries defined by these three parameter values is a Permitted Region (as shown in Fig. 2b), and beyond the boundaries defined by these three parameter values is an Inhibited Region (as also shown in Fig. 2b). The electrical supply system 60 may be configured to apply power-selected electrical energy to the first electrode 10. That is, the electrical supply system 60 may be configured to apply electrical energy according to a selected power level, such that the voltage applied will be as a result of the selected power and the resulting current (i.e. as per P = IV). To increase the voltage applied, e.g. to progress from left to right in the curve of Fig. 2a, the electrical supply system 60 may be configured to increase the selected power. In turn, this may cause a corresponding increase in the applied voltage and / or resulting current. Typically, increasing the power will increase the voltage. The amount by which the voltage will increase for a given increment in power may vary depending on the operating conditions of the vessel 1 (e.g. the region in which it is operating). To accommodate for variations in voltage / current arising from the applied power, the electrical supply system 60 may be configured to have limit values for each of current, voltage and power (Imax, Vmax, Pmax). These are the values shown in Figs. 2a and 2b. The electrical supply system 60 may be configured to inhibit the applied electrical energy exceeding any one of these limit values. For example, the electrical supply system 60 may be configured to reduce the electrical energy applied and / or inhibit increasing of the electrical energy being applied in the event that one of the parameter values is exceeded. For example, irrespective of the resulting current, the applied voltage may not exceed Vmax (and vice-versa). Likewise, the applied power may not exceed Pmax. The parameter values may be selected to inhibit electrical arcing occurring. For example, Vmax may be selected to be a voltage value which is below a voltage associated with causing electrical arcing to occur. Likewise, lmax may be selected to be a current value which is below that of a current arising due to electrical arcing occurring. Pmax may represent a compromise between both lmax and Vmax. Pmax may also be selected to be an input power value which is below that associated with causing electrical arcing to occur. Pmax may be selected to be at a power value lower than lmax * Vmax. As such, above a certain current level, Vmax may no longer be attainable due to Pmax being the limiting parameter (and vice-versa). By using all three limiting parameter values, the apparatus 100 may be more robust to controlling the application of electrical energy to different fluids in the vessel 1 (e.g. which may have different electrical properties). That is, the limiting factor for the electrical energy applied (e.g. the threshold value which inhibits application of further electrical energy) may vary depending on the properties of the fluid within the vessel 1. For example, at lower resistances, lmax is more likely to be limiting, whereas for higher resistances, Vmax is more likely to be limiting. Pmax may be more limiting for more average resistance values. By controlling the electrical supply system 60 in this manner, the vessel 1 may be operated to inhibit arcing occurring. For the graph of Fig. 2a, the input power may be increased overtime so as to move from left to right along the curve (e.g. from the resistive region into the plasma region). As the power is further increased, the curve will begin to approach at least one of these limiting values. As shown in Fig. 2a, lmax and Vmax may both be located at or below values prior to the electrical arc occurring (the arc actually occurring is shown by the far end of the curve where current increases exponentially quickly). Both lmax and Vmax may be located at or (slightly) beyond the Main Region of operation for the vessel 1. Pmax may straddle these two limits to be in region with relatively high current and voltage values, but not at either respective limit. In the example of Fig. 2a, Pmax is the limiting factor which prevents any further progression along the curve (e.g. into the arc region). For example, in this sense, the electrical supply system 60 may effectively supply voltage initially in a DC manner to the first electrode 10, with the voltage applied being relatively constant (unless it is being increased in the ramp up to normal mode operation) and the resulting current also being correspondingly constant. As the plasma generation begins to occur within the vessel 1, the electrical resistance conditions will vary more dynamically, as such, so too will the current response to applied voltage and the power limitations on the applied electrical energy. In which case, the voltage being applied will be in the form of a modulated DC voltage (e.g. with pulses at variable voltage depending on the resulting current). Additionally, or alternatively, it will be appreciated in the context of the present disclosure that resistance measurements could be used interchangeably or additionally to the I, V and P measurements mentioned above. For example, with knowledge of I and V, resistance could be deduced and used as an indicator of a health or operating condition for the vessel. Likewise, with knowledge of one of V or I and a resistance measurement, similar supply limitations could be imposed (e.g. in view of the known relationship as per Ohm’s law). To demonstrate how different current-voltage curves may interact differently with the different limit values, as well as to introduce other control mechanisms, reference will now be made to Fig. 3. How different properties for the supplied fluid affect vessel performance Fig. 3 shows a graph of voltage versus current values for five different example curves. Curve 1 is the same as that shown in Fig. 2a. For curves 2 to 5, only the relevant portion of the curve is shown to avoid overcrowding the graph. Also, for simplicity, all five curves shown follow the exact same trajectory (e.g. they run parallel to each other), but as will be appreciated, this may not be the case. Curves 2 and 3 are shown above curve 1, and curves 4 and 5 are shown below it. In other words, curves 2 and 3 exhibit higher current values for a given voltage than for curve 1, and curves 4 and 5 require higher voltage values to obtain the same current values as for curve 1. The present inventors have identified that different conditions for the fluid supplied to the vessel 1 may influence the ability of that vessel 1 to function as intended. Curves 2 and 5 show examples in which the vessel 1 may be unable to function satisfactorily. Curve 2 shows an example in which the electrical resistance is effectively too low. For example, the fluid within the vessel 1 may be too electrically conductive, e.g. its resistivity may be too low. In which case, the electrical energy applied to the first electrode 10 may result in a relatively high current for a given voltage. The present inventors have identified that there may be a threshold minimum voltage stress required across the vessel 1 (e.g. between the first and second electrodes 10, 20) in order to satisfactorily generate bubbles of plasma. With overly conductive fluid (e.g. as per curve 2), this voltage stress may not be achievable due to the resulting current being too high for a given voltage. For example, and as shown in Fig. 3, lmax may be a limiting factor, and so it may not be possible to enter a Main Region of operation with that fluid. Curve 5 shows an example in which the electrical resistance is effectively too high. For example, the fluid within the vessel 1 may be too electrically resistive, e.g. its conductivity may be too low. In which case, the electrical energy applied to the first electrode 10 may result in a relatively low current for a given voltage. The present inventors have identified that there may be limits to the maximum voltage to be applied to the vessel 1. With overly resistive fluid (e.g. as per curve 5), the required voltage to enter a Main Region of operation may not be achievable. For example, and as shown in Fig. 3, Vmax may be a limiting factor, and so it may not be possible to enter a Main Region of operation with that fluid. Curves 3 and 4 show examples in which the vessel 1 may function in a similar manner to that of curve 1, e.g. satisfactorily but at slightly different values. Curve 3 shows an example in which the electrical resistance is slightly lower than that for curve 1, e.g. due to the fluid being more electrically conductive. As a result, within the plasma region, the resulting current values may be slightly higher for a given applied voltage. The vessel 1 may still be able to generate plasma satisfactorily, and the Main Region of operation may occur at a slightly lower voltage (shown by MR3 in Fig. 3). For this curve, lmax may be the most applicable limiting factor to inhibit arcing. Curve 4 shows an example in which the electrical resistance is slightly higher than that for curve 1, e.g. due to the fluid being more electrically resistive. As a result, within the plasma region, the resulting current values may be slightly lower for a given applied voltage. The vessel 1 may still be able to generate plasma satisfactorily, and the Main Region of operation may occur at a slightly higher voltage (shown by MR4 in Fig. 3). For this curve, Vmax may be the most applicable limiting factor to inhibit arcing. In other words, the present inventors have identified that certain properties of the fluid supplied to the vessel 1 may provide a useful indicator as to how well the vessel 1 would be able to function using that fluid. For this, one or more different value ranges may be used. For example, a first value range may be an acceptable value range. With a parameter value for the fluid within the acceptable range, the apparatus 100 may be configured to use that fluid, e.g. because it is known that with a fluid parameter value in that range, the vessel 1 will be operable to generate a satisfactory output. For parameter values outside that acceptable range, the apparatus 100 may be configured to not use that fluid (in its current form), e.g. because it is known that the fluid with that parameter will be unable to function satisfactorily. Additionally, the present inventors have identified that notable parameter values may be different depending on the properties of the fluid used. For example, a first voltage threshold value (Vthi) at which plasma begins to generate may vary depending on the relevant parameter of the fluid. As another example, maximum acceptable values for current, voltage and / or power may also be variable in dependence upon the parameter of the fluid. Similarly, values associated with the Main Region of operation may be selected based upon on the value of the parameter of the fluid. Two particular fluid parameters of note are resistivity / conductivity and pH. Resistivity / conductivity From hereon in, reference will only be made to resistivity, but it will of course be appreciated that this also applies to conductivity, as the two are reciprocally related. The electrical supply system 60 may be configured to determine an indication of resistivity for a fluid based on obtained voltage and current data for the electrical energy applied to the first electrode 10. This may also be based on known values for the dimensions of the vessel 1, e.g. for the distance between first and second electrodes 10, 20 (and / or any resistance associated with intervening elements, such as the resistive element 40). For example, a controller of the apparatus 100 may store data (e.g. in the form of a look-up table) which links obtained voltage and current measurement values to an associated fluid resistivity. One advantage of using resistivity data is that this may be obtained using existing sensing functionality of the apparatus 100, e.g. based on voltage and current values from the electrical supply system 60. PH The apparatus 100 may be configured to determine an indication of acidity / basic the fluid is, e.g. to obtain an indication of pH for the fluid. For example, the apparatus 100 (the fluid supply system 70) may comprise a pH sensor for sensing a pH of the fluid supplied to the vessel 1. An obtained value for fluid pH may provide similar information to that obtained using resistivity. There may be a know relationship between pH and resistivity for the fluids used with the vessel 1, e.g. this may be stored by a controller of the apparatus 100 (e.g. in the form of a look-up table). The presence of additional salts in the fluid may cause a deviation from a neutral pH and these salts will also cause a corresponding decrease in resistivity. For example, in the case of tap water, a typical pH may be alkaline, and so a more alkaline pH may suggest a lower resistivity (and vice-versa). Additionally, or alternatively, pH may be used as an indicator to proton hydration reactions being able to occur, e.g. with lower pH providing an indication of more hydrogen being available for such reactions. Similarly, the apparatus 100 may be configured to monitor a pH change across the vessel 1 (e.g. difference between input and output pH), and operation may be controlled based on this change in pH across the vessel 1. Examples will be described in more detail below as to how operation may be controlled based on such obtained parameter values (e.g. resistivity / pH). Control of fluid supply The fluid supply system 70 may comprise a pump configured to deliver fluid to the vessel 1 under pressure. The pump may be configured to deliver a selected flow rate (e.g. volumetric flow rate) of fluid to the vessel 1. As already mentioned, the fluid supply system 70 may be configured to monitor one or more properties, such as pH, of the fluid it delivers. Additionally, or alternatively, the fluid supply system 70 may comprise a sensor configured to measure a resistivity of the fluid (e.g. upstream of the vessel 1). As set out above, properties of the fluid supplied to the vessel 1 may influence the ability of that vessel 1 to operate as intended. To provide greater versatility and / or reliability to the apparatus 100, the fluid supply system 70 may be configured to adjust relevant properties of the fluid it supplies. For example, the fluid supply system 70 may be configured to increase or decrease the amount of mineral(s) provided within the fluid. The mineral(s) may be provided in suspension or solution within the fluid. The mineral(s) may comprise transition metals. Minerals may be added to the fluid to increase electrical conductivity. For example, the apparatus may be configured to increase the supply of minerals to increase the electrical conductivity of the fluid, and vice-versa. The fluid supply system 70 is configured to supply a fluid, predominantly liquid, to the vessel 1. For this, the fluid supply system 70 may comprise a source of that fluid. This may be in the form of a reservoir of stored liquid (e.g. a tank, such as a water tank) and / or a connection to a supply of liquid (e.g. a mains water connection). In other words, the fluid supply system 70 may have a primary source of liquid it intends to use. Often, this may be tap water. The fluid supply system 70 may also comprise a store of one or more additional materials. The fluid supply system 70 may be configured to add said material(s) to the liquid to be supplied to the vessel 1. Said material(s) may be selected to enable the fluid supply system 70 to adjust a resistivity / pH of the liquid to be supplied. This may comprise increasing or decreasing the resistivity of the liquid supplied. The material(s) may comprise minerals, such as transition metals. The material(s) may be added to the fluid, e.g. in the form of a solution or a suspension. Additionally, or alternatively, the material could be added in the form a solid volume of that material (e.g. which may be eroded away in due course). For example, the fluid supply system 70 may be configured to bubble gas, such as carbon dioxide, through the liquid. This may alter the resistivity of that liquid. Where the liquid itself is already acidic (as is less common when using tap water), the addition of carbon dioxide may reduce resistivity, whereas if the liquid is alkaline, the addition of carbon dioxide may increase resistivity. Of course, it will be appreciated in the context of the present disclosure that alternative substances could be added to the liquid to adjust its resistivity. Additionally, or alternatively, the pH of the fluid may be controlled by selectively adding (or not) substances to the fluid. For example, the fluid supply system 70 may be configured to control the pH for the fluid to provide a pH in a desired range (e.g. suitable for proton hydration reactions to occur within the vessel). As another example, the fluid supply system 70 may be configured to reduce the concentration of additives within the liquid supplied, e.g. by diluting with more tap water (or deionised water), or by adding less to the liquid. In turn, this may also act to adjust the resistivity of the liquid. Another example which will be described in more detail below in relation to substance detection may be to incorporate within the liquid supplied to the vessel 1 some liquid which has already passed through vessel 1 and work management system (shown by a dashed line in Fig. 1). For example, the liquid which has already passed through the vessel 1 may provide a diluent effect to the liquid to be supplied to the vessel 1. In addition to adjusting the properties of the liquid supplied to the vessel 1, the fluid supply system 70 may also be configured to vary the flow rate of fluid supplied. For example, the fluid supply system 70 may be configured to perform a flushing operation. In the flushing operation, liquid may be pumped through the vessel 1, e.g. without any (or at least any high) electrical energy being applied thereto, and through the outlet to the work management system. This flushing operation may act to remove, from the vessel 1, any existing substances (e.g. existing fluids) that were contained therewithin. In other words, the fluid supply system 70 may be configured to remove existing liquid from the vessel 1 and to replace it with ‘new’ liquid. As will be described in more detail below, the fluid supply system 70 may be configured to flush the vessel 1 when performing a more significant adjustment of the fluid properties. For example, the entirety of the fluid therewithin may be replaced by a fluid with different properties (e.g. when a more significant property change is needed for said fluid). Likewise, fluid may be discarded if it is potentially dangerous. The fluid supply system 70 may be configured to adjust other properties of the fluid to be provided to the vessel 1. For example, the fluid supply system 70 may be configured to increase (or decrease) a temperature of the fluid to be supplied. The fluid supply system 70 may comprise a heater (e.g. or access to hotter fluid, such as from a hot water tank). The fluid supply system 70 may be configured to heat the fluid that is provided to the vessel 1. The fluid supply system 70 may be configured to selectively provide bubbles within the fluid to be provided to the vessel 1 and / or to increase (or decrease) the amount of bubbles within the fluid provided to the vessel 1. For example, the fluid supply system 70 may comprise a bubble generator, such as a CO2 bubbler, for adding bubbles into the fluid to be supplied. Additionally or alternatively, the fluid supply system 70 may have access to different stored fluids, where one is a fluid containing more bubbles, and increasing the amount of bubbles in the supplied fluid comprises utilising a higher proportion of the fluid with higher bubble proportion. Reference will now be made to Figs. 4 to 6, and how operation of the fluid supply system 70 and electrical supply system 60 may be controlled based on one or more detected properties of the vessel 1 and any fluid therein. Fault detection and vessel monitoring using resistance Fig. 4 shows a flow chart relating to the monitoring of an indication of electrical resistance. For this, the indication of resistance may provide a measure of the electrical resistance between the first electrode 10 and the second electrode 20, e.g. a measure of the electrical resistance brought about at least in part due to any intervening medium (e.g. fluid) within the internal volume 56 of the vessel 1. At step 400, an indication of resistance is obtained. This may be any suitable indication of resistance, it need not be a direct measurement of electrical resistance. For example, the indication of resistance may be obtained based on the applied voltage and resulting current, as measured by the electrical supply system 60 (e.g. with resistance as per R = V / l). Additionally or alternatively, other means for obtaining an indication of resistance may be obtained, such as using an alternative sensor (e.g. as part of the fluid supply system 70) and / or through inference based on sensing of another property such as a pH sensor, for example. Resistance too high At step 410, it is assessed whether or not the obtained indication of resistance is of a resistance value which is too high. For this, there may be an upper threshold resistance value. At a resistance value above this upper threshold, it is deemed that the resistance is too high. If the resistance is not too high, the method continues to step 420, but if it is too high, the method continues to steps 411 and 412. At steps 411 and 412, a liquid supply fault detection check and correction is performed. The liquid supply fault detection check is performed based on the obtained resistance value. If the resistance is too high, it may be inferred that the cause of this elevated resistance is to do with an absence of any electrically conductive material within the internal volume 56 of the vessel 1. For example, if there is no liquid present, the first and second electrodes 10, 20 may be separated by an air gap (as well as the optional resistive element 40). As such, this may cause a high electrical resistance. The upper threshold resistance value is selected to correspond to a value associated with an underfilled vessel 1. For example, this may be based on empirical data from obtaining a resistance value from an empty (or partially empty) vessel 1. In the event that the resistance is above the upper threshold value, it may be determined that a liquid supply fault has occurred. The apparatus 100, e.g. a controller thereof, may be configured to control operation of the apparatus 100 in the event that such a liquid supply fault is determined (e.g. in the event that the resistance is above a threshold value). For example, the apparatus 100 may be configured to output a signal (e.g. an alert signal) to indicate to an operator of the device that there is a fault with the liquid supply. The alert may be displayed on a display of the apparatus 100 and / or it may be sent as a message to a communications device (e.g. a mobile phone). Additionally or alternatively to outputting an alert signal, operation of the fluid supply system 70 and / or the electrical supply system 60 may be controlled in the event that it is determined that a liquid supply fault has been detected. The fluid supply system 70 may be controlled to increase delivery of fluid to the vessel 1. For example, a pump rate for the fluid supply system 70 may be increased, e.g. to try to deliver more liquid to the vessel 1. While trying to address the liquid supply fault, the resistance may be monitored. For example, one or more further indications of resistance may be obtained for the vessel 1. Based on the further indications of resistance, it may be monitored whether or not the liquid supply fault remains. In particular, in the event that the resistance value decreases (e.g. to a value below the upper threshold), it may be determined that the liquid supply fault has been resolved. For example, there may then be sufficient liquid within the vessel 1 for normal operation of the vessel 1 to be employed. Alternatively, if the resistance value remains above the upper threshold, it may be determined that the fault has not been resolved. While the liquid supply fault remains active (i.e. while it is still inferred that there is a fault with the liquid supply), the electrical supply system 60 may be configured to limit the delivery of electrical energy to the first electrode 10 (e.g. to avoid an unnecessary waste of energy). Electrical energy may still be supplied to the first electrode 10, e.g. occasionally and / or in small quantities, for the purpose of obtaining further resistance measurements. In other words, while it is determined that a liquid supply fault is in place, normal operation of the apparatus 100 may be inhibited. Instead, the apparatus 100 may be configured to try to increase the flow of liquid to the vessel 1 to try to address the fault and to reduce the amount of electrical energy applied to the first electrode 10. In the event that controlling the liquid supply system to increase liquid delivery does not address the liquid supply fault, the apparatus 100 may output an alert signal indicating that there is a fault with the liquid supply to the vessel 1. This output signal may be in the form of a message sent to a communications device, and / or it may be audibly and / or visually deployed from the apparatus 100. If it is determined that the fault is not resolvable through operation of the liquid supply system, further operation of the liquid supply system may be inhibited and / or application of electrical energy by the electrical supply system 60 may be inhibited. The controller of the apparatus 100 may be configured to repeatedly monitor the resistance values. That is, step 400 may be performed repeatedly during operation of the apparatus 100. For example, a first assessment of obtained resistance value may be performed prior to commencing start up of the vessel 1. In which case, prior to applying larger input power (e.g. a larger voltage) or at least increasing the input power, it may be first confirmed that there is sufficient liquid within the vessel 1. In the event that it is confirmed that liquid is present, e.g. if resistance is not above the upper threshold value at step 420, the start-up operation may continue. Throughout operation of the apparatus 100, e.g. even after start-up, the resistance may still be monitored to check that no liquid supply fault develops. The upper threshold value may be a fixed number for all operation of the vessel 1 or this threshold value may vary depending on the mode of operation (e.g. start-up / resistive or plasma generating). If it is determined that no liquid supply fault is present, i.e. if the resistance is below the upper threshold value, then operation of the apparatus 100 may continue. Resistance too low At step 420, it is assessed whether or not the obtained indication of resistance is of a resistance value which is too low. For this, there may be one or two different threshold resistance values, and this may depend upon the mode of operation for the vessel 1 (as described below). The resistance value(s) may be for ‘lower’ threshold(s), in that they are below the upper threshold value mentioned above. At a resistance value below the lower threshold(s), it is deemed that the resistance is too low. If the resistance is not too low, the method may return to step 40 (e.g. for further resistance monitoring), but if it is too low, the method continues to steps 421 and 422. At steps 421 and 422, two separate fault detection checks may be performed. The first fault detection check is for an electrical shorting fault. For this, it is determined whether the resistance is below a first lower threshold value. The first lower threshold value may be selected based on a level of electrical resistance which is unlikely to have resulted from the electrical energy (e.g. the applied voltage signal) travelling only towards the first electrode 10 from the electrical supply system 60. In other words, the first lower threshold value may be at a resistance value indicative of some form of electrical shorting occurring. The first lower threshold value may be a value which changes during different modes of operation of the vessel 1. For example, while the vessel 1 is in its start-up (resistive) mode of operation, the resistance value may be lower than when in the normal (plasma) mode of operation. Additionally or alternatively, in response to determining that the resistance value is below the first lower threshold, another parameter / metric may be observed to confirm that the low resistance is indicative of a shorting fault. For this, one or more metrics indicative of the vessel output may be monitored. For example, a COP for the vessel 1 or an indication of temperature / pressure of fluid exiting the vessel 1 may be determined. In the event that the other metric is below an expected threshold (e.g. has suffered a drop in its value), it may be determined that an electrical shorting fault has occurred. As one example, in the event that a vessel output metric has dropped by more than a threshold amount and the resistance is below the first lower threshold value, it may be determined that an electrical short has occurred (e.g. and that this is why the resistance and output are lower than expected). If it is determined that there is an electrical shorting fault, then at step 422, the electrical supply system 60 is configured to reduce or stop the application of electrical energy to the first electrode 10. Likewise, the fluid supply system 70 may be configured to reduce or stop the delivery of fluid to the vessel 1. An alert signal may be output by the apparatus 100 (e.g. as described above) to inform an operator of the apparatus 100 to check for an electrical short. In particular, the electrical short may typically be most likely to occur between the delivery of electrical energy to the first electrode 10 (e.g. via a connecting electrical conductor and / or the first electrode 10 itself) and the earthing connection for the second electrode 20 (e.g. to the earth connector 22). For example, the vessel 1 may comprise a bushing for electrically insulating the first electrode 10 / the electrical supply line to the first electrode 10 from the earth connector 22. The alert signal may contain an indication to check the status of the bushing. The second fault detection check is an arcing risk check. Forthis, it is determined whetherthe resistance is below a second lowerthreshold value. The second lower threshold value may be a different value to the first lowerthreshold value. The second lower threshold value may be selected based on a level of electrical resistance which is indicative of proximity to electrical arcing occurring. In other words, the second lowerthreshold value may be at a resistance value indicative of a drop in resistance arising due to conditions within the vessel 1 approaching an electrical arc occurring. The apparatus 100 may be configured to perform the arcing risk check once the vessel 1 is in its normal (plasma) mode of operation, e.g. this check may not be performed during the start-up (resistive) mode of operation. With reference to Fig. 2a, it will be appreciated that the risk of electrical arc occurring only becomes realistic when operating in the normal (plasma) mode, and towards the high voltage end of the normal mode. In the event that the apparatus 100 is not operating in that region, then no arcing risk checks may be performed. In other words, the arcing risk check may be performed in the event that the vessel 1 is operating in a region where electrical arcing could be possible (e.g. towards a high voltage limit). The arcing risk check may be performed together with a check of at least one other metric indicative of potential electrical arcing occurring. In other words, in the event that resistance is too low, another arc indicator may also be monitored to check for potential arcing occurring. For example, absolute values for voltage and / or current may be observed, and if above a threshold value, this may provide an indicator that potential arcing is near. If it is determined that there is an arcing risk, then at step 422, the electrical supply system 60 is configured to reduce or stop the application of electrical energy to the first electrode 10. For instance, in response to identifying that there is an arcing risk, the electrical supply system 60 may stop (or significantly reduce) delivery of any electrical energy to the first electrode 10. For example, a low level of electrical energy could be applied to enable measurement of resistance (e.g. to observe voltage and current values). This may not be to try to generate bubbles of plasma however. Additionally or alternatively, the fluid supply system 70 may be configured to deliver more liquid to the vessel 1. For example, the fluid supply system 70 may perform a flush operation. This may act to drive existing substances in there (which are close to permitting an electrical arc to occur) away from the internal volume 56 of the vessel 1. The fluid supply system 70 may also be configured to adjust one or more properties of the fluid delivered to the internal volume 56. For example, the fluid supply system 70 may be configured to deliver a fluid with higher resistance / resistivity to the vessel 1. This may comprise adding a component to the liquid being supplied to the vessel 1 so as to increase its resistance. For example, the fluid supply system 70 may be configured to add a mineral, such as a transition metal, to the fluid. An alert signal may be output by the apparatus 100 (e.g. as described above) to inform an operator of the apparatus 100 that the vessel 1 is close to a potential arcing event occurring. As described above, monitoring electrical resistance values may be used to provide fault detection monitoring for the apparatus 100. As will now be described with reference to Fig. 5, other electrical properties may be monitored in addition or as an alternative to resistance to provide operation monitoring for the apparatus 100. Fault detection and vessel monitoring using resistivity Fig. 5 shows a flow chart relating to the monitoring of electrical resistivity. For this, the indication of resistivity may provide a measure of the electrical resistivity of the fluid within the internal volume 56 of the vessel 1. For example, this may be for an indication of resistivity for the intervening medium between the first electrode 10 and the second electrode 20, e.g. a measure of the electrical resistivity brought about at least in part due to any intervening medium (e.g. fluid) within the internal volume 56 of the vessel 1. At step 500, an indication of resistivity is obtained. This may be any suitable indication of resistivity. As already mentioned, resistivity and conductivity may be used interchangeably, as they are reciprocally related. The indication of resistivity could therefore be an indication of conductivity or resistivity (the two provide the same information). In the following, the indication is of resistivity, and in particular resistivity for the fluid in the vessel 1. However, it will be appreciated that the measurement of resistivity may be obtained based on a measurement of electrical resistance, e.g. using voltage and current measurements. Any inference as to resistivity values may be obtained using the absolute resistance value. For example, as will be appreciated, a value for fluid resistivity may then be obtained from the resistance value in combination with knowledge of relevant vessel 1 dimensions (e.g. cross-sectional area and length separation between first and second electrodes 10, 20). Additionally or alternatively, other means for obtaining an indication of resistivity may be obtained, such as using an alternative sensor (e.g. as part of the fluid supply system 70) and / or through inference based on sensing of another property such as a pH sensor, for example. In the following, reference will be made to an indication of the resistivity of the fluid, but it will be appreciated that this indication may be any suitable parameter value from which resistivity may be determined or which is itself indicative of electrical resistivity. No absolute resistivity calculation may ever need to be performed. For example, this method could be performed using only values for electrical resistance (e.g. without converting that into a resistivity value), as the resistance values may themselves be sufficiently informative as to electrical resistance. Unsuitable fluid Based on the obtained indication of resistivity, steps 510 and 520 then assess whether this resistivity value is too high or too low. At step 510, it is determined whether resistivity is too high. For example, the resistivity value may be compared against an upper threshold value, and if it is greater than this upper threshold value, the fluid is deemed too resistive. Similarly, at step 520, it is determined whether resistivity is too low. For example, the resistivity value may be compared against a lower threshold value, and if it is below this lower threshold value, the fluid is deemed too conductive (not resistive enough). For both steps, the check is to see if the liquid will be suitable for plasma generation therein. If, at step 510, the resistivity is too high (e.g. above the upper threshold value), it may be determined that the fluid is unsuitable. At steps 511 and 512, the apparatus 100 may determine that the fluid is too resistive, and the fluid supply may be controlled to address this issue. For example, the fluid supply may be controlled to decrease the resistivity (e.g. by adding minerals to the fluid to increase its conductivity). The upper threshold value may be selected to be at a value above which the plasma generating process will not function satisfactorily. For example, at a resistivity above the upper threshold value, the current resulting from an applied voltage will be too low. In which case, the application of the voltage during the start-up mode may give rise to relatively weak resistive (l2R) heating of the liquid in the vessel 1 (and so it may be hard or impossible to generate any bubbles of gas therein). In turn, at a resistivity above this upper threshold value, it may not be possible to initiate the plasma generation within the vessel 1 and / or Vmax (or Pmax) may become limiting before the Main Region of operation has been achieved. If the resistivity is above the upper threshold value, it may be determined that new fluid is needed. In other words, for resistivity above the upper threshold value, the apparatus 100 may conclude that the fluid should be replaced rather than still trying to use that fluid during the process. For this, the fluid supply system 70 may be controlled to supply a lower resistivity fluid. For example, the fluid supply system 70 may adjust the fluid to reduce its resistivity, e.g. one or more substances may be added to the fluid to render it more electrically conductive. This may comprise performing a flush of the existing fluid from the vessel 1 so that it is replaced with more electrically conductive fluid. If, at step 520, the resistivity is too low (e.g. below the lower threshold value), it may be determined that the fluid is unsuitable. At steps 521 and 522, the apparatus 100 may determine that the fluid is too conductive, and the fluid supply may be controlled to address this issue. The lower threshold value may be selected to be at a value below which the plasma generating process will not function satisfactorily. For example, at a resistivity below the lower threshold value, the current resulting from an applied voltage will be too high. In which case, while the resistive (l2R) heating of the liquid in the vessel 1 may be relatively strong (and thus bubbles of gas may be generated within the liquid), it may not be possible to achieve a sufficient voltage stress across the bubbles due to the voltage being relatively low (due to the low resistivity). In turn, at a resistivity below this lower threshold value, it may not be possible to initiate the plasma generation within the vessel 1 and / or lmax (or Pmax) may become limiting before the Main Region of operation has been achieved. If the resistivity is below the lower threshold value, it may be determined that new fluid is needed. In other words, for resistivity below the lower threshold value, the apparatus 100 may conclude that the fluid should be replaced rather than still trying to use that fluid during the process. For this, the fluid supply system 70 may be controlled to supply a higher resistivity fluid. For example, the fluid supply system 70 may adjust the fluid to increase its resistivity, e.g. one or more substances may be added to the fluid to render it less electrically conductive. This may comprise performing a flush of the existing fluid from the vessel 1 so that it is replaced with more electrically conductive fluid. Resistivity monitoring may be repeated during use. For example, if the fluid is to be replaced (in steps 512 and 522), the resistivity for that fluid may be repeatedly monitored until the resistivity value is within an acceptable range, e.g. below the upper threshold value and above the lower threshold value. Catering for different resistivity values If, at steps 510 and 520, the resistivity is neither too high, nor too low (e.g. between the lower and upper threshold values), at step 530 it may then be determined whether the resistivity is within a preferred range of operation. The preferred range of operation may comprise a subset of the values between the lower and upper threshold values. The preferred range may comprise some of the values between the lower and upper threshold values, but not all of them. For example, the preferred range may be associated with resistivity values which yield the most efficient operation of the vessel 1 (e.g. which are close to optimal operating conditions). In other words, for resistivity values between the lower and upper threshold values, it may be possible to initiate the plasma generating process in the vessel 1, but for resistivity values within the preferred range, the output from the process may be above a threshold output criterion (e.g. the output may be at an elevated level). At step 530, it is assessed whether the resistivity is within the preferred range, or not. If the resistivity is outside the preferred range, at steps 531 and 532, it may be determined that operation of the vessel 1 is too inefficient (e.g. sub-optimal), and that the apparatus 100 should be controlled to improve the operating characteristics for the vessel 1. For example, at resistivity values above the preferred range (e.g. but below the upper threshold value), it may be harder and / or less efficient to provide resistive heating forthat fluid. In turn, generating gas bubbles in the fluid may require more energy and / or may take more time. For example, at resistivity values below the preferred range (e.g. but above the lower threshold value), it may be harder to achieve the required voltage stress across the fluid and / or the voltage stress which can be achieved may be lower. At step 532, the apparatus 100 may be controlled to adjust the operating characteristics for the fluid, e.g. to improve the operation of the vessel 1. One option to improve the operation comprises controlling operation of the fluid supply system 70. Controlling operation of the fluid supply system 70 may comprise supplying fluid at a different temperature (e.g. heated fluid), supplying fluid with a different amount of bubbles therein (e.g. increasing the amount of bubbles), and / or supplying fluid with different properties (e.g. with a difference in the type and / or amount of substance(s) therein). For example, substances, e.g. minerals, may be added, such as transition metals (either in solution or suspension within the fluid). In the event that the resistivity is above the preferred range of values, the fluid supply system 70 may pre-heat the fluid to be supplied to the vessel 1 and / or increase the amount of bubbles therein. For example, this may help to address the reduction in l2R heating associated with lower resistivity fluids, as the supplied fluid may be warmer (and so may be closer to bubble generation occurring therein) and / or more bubbled (and so may be easier to result in plasma formation occurring). Additionally, or alternatively, the fluid supply system 70 may be configured to adjust the fluid supplied to the vessel 1 to have a lower resistivity. For example, one or more substances may be added to that fluid or less / fewer substances may be added to the fluid supplied to the vessel 1 so that its resistivity drops. Where the resistivity is high, but not above the upper threshold, the fluid supply system 70 may apply a less severe change to the fluid properties. For example, the fluid supply system 70 may not perform a full flush of the vessel 1, but instead it may operate so that properties of fluid which is to be subsequently delivered to the vessel 1 may be adjusted accordingly. In the event that the resistivity is below the preferred range of values, the fluid supply system 70 may be configured to adjust the fluid supplied to the vessel 1 to have a higher resistivity. For example, one or more substances may be added to that fluid or less / fewer substances may be added to that fluid so that its resistivity increases. Where the resistivity is low, but not below the lower threshold, the fluid supply system 70 may apply a less severe change to the fluid properties. For example, the fluid supply system 70 may not perform a full flush of the vessel 1, but instead it may operate so that properties of fluid which is to be subsequently delivered to the vessel 1 may be adjusted accordingly. The operation of the fluid supply system 70 may be controlled differently depending on the mode of operation for the vessel 1. During the start-up operation, increasing temperature and / or bubble concentration within the fluid may be used, but during normal operation, this may be used less (or not at all). Likewise, during start-up, the resistivity of the fluid may be adjusted more than during normal operation. Another option to improve the operation comprises re-calibrating certain control parameters. The apparatus 100, and in particular the electrical supply system 60, may be controlled based on a number of different parameter values associated with the application of electrical energy to the first electrode 10. For example, as described above in relation to Fig. 2b, operation of the electrical supply system 60 may be controlled according to one or more of maximum values for current, voltage and power (Imax, Vmax and Pmax). Similarly, the apparatus 100 may store one or more other reference values, such as a voltage value (or range of values) at which it is expected that plasma generation will commence within the vessel 1 (Vthi), and / or voltage (or current) values associated with a Main Region of operation (e.g. a selected range for operating the vessel 1 in its normal mode, such as for particularly efficient operation of the vessel 1). The apparatus 100 may be configured to control one or more of these values (e.g. adjust the value(s)) based on the obtained resistivity value. For example, the apparatus 100 may provide an adjustment in value of parameter corresponding to the value of the resistivity and / or corresponding to a deviation in value of that resistivity from the preferred range of resistivity values. The adjustment in parameter value(s) for controlling the electrical supply system 60 (e.g. a magnitude and / or direction in the change) may be selected to account for different electrical properties arising due to the resistivity of the fluid being different relative to the preferred range of values. In the event that the resistivity is above the preferred range of values, one or more of the values may be adjusted to account for the higher resistivity. That is, the value(s) may be adjusted to account for the increased electrical resistance against applied voltage for that fluid, as compared to a selected / preferred range of such resistivity values. For example, Vmax, orVthi may be increased, as may be Pmax. The apparatus 100 may be configured to output an alert in the event that the voltage is above Vthi without plasma generation occurring within the vessel 1. The value Vthi may be increased for higher resistivity fluids, e.g. so that the alert is only output if there is still no plasma at a higher voltage. Similarly, as mentioned above, the apparatus 100 may output an alert and / or control operation accordingly in the event that Vmax or Pmax are exceeded, but this may only occur at higher V or P values for higher resistivity fluids. In the event that the resistivity is below the preferred range of values, one or more of the values may be adjusted to account for the higher resistivity. For example, lmax may be increased, as may Pmax, e.g. to account for a higher power being needed to obtain the required voltage values. A magnitude of the change(s) may correspond to a magnitude of the deviation from the preferred range of values. The parameters which are changed and / or the amount by which they are changed may be controlled differently depending on the mode of operation for the vessel 1. During the start-up mode, Pmax, Vmax and / or lmax may not be changed (or not changed as much), as these parameter values may be of less relevance to controlling operation. However, these parameter values may be adjusted (or adjusted more) based on resistivity data obtained during the normal mode of operation. Conversely, parameter values such as Vthi may be monitored and adjusted more during start-up operation than during the normal mode. For example, an initial resistivity indication (e.g. once the vessel 1 has been filled with fluid) may be obtained and used to set Vthi forthat start-up operation. In addition to setting such parameter values accordingly, the electrical supply system 60 may also be controlled to deliver electrical energy to the first electrode 10 according to said adjusted parameter values. For example, the electrical supply system 60 may be configured to permit the application of higher power and / or higher voltage electrical energy in the event that the resistivity value is above the preferred range. Likewise, the electrical supply system 60 may be configured to permit the application of higher power and / or higher current electrical energy in the event that the resistivity value is below the preferred range. In other words, where the resistivity of the fluid deviates from an expected / preferred value range, operation of the apparatus 100 may be controlled to account for such deviations in fluid properties. For example, where tap water is to be used for fluid, it will be appreciated that the particular constituents of that tap water may vary between different regions. The apparatus 100 may be designed to adapt to such differences in fluid by adjusting operation accordingly. Such changes in operating conditions may permit a range of different fluid properties to still allow the apparatus 100 to function. The apparatus 100 may have up to three different ranges for fluids it receives: (i) a first range (above the upper threshold or below the lower threshold), where the fluid is not used (e.g. it may be flushed and replaced with a different fluid), (ii) a second range (between the lower and upper thresholds but outside the preferred range), where the fluid is used but operation of the apparatus 100 may be adjusted accordingly (e.g. to accommodate for deviation in fluid properties from an expected value range), and (iii) third range (within the preferred range), where the fluid is as expected / preferred and normal operation of the apparatus 100 is used (without needing fluid-based adjustment). As described above, these assessments / changes may be made during either start-up or normal modes of operation. As will now be described, one particular benefit of using resistivity measurements may be for controlling the start-up mode. Controlling start up based on resistivity At step 540, it is determined whether the approach for start-up of the vessel 1 should be changed. This assessment may be performed initially, e.g. prior to start-up. It may no longer need to be performed once the vessel 1 is in its normal mode of operation. The present inventors have identified that different approaches for the start-up operation of the vessel 1 may be beneficial depending on one or more properties of the fluid within the vessel 1. One parameter for consideration is an indication of a temperature of the fluid within the vessel 1. Another parameter for consideration is a resistivity of the fluid. The apparatus 100 (e.g. the electrical supply system 60) may be configured to provide two or more different profiles for start-up of the vessel 1. While it may be preferable to achieve a quicker start-up (i.e. so that the plasma generating mode is reached quicker), a quick start-up may not always be achievable and / or efficient. The apparatus 100 may be configured to provide a quicker (or more aggressive) start-up profile for hotter and / or less resistive fluid in the vessel 1. For this, the apparatus 100 may increase the magnitude of the applied electrical energy more quickly. For example, the applied energy may be controlled according to a selected power level, and that selected power level may be increased more rapidly in the quicker start-up mode. As a result, it may take less time until greater voltages are being applied to the first electrode 10, e.g. and so Vthi may be reached more quickly. The apparatus 100 may be configured to provide a slower (and less aggressive) start-up profile for cooler and / or more resistive fluid in the vessel 1. For this, the apparatus 100 may increase the magnitude of the applied electrical energy less quickly. For example, selected power level may be increased more slowly in the slower start-up mode. As a result, it may take more time until greater voltages are being applied to the first electrode 10, e.g. and so Vthi may be reached more slowly. The present inventors have identified that a value for Vthi may itself vary depending on the start-up profile taken, e.g. depending on a ramp rate of the applied electrical energy. For the slower ramp rate, Vthi may be at a lower value than for the quicker ramp rate. At step 540, it is determined what profile to adopt for the start-up operation. For example, this may comprise comparing an obtained indication of temperature against a threshold value. If above the threshold temperature, a quicker start-up may be employed than if below the threshold temperature. Additionally or alternatively, at step 540, the indication of resistivity may be used to identify what profile to adopt for the start-up operation. If the resistivity is above a start-up resistivity threshold, then the quicker start-up may be employed than if the resistivity was below said threshold. This ramping process for start-up may also be controlled based on an end value for the ramp. For example, there may be an ‘end-voltage’, at which it is expected that the start-up process will be complete (and plasma generation will be occurring within the vessel 1). The end value forthe voltage may be selected based on the obtained resistivity value forthe fluid, e.g. to account for any difference in this voltage arising due to the use of more / less resistive fluid. This end-voltage may be selected based on a value of the temperature of the fluid in the vessel 1. At steps 541 and 542, it is determined how the start-up should be performed (e.g. what the relevant parameter metrics are and which start-up profile to use) and the electrical supply system 60 is configured to control the delivery of electrical energy to the first electrode 10 according to the determined start-up approach. For example, the start-up may comprise following either a first ramp rate or a second ramp rate depending on an obtained indication of resistivity and / or temperature for the fluid. The first ramp rate may be quicker (e.g. more aggressive) than the second. The controller may select the first ramp rate if a temperature and / or resistivity forthe fluid is above a selected value, e.g. above which a quicker acceleration should still work. The controller may select the second ramp rate if a temperature and / or resistivity for the fluid is below a selected value, e.g. above which a quicker acceleration should still work. Ongoing monitoring Resistivity values forthe fluid may be monitored during normal mode operation of the vessel 1 as well. For example, once the start-up process is complete, resistivity may still be monitored in the manner mentioned above in relation to steps 510 to 532. Additionally or alternatively, resistivity monitoring may be employed as an additional fault detection mechanism. For this, the resistivity may be monitored to detect a sudden and / or significant change in value associated with a fault of the apparatus 100. For example, the apparatus 100 may determine that there is a fault in the event that there is a change in resistivity, e.g. such that it was within an acceptable range (e.g. the preferred range and / or between the lower and upper threshold values) and it then moves outside of that range. In this event, the apparatus 100 may output an alert signal indicating that there is a possible fault with the apparatus 100. For example, the fault may be due to the fluid being used and / or the application of electrical energy to that fluid. In this event, the electrical supply system 60 may be configured to stop or reduce the application of electrical energy. For example, one or more properties of the fluid may have changed (which may be problematic and / or the equipment may have degraded, e.g. the first electrode 10). As such, in addition to regulating resistivity to ensure satisfactory operation of the vessel 1, resistivity may additionally or alternatively be monitored for fault detection. Another parameter which may be used for fault detection is the wavelength of light being emitted within the vessel 1, as will now be described with reference to Fig. 6. Fault detection and vessel monitoring using wavelength of emitted light The present inventors have also identified that operation of the apparatus 100 may be controlled based on an observation of the wavelengths of light being emitted within the vessel 1. For instance, the present inventors have identified that particular wavelengths of emitted light may be associated to the presence of certain substances within the vessel 1. As will be appreciated, each wavelength of emitted light may be associated with a certain energy level transition within a given atom or molecule. In turn, each wavelength of emitted light may provide some indication as to what atoms / molecules are present, as well as what transitions are occurring. The present inventors have identified that monitoring the wavelengths of emitted light may provide indication as to whether the vessel operation is satisfactory, or if any unwanted substances may be present. For this, the apparatus 100 may comprise one or more sensors configured to detect wavelengths of light being emitted within the vessel 1. For example, there may be a sensor located within the vessel 1, or a sensor may be located somewhere with line of sight to the internal volume 56 of the vessel 1. For example, the sensor may be separated from the internal volume 56 by a portion of transparent (or at least substantially transparent) material. As one example, at least a portion of the housing 50 of the vessel 1 may comprise an optical window, e.g. a transparent region. The sensor may be located adjacent to that window. Alternatively, the sensor may be located within the vessel 1, and it may be shielded from the internal conditions of the vessel 1 by some transparent material. A controller of the apparatus 100 may be configured to obtain wavelength data from the sensor. For example, the controller may be configured to obtain an indication of what particular wavelengths of light are being emitted. This may be for each wavelength emitted or it may be for a subset of the emitted wavelengths. The controller may be configured to obtain an indication of the wavelengths of light being emitted, as well as an indication of the intensity of each particular wavelength. For example, the intensity may provide an indication of how many such wavelengths are being emitted (e.g. how many of those energy level transitions are occurring within the vessel 1). In other words, the controller may be configured to obtain an indication of which particular wavelengths are occurring, as well as how much each of those wavelengths is occurring. Reference will now be made to Fig. 6 for an example of processes for controlling operation of the apparatus 100 using wavelength data. Fig. 6 starts at step 600 by obtaining an indication of one or more wavelengths of light being emitted within the vessel 1. This may occur as mentioned above, e.g. the indication may contain a number of different wavelengths which are being emitted, as well as an intensity of each of these emissions. Detecting harmful substances At step 610, it is determined whether or not the emissions are associated with any unwanted substances. For this, a controller of the apparatus 100 may have access to a data store which contains a list of wavelengths associated with unwanted substances (e.g. atoms / molecules). As will be appreciated, each substance may give rise to a number of different energy level transitions, where each such transition has a specific wavelength associated therewith. The apparatus 100 may have one or more substances which are unwanted. The number of unwanted substances, as well as what those substances are, may depend upon the particular use case for the apparatus 100. What the particular unwanted substances are may vary depending on the particular fluid which is used. For example, if tap water is to be used, the different substances present may vary depending on location and setting for the usage, and so this may influence what the unwanted substances in question may be. As one example, substances to be avoided may comprise substances whose presence may have damaging effects to the apparatus 100 itself. For example, the presence of fluorine may be associated with a greater likelihood of damaging the apparatus 100. In which case, it is beneficial for the apparatus 100 to avoid operating on fluid which contains larger quantities of those unwanted substances (e.g. fluorine). At step 610, an initial unwanted substance check is performed. For this, any detected wavelengths occurring within the vessel 1 may be compared against the stored list of unwanted wavelengths (e.g. wavelengths associated with unwanted substances). In the event that there is a match (i.e. that one or more of the detected wavelengths occurring within the vessel 1 corresponds to a wavelength associated with an unwanted substance), the method continues to step 611. If not, the method continues to step 620. At step 611, it is known from the assessment at step 610 that there is at least one match where a wavelength within the vessel 1 corresponds to a wavelength associated with an unwanted substance. At step 611, the significance of this match is further investigated. For this, it may be determined how much of the unwanted substance is present and / or how likely it is that it is the unwanted substance that is present. The assessment at step 611 may be based on an obtained intensity of the light at each particular wavelength. This may comprise determining whether the intensity for any particular wavelength is above a threshold value. If the intensity is above threshold value, it may be determined that there is too much of the unwanted substance present. In which case, the method may proceed to step 612. Additionally, or alternatively, at step 611, the assessment may be based on the presence or absence of other wavelengths of light. For example, for a given substance present in the fluid, there may be a plurality of different wavelengths associated with energy level transitions for that substance. Based on the different transitions involved, and the respective likelihood of each of these occurring, it may be possible to predict an expected proportion of each different transition which may occur. In other words, there may be a particular signature of transitions for each given unwanted substance. The assessment at step 611 may comprise determining whether the signature for an unwanted substance is present based on a plurality of different wavelength transitions. For example, this may reduce the likelihood of a false identification occurring where the particular wavelength observed may be associated with another substance (e.g. which is not an unwanted substance). If at step 611 it is determined that the unwanted substance is present in a substantial quantity and / or with a substantial significance of it being present, it is determined that this poses a risk to operation of the apparatus 100. In which case, the method proceeds to step 612, where action is taken to try to avoid the presence of this substance causing any damaging effects. For this, the electrical energy applied to the electrode may be stopped or reduced. Likewise, the fluid within the vessel 1 may be flushed out. An alert may be output that the fluid supply needs to be checked. For example, the apparatus 100 may not be operated until it can be confirmed that the unwanted substance is not present. Additionally or alternatively, the method may comprise supplying a different fluid to the vessel 1, e.g. one which is known not to contain the substance of interest. For example, fluid which had previously passed through the vessel 1 and which did not contain the substance of interest may be reused (e.g. it may be provided to the vessel 1 for plasma generation thereof again). The method may comprise ongoing monitoring to check that the unwanted substance does not become present again. If at step 611 it is determined that an insufficient amount of the substance of interest is present or that the wavelength of interest may correspond to a different substance, the method may proceed to step 613. At step 613, operation is controlled to provide ongoing monitoring of this situation. For example, wavelengths of light may be repeatedly obtained and checked to see if they do contain any selected wavelengths. Additionally or alternatively, one or more of the power supply and the fluid supply may be controlled accordingly. For example, the power applied may be reduced, and / or the fluid supplied may have one or more properties adjusted. For example, the fluid may be diluted with substance known to contain no unwanted components. In other words, at steps 610 to 613 it is determined whether or not an unwanted substance is present, and in such a way as to potentially cause damage to the apparatus 100. If such an unwanted substance is present, then the apparatus 100 may be controlled to prevent such damage occurring. This may comprise stopping operation of the apparatus 100, e.g. until it is deemed safe to use again. Where such an unwanted substance is not found to be present, then operation of the apparatus 100 may continue. In which case, the situation may be monitored to check that no further indications of a substance of interest occur and / or the operation of the apparatus 100 may be controlled to be within saferoperating conditions. Detecting intended substances While steps 610 to 613 relate to detecting potential fault conditions forthe vessel 1 based on obtained wavelengths, the obtained wavelengths may additionally or alternatively be utilised to determine whether operating conditions forthe vessel 1 are as desired. For this, at step 620 it is determined whether or not any wanted wavelengths are present. This assessment may be very similar to that performed at step 620, with the exception that stored wavelengths may be used which correspond to wanted substances (not unwanted substances). For example, for any given fluid supplied to the vessel 1, there may be a number of substances which are expected to be present (such as hydrogen, nitrogen, carbon, oxygen etc.), and observing energy level transitions associated with these substances may confirm that the plasma generation process is occurring satisfactorily within the vessel 1. If at step 620 it is determined that there are one or more wavelengths present which correspond to a wanted substance, the method proceeds to step 621. As with step 611, step 621 may comprise further determining the quantity of those substances which is present and / or how likely it is that the obtained wavelengths correspond to the substance in question. Again, at this stage, this assessment relates to substances (e.g. energy level transitions) which are wanted. If it is determined at step 621 that a wanted substance is present, and that this substance is present in a satisfactory quantity (e.g. the intensity associated with the wavelength(s) forthat substance is above a threshold), this may be taken as an indicator that the process is functioning satisfactorily. In which case, the method may continue to step 622, where operation of the apparatus 100 is controlled with the same operational parameters, e.g. as it is deemed that these parameters are producing the desired results. If it is determined at step 621 than the wanted substance(s) is not present, or is not present in satisfactory quantity, the method may proceed to step 623. At step 623, the electrical supply system 60 and / or fluid supply system 70 may be controlled accordingly. For example, the amount of electrical power applied may be increased (e.g. to try to cause the desired energy level transitions to occur within the vessel 1) and / or the fluid supply may be adjusted so that it is more likely that the fluid being used will yield the desired energy transitions (e.g. by adding one or more substances to the fluid supplied). Detecting informative substances In a manner analogous to that at steps 610 and 620, the apparatus 100 may also store data associated with other known substances that may be present in the vessel 1. These other substances (and their known signatures / transition levels) may not necessarily be wanted or unwanted, but they may nevertheless provide some information as to the particular operating conditions within the vessel 1. From hereon in, these will be referred to as ‘informative substances’. At step 630, it is determined whether any informative substances are present. Again, this may comprise comparing obtained wavelength(s) against known wavelengths. The informative substances may comprise substances which provide a particular indication of what energy level transitions are occurring within the vessel 1. For example, they may provide an indication of what substances are present within the fluid supply and / or they may provide an indication of how well the plasma generating process is functioning. Some energy level transitions may start occurring before others. For example, the fluid may comprise a plurality of different substances, e.g. oxygen, nitrogen, carbon and / or hydrogen, and some may start undergoing the relevant energy level transitions before others. As the process continues, e.g. as operating conditions approach higher levels (e.g. for applied energy, temperature, pressure etc.), different energy level transitions may occur for the known substances in the fluid. The different wavelengths present may therefore provide an indication of the current mode of operation (e.g. how close to the Main Region it is). If at step 630 it is determined that an informative substance is present (e.g. that informative wavelength(s) are present), the method may proceed to step 631. At step 631, the process may be controlled accordingly. For example, there may be stored data linking particular detected substances to a corresponding operating condition to be applied. At step 631, the electrical energy applied to the electrode and / or the fluid supplied to the vessel 1 may be controlled based on the detected informative substance. This may comprise following stored protocols, e.g. to increase power and / or to adjust at least one property of the fluid depending on the substance present. In other words, at step 631, the apparatus 100 is controlled based on the obtained information relating to what substances are present within the vessel 1. This control is to move the apparatus 100 towards its Main Region of operation, e.g. and where this is determined based on what substances are detected to be present in the vessel 1. If at step 630 no informative wavelengths have been detected, the method may proceed to step 632. Here, the apparatus 100 may be controlled to try to cause such energy level transitions to occur. For example, the power applied may be increased and / or one or more properties of the fluid may be adjusted. In other words, much like during start-up conditions for the vessel 1, at step 632 the apparatus 100 may be controlled to try to generate more bubbles of plasma (and associated energy release). The wavelengths being emitted may be repeatedly monitored during operation of the vessel 1. As will be appreciated in the context of the present disclosure, this light (and associated wavelengths) will typically only be emitted during the normal mode of operation, e.g. once plasma generation has started occurring. Prior to this, there may be little to no wavelength emissions. As a result, during start-up conditions, the primary monitoring may just be to see if any wavelengths are being emitted. If not, the start-up process will continue (e.g. increasing power applied). Once the normal mode of operation is underway, the steps 610 to 613 may be for instantaneous fault detection. Steps 620 to 632 may all relate to trying to guide the apparatus 100 towards its desired operating conditions (e.g. the Main Region). Low levels of wanted wavelengths and / or the informative wavelengths may be relevant as the process approaches the Main Region (e.g. to detect progress). Once the desired quantities of the wanted substances are present, the informative wavelengths may be less useful, as the main parameters of note may be that the wanted wavelengths are occurring in a satisfactory quantity and that no (or not enough) unwanted wavelengths are occurring. Purification system using vessel and monitoring wavelength of emitted light In the examples described so far, the apparatus 100 may be used primarily for the purposes of generating a heated fluid from which work may be extracted. For example, the work extraction system 80 may comprise any suitable arrangement for extracting useable work from heated steam, such as through a steam engine or a heat exchange apparatus. Another beneficial use of the apparatus 100 disclosed herein may be for liquid purification. That is, a liquid may be supplied to the vessel 1, and through operation of the vessel 1 for generating bubbles of plasma, one or more substances may be removed from the liquid supplied to the vessel 1. For this, the exothermic reactions occurring within the vessel 1 may act to consume or change form of some of the substances within that fluid. For example, the conditions within the vessel 1 may cause larger molecules to be split into smaller component parts, where these smaller component parts may then bind to other atoms / molecules present, thereby to take the form of less harmful substances. For example, one or more Fenton reactions may occur, as may one or more single-atom catalyst (‘SAC’) reactions or proton hydration reactions. As a consequence of such reactions occurring, substance oxidations may also occur, thereby consuming those substances. In turn, this may act to at least partially remove those substances from the fluid, and so the fluid which is output from the vessel 1 may have been purified. For example, the heated fluid may exit predominantly in the form of a gas (e.g. steam). That gas may be used for the extraction of work, with the fluid which remains after having been used for work extraction (typically a liquid) being a purified version of the liquid supplied to the vessel 1. Embodiments of the present disclosure may therefore provide fluid purification apparatuses and methods. For this, the fluid supply system 70 may be configured to supply a liquid to be purified to the plasma generating vessel 1. For example, purification may occur due to one or more of the reactions occurring within the vessel 1 (e.g. Fenton, proton hydration and / or SAC reactions). The plasma generating vessel 1 may be configured to operate as described above, and to provide, as its output, a heated purified fluid. The apparatus may be configured to re-use some of the fluid output from the vessel 1. For example, the apparatus may be configured to recircle some of the fluid back to the vessel 1 for further purification thereof. A controller of the apparatus may be configured to utilise one or more of the methods disclosed herein for controlling operation of the vessel 1. In particular, the controller may be configured to monitor the wavelengths of light being emitted within the vessel 1 to identify whether or not substances of interest are being removed from the fluid within the vessel 1. The controller may be configured to monitor the wavelengths of light being emitted within the vessel 1 to determine whether or not the substance of interest is present. For example, the controller may be configured to determine that the substance of interest is present in the event that one or more wavelengths of light associated with that substance are detected. The controller may be configured to determine that, in response to detecting said wavelengths of light being emitted, at least some of that substance is being removed by the plasma generation and subsequent light release. The controller may be configured to monitor emission of wavelengths of light within the vessel 1 and to control operation of the apparatus so that the desired wavelengths are being emitted. For example, the apparatus may operate according to steps 620 to 623 in particular in Fig. 6 to detect that the desired wavelengths of light are being emitted within the vessel 1. The apparatus may be configured to continue using the liquid within the vessel 1 while the relevant wavelengths of light are detected. For example, the apparatus may be configured to output purified fluid in the event that no further emissions are detected which are associated with the substances to be removed. Additionally or alternatively, the apparatus may comprise one or more sensors configured to detect the properties of the fluid. For example, the sensor may be configured to detect what substances are in the fluid, e.g. whether or not the fluid contains a substance to be removed. The controller may be configured to control operation of the vessel 1 based on data output from said sensor. For example, the apparatus may be configured to generate bubbles of plasma within the vessel 1 until sensor data indicates that the substance(s) to be removed have been removed from the fluid. For example, the apparatus may be configured to recirculate fluid through the vessel 1 until the sensor indicates that the substance to be removed has been removed. Alternatives and variants It is to be appreciated in the context of the present disclosure that the precise combination of method steps set out above for each flow chart do not need to be considered limiting. Similarly, while different methods are shown in the flowcharts, it will also be appreciated that these methods may be combined, e.g. performed simultaneously. Also, it will be appreciated that not all of the method steps need to be implemented together. Embodiments may contain methods which only use one or some of the method steps. For example, any one of the question boxes in the flow charts may be used on its own or in combination with other question boxes. It will also be appreciated that, while reference has generally been made to certain control parameters and thresholds, the values for these may be variable for different use cases. Similarly, while embodiments described above include use of lmax, Vmax and Pmax, it will be appreciated that beneficial effects may be obtained using one, two or three of these values. It will be appreciated from the discussion above that the examples shown in the figures are merely exemplary, and include features which may be generalised, removed or replaced as described herein and as set out in the claims. With reference to the drawings in general, it will be appreciated that schematic functional block diagrams are used to indicate functionality of systems and apparatus described herein. In addition, processing functionality may also be provided by devices which are supported by an electronic device. It will be appreciated however that the functionality need not be divided in this way, and should not be taken to imply any particular structure of hardware other than that described and claimed below. The function of one or more of the elements shown in the drawings may be further subdivided, and / or distributed throughout apparatus of the disclosure. The function of one or more elements shown in the drawings may be integrated into a single functional unit. As will be appreciated by the skilled reader in the context of the present disclosure, each of the examples described herein may be implemented in a variety of different ways. Any feature of any aspects of the disclosure may be combined with any of the other aspects of the disclosure. For example, method aspects may be combined with apparatus aspects, and features described with reference to the operation of particular elements of apparatus may be provided in methods which do not use those particular types of apparatus. In addition, each of the features of each of the examples is intended to be separable from the features which it is described in combination with, unless it is expressly stated that some other feature is essential to its operation. Each of these separable features may of course be combined with any of the other features of the examples in which it is described, or with any of the other features or combination of features of any of the other examples described herein. Furthermore, equivalents and modifications not described above may also be employed without departing from the invention. Certain features of the methods described herein may be implemented in hardware, and one or more functions of the apparatus may be implemented in method steps. It will also be appreciated in the context of the present disclosure that the methods described herein need not be performed in the order in which they are described, nor necessarily in the order in which they are depicted in the drawings. Accordingly, aspects of the disclosure which are described with reference to products or apparatus are also intended to be implemented as methods and vice versa. The methods described herein may be implemented in computer programs, or in hardware or in any combination thereof. Computer programs include software, middleware, firmware, and any combination thereof. Such programs may be provided as signals or network messages and may be recorded on computer readable media such as tangible computer readable media which may store the computer programs in non-transitory form. Hardware includes computers, handheld devices, programmable processors, general purpose processors, application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), and arrays of logic gates. Any controller described herein may be provided by any control apparatus such as a general purpose processor configured with a computer program product configured to program the processor to operate according to any one of the methods described herein. In addition, the functionality of the controller may be provided by an ASIC, an FPGA, by a configuration of logic gates, or by any other control apparatus. Other examples and variations of the disclosure will be apparent to the skilled addressee in the context of the present disclosure.

Claims

s1. A fluid purification apparatus comprising:a plasma generating vessel;5 a fluid supply system configured to supply a fluid to be purified to the plasma generatingvessel, and wherein the plasma generating vessel comprises an electrode configured to apply electrical energy to liquid within the vessel to generate one or more bubbles of plasma therein; anda controller configured to control operation of the apparatus based on an obtained indication of at least one wavelength of light being emitted within the vessel, wherein the controller is configured10 to control operation of the apparatus based on a comparison of the obtained indication of at least one wavelength of light being emitted against one or more selected wavelengths.

2. The apparatus of claim 1, wherein the one or more selected wavelengths comprise one or more wavelengths associated with at least one unwanted substance; and15 wherein comparing the at least one wavelength against the one or more selectedwavelengths comprises determining whether a said unwanted substance is present in the vessel based on the at least one wavelength of light being emitted within the vessel.

3. The apparatus of claim 2, wherein the controller is configured to stop or reduce the application of power to the electrode in the event that it is determined that an unwanted substance is present in the vessel.

4. The apparatus of claim 2 or 3, wherein the controller is configured to increase the flow of fluid through the vessel in the event that it is determined that an unwanted substance is present in the 25 vessel.

5. The apparatus of claim 4, wherein the controller is configured to flush the vessel with liquid.

6. The apparatus of claim 5, wherein the controller is configured to flush the vessel with liquid30 until the wavelength associated with the unwanted substance is no longer identified within the vessel.

7. The apparatus of any of claims 2 to 6, wherein comparing the at least one wavelength of light being emitted within the vessel against the one or more selected wavelengths comprises determining whether an intensity of said one or more selected wavelengths within the vessel is above a threshold35 intensity.

8. The apparatus of claim 7, wherein the controller is configured to stop or reduce the application of electrical energy to the vessel in the event that the intensity is above the threshold08 01 26intensity.

9. The apparatus of claim 7 or 8, wherein the controller is configured to adjust at least one property of the fluid supplied to the vessel in the event that the intensity is below the threshold 5 intensity.

10. The apparatus of any preceding claim, wherein the one or more selected wavelengths comprise one or more wavelengths associated with at least one wanted substance; andwherein comparing the at least one wavelength against the one or more selected 10 wavelengths comprises determining whether a said wanted substance is present in the vessel basedon the at least one wavelength of light being emitted within the vessel.

11. The apparatus of claim 10, wherein the controller is configured to continue operation of the apparatus in the event that the wanted substance is present.1512. The apparatus of claim 10 or 11, wherein the controller is configured to increase the power applied to the vessel and / or adjust at least one property of the fluid supplied to the vessel in the event that an intensity of the wavelength of light associated with the wanted substance is below a threshold intensity.

13. The apparatus of claim 12, wherein the controller is configured to continue with existing operational parameters for controlling operation of the apparatus in the event that the intensity of the wavelength of light associated with the wanted substance is above the threshold intensity.25 14. The apparatus of any preceding claim, wherein the one or more selected wavelengthscomprise one or more wavelengths associated with at least one informative substance or energy level transition; andwherein the controller is configured to control operation of the apparatus based on whether or not any informative substances are present.3015. The apparatus of claim 14, wherein controlling operation of the apparatus based on whether any informative substances are present comprises adjusting the operational parameters for the apparatus based on the informative substances present in the vessel.35 16. The apparatus of any preceding claim, wherein the controller is configured to adjust the flowof liquid and / or increase the amount of electrical energy applied to the electrode in the event that the intensity of wavelengths detected and / or the intensity of selected wavelengths detected is below a threshold value.08 01 2617. The apparatus of any preceding claim, wherein the apparatus is operable to re-use fluid which has already passed through the vessel.5 18. The apparatus of claim 17, wherein the controller is configured to selectively re-use fluidbased on the obtained indication of one wavelength being emitted.

19. The apparatus of claim 17 or 18, wherein the controller is configured to increase the use of recycled fluid in the event that it is determined that one or more unwanted substances are present 10 in the vessel.

20. A method of controlling operation of a fluid purification apparatus, wherein the fluid purification apparatus comprises: (i) a plasma generating vessel, and (ii) a fluid supply system configured to supply a fluid to be purified to the plasma generating vessel, and wherein the plasma 15 generating vessel comprises an electrode configured to apply electrical energy to liquid within the vessel to generate one or more bubbles of plasma therein, wherein the method comprises:controlling operation of the apparatus based on a comparison of an obtained indication of at least one wavelength of light being emitted within the vessel against one or more selected wavelengths.

21. A fluid purification method comprising:operating a fluid supply system to supply a fluid to be purified to a plasma generating vessel; applying electrical energy to liquid within the vessel using an electrode of the plasma generating vessel to generate one or more bubbles of plasma therein;25 obtaining an indication of at least one wavelength of light being emitted within the vessel; andcontrolling operation based on a comparison of the obtained indication of at least one wavelength of light being emitted within the vessel against one or more selected wavelengths.

22. A computer program product comprising computer program instructions configured to 30 program a controller to control operation of an apparatus to implement the method of claims 20 or21.A

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