Apparatus and method for lithium enrichment

Abstract

A beam of lithium plasma comprising 6Li and 7Li is generated using a helicon plasma source 105, and 6Li is selectively heated within the beam using a plasma heating module 130. The beam is conveyed al

Description

Field of Invention The invention relates to an apparatus for 6Li enrichment and a method of enriching 6Li, Background Isotopes of lithium have a range of industrial uses. Stable isotopes of lithium include 6Li and 7Li. is in demand to support nuclear, fusion reactors that operate based on the reaction between deuterium and tritium, since it is proposed to breed tritium during operation of a fusion reactor using the reaction between a neutron and 6Li. Many fusion plant designs based on the deuterium and tritium reaction use breeders that require enriched 6Li -i.e., where the 6Li isotope is provided at a higher concentration than in naturally occurring lithium (which generally comprises a mix of 6Li and 7Li isotopes). Naturally occurring lithium typically comprises approximately 7.5% 6Li and 92.5% 7Li (i.e. 0.075 particle fraction of 6Li). Established processes for enriching lithium include the COLEX process. It is a chemical method based on 6Li having a greater affinity for mercury within an aqueous solution, and the separation of an amalgam of 6Li and mercury from 7Li. The COLEX process is prohibited in many nations owing to safety and environmental concerns. Summary According to a first aspect there is disclosed a method of enriching 6Li, comprising: generating and discharging a beam of lithium plasma comprising 6Li and 7Li, using a plasma source; selectively heating 6Li within the beam of lithium plasma in a plasma heating module; and conveying the beam of lithium plasma comprising selectively-heated 6Li along an arcuate path within a drift-orbit magnetic field to drive separation of 6Li from 7Li along the arcuate path. The plasma source may be a helicon plasma source. The beam of lithium plasma comprising selectively-heated 6Li may be conveyed along the arcuate path within the drift-orbit magnetic field to drive separation of 6Li from 7Li along the arcuate path in a drift direction normal to the arcuate path and define an enriched sLi region of the beam for collection. The lithium plasma comprising 6Li and 7Li may consist of 6Li and 7Li. The drift-orbit magnetic field may be generated in a drift-orbit separation module which defines the arcuate path and a magnetic array may generate the drift-orbit magnetic field around the arcuate path. It may be that, at a location between the plasma heating module and a drift-orbit separation module that defines the arcuate path, the beam of lithium plasma has: a particle flux of 1018-1022 partic!es / m2s; and / or - a mass flux of 10*8 - 10^ kg / s; and / or a density of at least 1016 particles / m3; and / or a density of at least 1019 particles / m3. The particle flux may be at least 1021. for example at least 1022. The density may be at least 1019 particles / m3, for example 1020 particles / m3. The particle flux, mass flux, and / or density represents a throughput of lithium plasma (e.g. through an associated apparatus). It may be that, at a location between the plasma heating module and a drift-orbit separation module that defines the arcuate path, the beam of lithium plasma has a cross-sectional profile having: a major dimension of 0.025m - 0.2m; and / or an area of 0.5e-3 m2 - 30e-3 m2. The major dimensions may be 0.01m-0.3m, for example 0.025-0.2m, 0.04m - 0.15m, or 0.05-0.12m. The area may be 0.8e-3 m2 - 70e-3 m2, for example, 0.5e-3 m2 - 30e-3 m2, 1e-3m2 - 20e-3m2 or 1e-3 m2 -11e-3 m2. It may be that the ®Li is selectively heated by ion cyclotron resonance heating in the plasma heating module in the presence of a heating module magnetic field having a heating module magnetic field strength. It may be that the plasma heating module generates radio-frequency waves tuned to a target frequency for selective heating of 6Li that corresponds to the ion cyclotron frequency of ®Li for the heating module magnetic field strength. The generation may be done using a radio-frequency generator coupled to a radio-frequency antenna. The plasma heating module may be configured to operate the radio-frequency antenna at a power of 0.1-30kW and / or at a frequency of 200kH-2000 kHz. For example, the radio-frequency antenna may be operated at a power of 1-30 kW, for example 2-20kW, 5-20kW, 10-20kW or approximately 15 kW. The radio-frequency antenna may be operated at a frequency of 300kHz - 2000kHz, 200kHz - 1500kHz, 300kHz - 1500kHz, 300kHz - 1000kHz, 300kHz - 500kHz, or approximately 400kHz - kHz. It may be that the selective heating in the plasma heating module causes heating of sLi within the plasma beam by a temperature difference of at least 30eV, for example at least 50eV, at least 60eV, 30-65eV or 30-60eV. The use of eV (electronvolts) as a measure of temperature is considered conventional in the field, but such definitions of a temperature and temperature difference with reference to eV may interchangeably be used and defined herein as an energy or energy difference in eV, or otherwise converted to suitable units of energy or temperature. It may be that the drift-orbit magnetic field is provided by permanent magnets and has a magnetic field strength of 0.15T-0.25T. It may be that, at a location between the plasma heating module and a drift-orbit separation module that defines the arcuate path, the beam of lithium plasma has: a cross-sectional profile having a major dimension of 0.04m -0.15m; and / or a density of at least 1018 particles / m3 or a mass flux of at least 1.7 kg / s. It may be that the separation of 6U from 7Li along the arcuate path is confined about the drift direction so that, in a cross-section of the beam at a downstream end of the arcuate path, an angular distribution of eLi around an annular region of the beam relative to a beam centre is non-uniform and concentrated in the enriched 6Li region. In a cross-section of the beam at a downstream end of the arcuate path, the enriched 6U region may have the form of a tail extending along the drift direction beyond an original cross-sectional profile of the beam upon receipt in the drift-orbit separation module. The “tail" form of the enriched 6Li region may be defined with respect to its angular location, or it’s lateral extent, for example. The enriched eLi region may have an acute angular extent relative to a centre of the beam and about the drift direction. The acute angular extent about the drift direction may be 5-40°, for example 5-30°, or 5-20°. The enriched 6Li region may have a lateral extent corresponding to a lateral width of the beam upon receipt into the drift-orbit separation module. The lateral direction may correspond to the radius of curvature of the arcuate path. The enriched 6Li region exclude a central portion of the beam, which may correspond to a profile of the beam upon receipt into the drift-orbit separation module. For example, the enriched eLi portion of the beam may be an annular segment having an acute angular extent about the drift direction, or a limited lateral extent corresponding to a lateral width of the beam upon receipt into the drift-orbit separation module. According to a second aspect there is disclosed an apparatus for 6Li enrichment, comprising: .a plasma source configured to generate a beam of lithium plasma; a plasma heating module configured to receive the beam of lithium plasma from the plasma source, and configured to selectively heat eU within the lithium plasma; and a drift-orbit separation module configured to receive the beam of lithium plasma 5 from the plasma heating module along an arcuate path, wherein the drift-orbit separation module comprises a magnetic array configured to generate a drift-orbit magnetic field around the arcuate path to drive separation of 6Li from 7Li along the arcuate path. The apparatus may comprise a source of lithium for the plasma source. The source of lithium may comprise (e.g., consist of) 6Li and 7Li (the stable isotopes of lithium). 10 The plasma source may be a helicon plasma source (e.g., configured to generate the beam of lithium plasma using helicon waves). It may be that the magnetic array of the drift-orbit separation module is configured to generate a drift-orbit magnetic field around the arcuate patfi to drive separation of 6Li from 7Li along the arcuate path in a drift direction normal to the arcuate path, to define 15 an enriched 6Li region of the beam for collection. As described elsewhere herein, a magnetic array of a module as described herein (e.g. of a plasma source, vaporisation module, plasma generation module, plasma heating module, drift-orbit separation module) may be provided by a magnetic array of the apparatus that extends over multiple such modules (e.g. a multi-module magnetic array). 20 Accordingly, it may be that the apparatus comprises a magnetic array (e.g., a single magnetic array) which generates a magnetic field corresponding to one or more of the respective magnetic fields discussed herein (e.g., any of the magnetic fields of the respective modules as discussed below, for example a first magnetic field associated with a vaporisation module, a second magnetic field associated with a plasma generation 25 module, a third magnetic field associated with a plasma heating module, a fourth magnetic field associated with the drift-orbit separation module). The apparatus may be defined as comprising such a magnetic array which provides the magnetic array or magnetic fields of or associated with the respective modules, and discussion of a magnetic array of a particular module is to be understood as including that the magnetic 30 array may be provided by such a (multi-module) magnetic array of the apparatus. The apparatus may comprise a source of lithium for the helicon plasma source. The source of lithium may comprise 8Li and 7Li (the stable isotopes of lithium). The apparatus may define a beam path for the lithium plasma through the helicon source, plasma heating module and the drift-orbit separation module, including the arcuate path 35 of the drift-orbit separation module. The beam path may be linear in the plasma source and the plasma heating module. The arcuate path may have a radius of curvature about an arc axis (which may be orthogonal to the beam path, e.g., orthogonal to a linear portion of the beam path within the plasma source and / or plasma heating module). An angular extent of the arcuate path (i.e. an angular extent through which the arcuate path extends, e.g., with respect to the arc axis) may be 90o-360°, for example 135°-270°, for example 180°-270°. Where the angular extent is close to360° a full revolution (e.g. 270° or more, for example 270°-720° or 270°-540°) Separation of “Li from 7Li along the arcuate path may be along a drift direction which is normal to the arcuate path and parallel with an arc axis. The arc axis may intersect a centre of curvature of the arcuate path (or intersect centres of curvature of the arcuate path in the case of a helical arcuate path). The arc axis may be normal to the radius or radii of curvature of the arcuate. The arc axis may be orthogonal to a plane in which the arcuate path lies. The apparatus may be configured so that at a location between the plasma heating module and the drift-orbit separation module, the beam of lithium plasma has: a particle flux of 1018-1022 partides / m2s; and / or a mass flux of 10'8 - 10'* kg / s; and / or a density of at least 1016 partides / m3; and / or a density of at least 1019 partldes / m3. The partide flux may be at least 1021, for,example at least 1022. The density may be at least 1019 partlcles / m3, for example 1020 particles / m3. The particle flux, mass flux, and / or density represents a throughput of lithium plasma through the apparatus. As noted elsewhere herein, drift-orbit separation of other species has been proposed at relatively low throughput on the basis of avoiding particle collisions and enabling separation. A higher throughput as described herein leads to a higher yield of enriched “Li suitable at a commercial scale, and has been shown to be achievable for lithium. It may be that the apparatus comprises a controller to control the plasma source so that the beam of lithium plasma has a partide flux, mass flux and / or density as specified above. It may be that the plasma heating module and drift-orbit separation are configured to contain the beam of lithium plasma such that, in conjunction with control of the plasma source, the beam of lithium plasma has the specified particle flux, mass flux and / or density at the location. The apparatus may be configured so that, at a location between the plasma heating module and the drift-orbit separation module, the beam of lithium plasma has a cross-sectional profile having: a major dimension of 0.025m - 0.2m; and / or an area of 0.5e-3 m2-30e-3m2. The major dimensions may be 0.01m-0.3m, for example 0.025-0.2m, 0.04m - 0.15m, or 0.05-0.12m. The area may be 0.8e-3 m2 - 70e-3 m2, for example, 0.5e-3 m2 - 30e-3 m2, 1 e-3m2 - 20e-3m2 or 1 e-3 m2 -11 e-3 m2. Such geometric parameters of ttie plasma cross-section are considered to relate to the stability of the beam of lithium plasma in the drift-orbit separation module, and whether turbulence effects for one or both isotope species (i.e., 6Li and 7Li) or diffusion effects come to dominate, or whether the drift separation effect of 6Li dominates to drive separation along the curvilinear path. The apparatus may comprise a guide tube extending through the plasma heating module and configured to discharge the beam of lithium plasma to the drift-orbit separation module at a discharge end, wherein the discharge end has a cross-sectional profile having a major dimension of 0.03-0.3m, and / or an area of 0.0025-0.3m2. The major dimension may be 0.03 - 0.3 m, for example 0.03 - 0.2 m 0.05 - 0.15 m, or 0.07 - 0.13 m, for example approximately 0.1m. . The area may be 0.0007 - 0.07 m2, for example 0.0007 - 0.03 m2, 0.002 - 0.02 m2, 0.003 - 0.015 m2, or approximately 0.008m2 The major dimension may be a diameter, a major axis of an ellipse, a long side of a rectangular cross-section. The geometry of the guide tube influences the geometric parameters of the beam of lithium plasma in the drift-orbit separation module, and is thereby associated with the effects discussed above. The guide tube of the plasma heating module may terminate at the discharge end and the drift-orbit separation module may have no corresponding guide tube, such that a passageway for the beam of lithium plasma expands upon entering the drift-orbit separation module (the passageway being defined by the guide tube in the plasma heating module, and being defined in the drift-orbit separation by a curvilinear housing of the drift-orbit separation module). The plasma source may comprise a guide tube for the beam of lithium plasma, which may be a separate guide tube or the same guide tube as the guide tube of the plasma heating module. Where different guide tubes are provided, there may be an axial separation between them. It may be that the magnetic array of the drift-orbit separation module is configured so that the drift-orbit magnetic field has a magnetic field strength of 0.1T-1T along the curvilinear path. It may be that the magnetic array of the drift-orbit separation module is configured so thatthe drift-orbit magnetic field has a magnetic field strength of 0.15T-1T or 0.1T-0.75T, for example 0.15T-0.75T, 0.15T-0.5T, or 0.15T-0.25T along the curvilinear path. The magnetic array of the drift-orbit separation module may be configured to provide a magnetic field of uniform magnetic field strength within the drift-orbit separation module. A non-uniformity of the magnetic field strength may be 10% or less’, for example 7.5% or less or 5% or less. The non-uniformity may be evaluated over a cross-section within the drift-orbit separation module (e.g. normal to the lithium pathway). The non-uniformity may be evaluated in a region of the cross-section, for example a region corresponding to the profile of an upstream guide tube discharging the plasma beam to the drift-orbit separation device, or a region having a predetermined diameter (e.g., e.g. a circular region concentric with the lithium pathway and of a predetermined diameter, for example 0.1m; or half of a diameter of a housing of the drift-orbit separation module). It may be that the magnetic array of the drift-orbit separation module comprises permanent magnets which are configured to generate the drift-orbit magnetic field. The definition that the drift-orbit separation module comprises permanent magnets to generate the drift-orbit magnetic field is intended to indicate that the magnetic field is only generated by such permanent magnets (i.e., such that the magnetic array of the drift-orbit separation module is free of electromagnets for the purposes of generating the magnetic field). The same applies to definitions elsewhere herein with respect to the magnetic arrays of the plasma source and the plasma heating module. It may be that the plasma source comprises a magnetic array configured to generate a plasma source magnetic field. It may be that the magnetic array of the plasma source is configured so that a magnitude of a strength difference between the plasma source magnetic field strength relative to the drift-orbit magnetic field strength is 10% or less. It may be that the plasma heating module comprises a magnetic array configured to generate a heating module magnetic field. It may be that the magnetic array of the plasma heating module is configured so that a magnitude of a strength difference between the heating module magnetic field relative to the drift-orbit magnetic field is 10% or less. The magnetic array of the plasma source may comprise permanent magnets which are configured to generate the plasma source magnetic field. The magnetic array of the plasma heating module may comprise permanent magnets which are configured to generate the heating module magnetic field. It may be that the plasma heating module is an ion cyclotron resistance heating module comprising a radio-frequency antenna configured to selectively heat 6Li within the beam of lithium plasma. It may be that the plasma heating module comprises a magnetic array configured to generate a heating module magnetic field having a heating module magnetic field strength. It may be that the plasma heating module further comprises a radio-frequency generator coupled to the radio-frequency antenna and configured to generate radiofrequency waves tuned to a target frequency for selective heating of 6Li, wherein the target frequency corresponds to the ion cyclotron frequency of ®Li for the heating module magnetic field strength. The plasma heating module may be configured to operate the radio-frequency antenna at a power of 0.1-30kW and / or at a frequency of 200kH-2000 kHz. For example, the radio-frequency antenna may be operated at a power of 1-30 kW, for example 2-20kW, 5-20kW, W-20kW or approximately 15 kW. The radio-frequency antenna may be operated at a frequency of 300kHz - 2000kHz, 200kHz - 1500kHz, 300kHz - 1500kHz, 300kHz - 1000kHz, 300kHz - 500kHz, or approximately 400kHz - kHz. It may be that the helicon plasma source comprises a helicon antenna configured to emit radiofrequency (RF) waves to generate plasma within a plasma containment, and a magnetic array around the plasma containment to confine the plasma within the plasma containment and define a path for discharging the beam of lithium plasma from the plasma containment. The plasma containment may be or may comprise a guide tube of the plasma source in accordance with any statement herein. It may be that the helicon plasma source is configured to operate the helicon antenna at a power of 0.1kW-5kW, for example 0.1kW-3kW, 0.1kW-2kW, or 0.5kW - 1.5kW, such as approximately 1 kW. It may be that the helicon plasma source if configured to operate the helicon antenna at a frequency of between 10000kHz and 15000kHz, for example 12000kHz - 14000kHz, such as approximately 13000kHz. z The apparatus may be configured so that selective heating in the plasma heating module corresponds to heating of 6Li within the plasma beam by a temperature difference of at least 30eV, for example at least 50eV, at least 60eV, 30-65eV or 30-60eV. The apparatus may be suitably configured to provide the temperature difference by suitable selection or control of an RF power input for the plasma heating module for the prevailing conditions of the plasma, which may be determined by other aspects of the configuration of the apparatus (for example, the particle flux, mass flux or density of the plasma, the thermal velocity of the plasma, the geometry of the plasma heating module and the plasma beam). A higher temperature difference may promote separation effects as discussed herein. However, maintaining a temperature difference so that the 6Li temperature is below the second ionisation temperature by a margin may inhibit a risk of some ions going beyond the second ionisation temperature as discussed elsewhere herein. The temperature difference that a plasma heating module is configured to provide to 6Li may be determined indirectly by reference to a monitored power input, according to a temperature derivation method discussed below. The temperature derivation method includes (i) determining an RF input power of the antenna (an incident power), (ii) determining an RF power retumed.to the signal generator circuit (a returned power), (iii) determining a baseline RF power loss associated with the signal generator circuit (iii) determining an applied RF power based on the RF input power, less the RF power returned to the signal generator, and less the baseline RF power loss; (iv) determining a parameter relating to the density of the plasma beam at the plasma heating module; (v) determining the temperature difference provided to 6Li based on the applied power and the parameter relating to the density of the plasma beam. An example method for determining the quantities discussed above is as follows. The RF input power and the RF power returned to the signal generator circuit may be determined as monitoring outputs from the signal generator (i.e. determined by a signal generator) as is known in the art. The baseline RF power loss may be determined by running the plasma heating module at operating conditions to couple with 6Li, but without providing 6Li plasma. For example, a plasma of 7Li may be provided to the plasma heating module, such that there is no coupling at the ion cyclotron frequency (of 7Li) that would result in power input to the plasma. Accordingly, the baseline RF power loss associated with the signal generator and associated circuitry can be determined, and applied to determine the energy input the plasma, the thermal velocity and therefore the temperature difference. An example calculation is as follows: A return loss for a system operating with plasma comprising 7Li ions (only) will be: RLB(dB) = 101oglo(Pj / Pr) Where Pj / Pr is the ratio of incident to returned power and RLB is the baseline RF power loss when no power is entering the plasma. From this it follows that: Pi rlb -L = 10 10 The power lost to the RF circuit is then p. p. = p. - p = p.__!__ Hoss ri rr ri lOio Running the system with a lithium plasma comprising eLi and 7Li ions, such that there is coupling into the 6Li for selective heating, we have: RL low = Pi / pr(2) Where the return loss here (RL) is not equal to the baseline return loss (RLB). We then have the power inputted into the plasma as: Pplasma “ P| ^r(2) ^loss As the returned power here will contain the power loss in the RF circuit, this is to be negated. From before it follows that ( P' P‘ / 1 1 \ Pplasma ~ “i Pr(2) - I Pj 1 ~ - Pr(2) — Pi I rl I \ 10 io / 10 io \10 io 1010 / This “plasma" power is taken to be applied evenly among the 6Li within the volume of the antenna. As such we have: mLi6 = PLisVantenna = pLisAantennakntenna Where pL1Js the mass density of the 6Li plasma and Vantenna is the spatial volume of the antenna, made up of product of antenna area and length. The time of flight of the Lithium within the antenna is t = vthlantenna Where vth is the thermal velocity of the Lithium-6. From this we can determine the energy per unit mass inputted to the plasma: 1 11., n / 1 1 V *11 REb IC I vth1antenna HI RL^ RL I vth _ \10 io IQio / _ \1Q io IQio / P Li 6 ^antenna Untenna Pl16 Aantenna This energy can be used trivially to calculate the temperature increase of the Lithium-6. The temperature difference may be determined using a spectrometer. For example, a spectrometer may be positioned and configured (e.g. itself or in conjunction with a controller) to determine an energy (temperature) of the plasma. A spectrometer may be positioned to downstream of the plasma heating module to receive emitted photons-for example through a maintenance port as discussed elsewhere herein. Through the action of electrons recombining with ions, photons will be released from the plasma and their frequency detected by the spectrometer. The excitation energy can then trivially be derived based on the Plack relation E=hf (energy equal to Planck’s constant multiplied by frequency). The energy (temperature) can be compared with an input energy of the plasma upstream of the plasma heating module (which may also be determined using a spectrometer method as described above, or otherwise derived). it may be that the drift-orbit separation module comprises a magnetic array of permanent magnets configured to provide the drift-orbit magnetic field having a magnetic field strength of 0.15T-0.25T. It may be that the apparatus is configured so that, at a location between the plasma heating module and the drift-orbit separation module, the beam of lithium plasma has: a cross-sectional profile having a major dimension of 0.04m -0.15m; and / or a density of at least 1018 particles / m3 or a mass flux of at least 1.7 kg / s. The apparatus may comprise a collector module disposed at an arcuate end of the driftorbit separation device opposite to the plasma heating module, wherein the collector module is configured to delimit a central region and an enriched 6Li region for separate collection of enriched 6Li. It may be that the central region is disposed around an intersection of the arcuate path and the drift-orbit separation module. It may be that the enriched 6Li region comprises an annular region or annular segment disposed around the central region. It may be that the enriched 6Li region comprises an annular segment to capture a tail of 6Li drifted from a central region along a drift direction normal to the arcuate path. It may be that the enriched 6Li region has an acute angular extent relative to a centre of the beam and about the drift direction. The acute angular extent about the drift direction may be 5-40°, for example 5-30°, or 5-20°. The enriched 6Li region may have a lateral extent corresponding to a lateral width of the beam upon receipt into the drift-orbit separation module. The lateral direction may correspond to the radius of curvature of the arcuate path. The plasma source may comprise a lithium vaporisation module and a plasma generation module. The lithium vaporisation module may be configured to heat lithium to generate a lithium vapor. The plasma source may be configured to guide the lithium vapor from the lithium source to the plasma generation module. The plasma generation module may comprise a radio-frequency antenna configured to heat the lithium vapor to provide a lithium plasma, and may be configured to discharge the lithium plasma in the beam of lithium plasma to the plasma heating module. Any feature defined herein with respect to the method of the first aspect is equally applicable and extends to the apparatus of the second aspect. A method in accordance with any statement herein with respect to the first aspect may be performed using an apparatus in accordance with any statement herein with respect to the second aspect. A feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore, except where mutually exclusive any feature described herein may be applied to any aspect and / or combined with any other feature described here. Brief Description of the Drawings Examples will now be described by way of example only, with reference to the accompanying drawings, in which: Figure 1 schematically shows a plan view of an example apparatus for lithium enrichment; Figure 2 schematically shows a side view of the example apparatus of Figure 1; Figure 3 schematically shows a cross-sectional side view of a lithium vaporisation 5 module of the example apparatus of Figures 1 -2; Figure 4 schematically shows a cross-sectional cutaway side view of a plasma generation module of the example apparatus of Figures 1-2; Figure 5 schematically shows a cross-sectional cutaway side view of a plasma heating module of the example apparatus of Figures 1-2; 10 Figure 6 schematically shows a cross-sectional cutaway plan view of a drift-orbit , separation module of the example apparatus of Figures 1-2; Figures 7 and 8 schematically show perspective and perspective cutaway views of an example implementation of the apparatus of Figures 1-2; Figures 9-11 are contour plots of lithium isotope concentration in three respective 15 simulation cases; Figures 12 and 13 are plots of enriched lithium yield vs 6Li temperature for different plasma densities; and Figure 14 is a plot of enriched lithium yield vs magnetic field strength for a set of simulation cases. 20 Detailed Description h number of mass and isotope separation techniques have either been proposed for lithium enrichment, or may be applicable to lithium enrichment as an alternative to the COLEX and other liquid phase chemical exchange processes. These include 25 Displacement Chromatography (DC), atomic vapor laser isotope separation (AVLIS), laser excitation (SILEX), thermal diffusion (of fluidic lithium isotopes), radiofrequency spectroscopy, and electromigration (electrolysis of a lithium-containing electrolyte fluid, for 6Li accumulation collection at the anode). Ion Cyclotron Resonance Heating (ICRH) is a plasma heating technology. The primary 30 industrial uses of ICRH are not for isotope separation, but for plasma heating, for example In tokamaks and other applications in nuclear fusion and fission. ICRH operates based on scientific principles associated with ion gyration (e.g. orbiting of magnetic field liens) at a respective ion cyclotron frequency, within a plasma conveyed along the axis of a magnetic field. The ion cyclotron frequency is the natural frequency at which ions in the plasma gyrate (e.g. orbit) with respect to magnetic field lines Ions adopt a spiral or helical orbit around the magnetic field liens at the gyroradius. 5 ICRH systems employ a radiofrequency (RF) antenna to subject the ions within a plasma to RF waves at a frequency matching the cyclotron frequency, such that energy is transferred to the ions to cause heating. ICRH therefore relies on the heated ions colliding with other ions in the plasma to distribute the energy and heat the plasma as a whole. 10 However, ICRH has also been proposed for ion separation, based on the principle that the gyroradius (or Larmor radius - the radius at which ions orbit magnetic field lines at the cyclotron frequency) will differ for different ions. Ions orbiting at a greater gyroradius can be collected separately from ions orbiting at a lower gyroradius. For example, collector arrangements have been proposed whereby shielding screens are provided in 15- a plane normal to the direction of the magnetic field (i.e., normal to an axial direction of the ICRH module), with gaps between them Collector plates are disposed axially behind the shielding screens and oriented perpendicular to those screens, such that collection surfaces of the collector plates are offset from a profile of the shielding plates by a minimum distance in the normal direction (i.e., normal to the direction of the magnetic 20 field). With this arrangement, only ions with a gyroradius larger than the minimum J distance may move past the screens and subsequently be deposited on the collector plates, whereas ions with a lower gyroradius and which pass through a gap between the screens cannot orbit with sufficient radius to impact the collector plates. The gyroradius (fl) is proportional to the mass of the partide (m) and the velocity (v) of 25 the particle perpendicular to the direction of the magnetic field, and is inversely proportional to the charge of the partide (q) and the magnetic field strength (B). mv TL^ Accordingly, the gyroradius can differ between ions owing to their mass and / or their charge. The gyroradius can also be caused to differ as the ions are differently heated 30 by the RF waves. The energy transfer from the RF waves causes the respective ion to absorb more energy, which drives the ion to move at a greater velocity and thereby a larger gyroradius (at a constant cydotron frequency, assuming a constant charge and magnetic field strength). This velocity difference and an associated change in gyroradius promotes collisions between ions and consequent deflections of partides. Such collisions can further cause selective variations (e.g. increases) of axial ion speed, further promoting collisions. In contrast to ICRH for bulk plasma heating (which benefits from collisions for heat distribution and uniform heating), ICRH techniques as adapted for ion separation rely on a substantially collisionless plasma beam, such that the respective ions are permitted to adopt different gyroradii over a length of the ICRH device based on their properties and any selective heating by RF waves, without being diverted by such collisions. Therefore, while ICRH separation techniques have been proposed at small scale for a variety of separation applications (i.e., for separating different species of ions), it is thought that ICRH separation at greater throughput would result in increased collisions that would adversely impact separation performance. . “Throughput" as referenced herein relates to the amount of plasma being conveyed through a system, for example per unit time. Various parameters may be related to throughput, including particle density (particles / m3), particle flux (particles / m2s), particle flow rate (particles / s) and mass flow rate (kg / s) (e.g., at a measurement location along an apparatus). As such, it is considered that there is a technical prejudice in the art against pursuing plasma separation techniques for high throughput applications (e.g., for species that would require a relatively high yield and therefore throughput for industrial application). It is considered that 6Li enrichment is one such use case. For example, basic calculations reveal that at least 110kg of enriched Li (e.g., enriched to 0.2-0.4 particle fraction of 6LI) would be required per GigaWatt (GW) per year, for any fusion power plant operating based on the deuterium and tritium reaction. Nevertheless, the inventors’ research has shown that high throughput lithium enrichment using plasma techniques can be implemented. The inventors propose a system that combines lithium plasma generation, selective jheating of 6Li, and drift-orbit separation of 6Li and 7Li isotopes. It is known that the path of charged particles (e.g., ions orbiting about a gyroradius) are caused to drift in a drift direction which is perpendicular to the direction of a magnetic field B and also perpendicular to the direction of the gradient of the magnetic field. This is because the gyroradius varies slightly around the orbit, as it moves through the locally varying magnetic field. In a drift-orbit separation device (or module, as used herein), a curvilinear magnetic field is provided, for example having a radius of curvature R about a centre of curvature. The direction of the magnetic field B is along the curvilinear path of the magnetic field. The magnetic field may be of substantially uniform strength along the curvilinear path. Nevertheless, owing to the curvilinear nature of the path, there is an effective gradient of the magnetic field (grad B, or VB) which is parallel with the radius of curvature. 5 Accordingly, the drift direction of an ion will be locally perpendicular to the curvilinear path and perpendicular to the local radius of curvature. In the example of a curvilinear path provided in a generally horizontal plane and arcuate about a vertical axis, the drift direction of ions will be vertical. Electrons will drift in an opposing direction, leading to a self-induced magnetic field E. 10 ICRH ion separation is based on establishing different gyroradii of species in a collisionless environment, leading to an annular separation effect with one species enriched in an annular region around a central core. The interface and overlap between these regions are such that the separation effect is prone to disruption from collisions as discussed above. In contrast to ICRH separation, the inventors have considered that 15 drift-orbit separation provides separation along a local vector - the “drift direction" - which remains substantially constant relative to the direction of the magnetic field (for example, vertically in the case of a curvilinear path provided in a horizontal plane and arcuate about a vertical axis). Without wishing to be bound by theory, the inventors have found that this more focussed 20 mode of separation (e.g., continuous drifting along the drift direction perpendicular to the magnetic field, as opposed to separation based on differing gyroradii, as in ICRH separation) may be less susceptible to disruption by collisions. As will be set out below, the inventors have found that, in the case of 6Li enrichment, toe drift effect can become established as a dominant aspect of particle motion in a drift-orbit 25 separation module even at relatively high throughput, overcoming competing effects as may be caused by particle collisions and any turbulent interaction of particles owing to ExB drift (also known as E-cross-B-drift, which is associated with movement of charged . particles in toe presence of both an electric field E (e.g. self-induced by opposing drift of ions and electrons) and a magnetic field B). 30 These findings have been demonstrated by simulation of 6Li and 7Li motion in a drift-orbit separation module of an example enrichment apparatus, over a range of different conditions. The example enrichment apparatus 100 is schematically shown in Figure 1. The example apparatus comprises, in flow order with respect to a direction of lithium movement in use:, a plasma source 105, a plasma heating module 130, and a drift-orbit separation module 140. In this example the plasma source 105 comprises a lithium vaporisation module 110 which receives a source of lithium and a separate plasma generation module 120, but in other examples the plasma source may receive the source 5 of lithium directly. An apparatus according to the disclosure may have a controller as shown in Figure 1, which in this example is operatively coupled to each of the lithium vaporisation module 110, plasma generation module 120 and plasma heating module 130. In other examples, there may be multiple controllers independent configured to control respective modules 10 or sub-systems of respective modules. In this highly schematic view, the lithium vaporisation module 110, plasma generation module 120 and plasma heating module 130 are schematically shown as boxes along a lithium pathway 102 which extends through the apparatus, and the drift-orbit separation module 140 is shown having a generally arcuate extent along that pathway. Figure 1 is 15 a plan view (i.e. normal to a vertical axis), and the arcuate extent of the drift-orbit separation module is with respect to a vertical axis. The further description may refer to relative positions of components as “upstream” or “downstream’’, which is intended to refer to relative positions along the lithium pathway 102 extending from the lithium vaporisation module 110 (upstream) t the drift-orbit separation module 140 20 (downstream). Figure 2 shows the same modules 110, 120, 130, 140 as depicted in Figure 1 from a side view, simply to indicate that in this example the modules are oriented so that the lithium pathway 102 is generally horizontal, with an arcuate portion within the drift-orbit separation module 140 lying generally in a horizontal plane. While this may be a practical and convenient orientation for installation, it will be appreciated that other 25 orientations of the equipment are possible. In particular, the arcuate portion of the lithium pathway 102 within the drift-orbit separation module need not lie in a horizontal plane. In .contrast, it is envisaged that the drift-orbit separation module 140 may be configured to define a helical portion of the lithium pathway about a helix axis. Such an arrangement would enable relatively large angular extents of the drift-orbit separation module and may 30 be necessary for when the angular extent is greater than approximately 270°. As shown in Figures 1 and 2, the modules 110,120,130,140 of the apparatus 100 are optionally interconnected by connecting tubes 104, some of which are schematically shown with maintenance ports 106. For example, such maintenance ports may be provided for internal access and / or to control gate valves that shut-off fluid 35 communication between the respective modules, and / or permit their separation. Although the connecting tubes appear to have a narrower cross-sectional profile than the adjacent modules in Figure 1, they may have the same or a corresponding cross-sectional profile as housings for such modules, with the modules appearing to be larger by virtue of magnetic arrays disposed outside such housings (as discussed below). The apparatus 100 may be configured to maintain a sealed internal volume that is subject to very low pressure (e.g., vacuum) conditions (excepting the lithium). Each module is described herein as having a respective vacuum housing coupled to a respective adjacent vacuum housing of the next module or of the connecting tubes 104. The vacuum housings may be configured to maintain the sealed internal environment, when connected to the respective adjacent vacuum housing (or when sealed by a respective gate, valve). In other examples, there may be other arrangements of joined vacuum housings (e.g., dis-connectable at junctions with the connecting tubes 104, optionally while maintaining a vacuum in part of the internal volume using gate vales), or there may be a single vacuum housing (e.g., which may nevertheless be formed of multiple parts). Figure 3 is a cross-sectional side view of the example lithium vaporisation module 110 comprises a housing 112 configured to define part of the sealed internal volume of the apparatus 100. In this example, the housing comprises tubular elements (e.g;, cylindrical), including a tubular element oriented around (e.g., coaxial with) a longitudinal portion 108 of the lithium pathway 102 through the apparatus, and an end element configured to define one end of the sealed internal volume and house a lithium source and vaporisation device. In this example, the vaporisation device is an effusion cell 114 configured to receive the lithium source in a crucible 115, and configured to heat the lithium source in the crucible to vaporise it, causing ejection of lithium in vapor form through an opening of the crucible, such as an orifice in an orifice plate closing the crucible. The lithium source may be received in solid granular form (e.g., pellet form). The vaporisation device may be configured to receive the lithium source in batches or in a continuous process (e.g. via a feeding apparatus). For small scale batch operation, a loading opening may be formed in the vaporisation device to load the lithium source, for example by removing the orifice plate. Although the vaporisation device is shown wholly within the housing, it will be appreciated that in other examples the vaporisation device may be partly outside of the housing. For example, a controller, power control module, or part of a heating apparatus for the vaporisation device may be disposed outside of the housing and operatively coupled to a remainder of the vaporisation device. However, other forms of lithiurp vaporisation may be used, and in other implementations an apparatus may be provided which is configured to receive vaporised lithium from another (e.g. a separate) source or apparatus. The example vaporisation module 110 optionally comprises a portion of a guide tube 160 disposed within the housing and having an open end opposing the vaporisation device 114, configured to receive lithium vapour along the portion 108 of the lithium pathway 102. The guide tube may be made of any suitable material, and in this example comprises quartz, selected for its combination of thermal stability, electrical insulation and mechanical performance. The example guide tube 160 extends along the lithium pathway 102 beyond the vaporisation module 110 as will be discussed further below. Portions of the guide tube 160 outside of the vaporisation module 110 are indicated in dashed lines, as are portions of a connecting tube 104 that is coupled to the housing 112 of the vaporisation module 112. The vaporisation module 110 optionally comprises a first magnetic array 116 disposed around the housing as shown. In this example, the first magnetic array 116 is provided towards a downstream end of the lithium vaporisation module, and overlaps with a longitudinal extend of the guide tube 160. In various examples, the first magnetic array 116 may be an electromagnetic array or a permanent magnet array (e.g., consisting only of permanent magnets, with no electromagnets), and may be provided by a multiple-module magnetic array as discussed elsewhere herein. In this particular example, the first magnetic array 116 is a permanent magnet array. The first magnetic array 116 is configured to establish a first magnetic field within the housing around the longitudinal portion 108 of the lithium pathway, having a first magnetic field strength (also referred to herein as a vapor module magnetic field strength) as will be further defined below. The provision of a magnetic field serves to contain the vapor to form a beam around and along the lithium pathway 102. While the particular example of Figure 1 includes a first magnetic array 116 disposed around the housing of the vaporisation module 110, in other implementations there may be no magnetic array at this location and the first magnetic array along the length of the lithium pathway may be provided relatively more downstream, for example at the plasma generation module. Further, instead of magnetic arrays individually associated with the respective modules, there may be a single magnetic array extending over multiple modules, as will be described separately below. Figure 3 shows ejection of the vapor along a direction that is inclined upwardly towards the longitudinal portion 108 of the lithium pathway 102 - in this example the indine is approximately 10Q relative to the horizontal, and the longitudinal portion 108 of the lithium pathway 102 is substantially horizontal. ' This inclined arrangement is considered to be one option for advantageously establishing a one way flow of lithium from the vaporisation device and along the lithium pathway, since the heated lithium vapor generally flows upwardly from the crucible into the internal space within the housing, such that a return flow into the crucible may be naturally prevented. However, in other examples such an inclined arrangement may not be utilised. For example, the vaporisation rate from the lithium source may itself prevent a significant return flow. As shown by further arrows in Figure 3, the lithium vapor then turns to flow in a direction parallel with the lithium pathway 102 (e.g. parallel with the longitudinal portion 108). Figure 4 schematically shows a cross-sectional cut-away side view of the plasma generation module 120 along the longitudinal portion 108 of the lithium pathway 102. The plasma generation module is configured to receive the lithium vapour from the vapour generator, and to ionize the lithium vapour to form a plasma. The plasma generation module comprises a housing 122 configured to define a respective part of the sealed internal volume of the apparatus 100. In this example, ttie housing is tubular (e.g., cylindrical), and is disposed between adjacent connecting tubes 104 as described above. The housing 122 is open at both ends for fluid communication with the remainder of the sealed internal volume of the apparatus 100 (in this example via the connecting tubes 104). In other examples, the housing 122 may be directly coupled to one or more housings of adjacent modules, without such connecting tubes, or a common housing may extend between multiple modules as discussed elsewhere herein. The guide tube 160 as described above with respect to Figure 3 extends through the plasma generation module, and in this example is coaxially aligned with the longitudinal portion 108 of the lithium pathway and with the housing 122. As shown in Figure 4, an antenna 124 is disposed around an outer surface of the guide tube 160 and within the housing 122. In this example, the plasma source is a helicon plasma source and the antenna 124 is a helicon antenna, although other plasma sources may be utilised. The example antenna is configured to generate radiofrequency waves tuned to a target frequency configured for ionization and plasma generation. The antenna is coupled to a signal generator controlled by a controller (e.g., the controller 180 as described above with respect to Figure 1, or a dedicated controller) to cause the antenna to generate the radiofrequency waves at the target frequency. In this example the target frequency 13.56MHz, but a target frequency may be set based on parameters of the plasma generation module and its operation (e.g., magnetic field strength, density of the lithium plasma). Such signal generation arrangements are known in the art, and for example may include supplying an AC signal to the antenna at a specified frequency matched to the antenna to cause radiation of radiofrequency waves at the target frequency. The signal generator may be a single frequency generator (e.g., configured to produce a stable, unmodulated signal at a specific frequency), which may otherwise be referred to or known as a continuous wave generator. Otherwise, the signal generator may be a variable signal generator (which may be referred to or known as a function generator or arbitrary waveform generator (AWG)) capable to output a signal at variable frequencies, but configured (e.g., controlled) for operation with the plasma generation module to target a target frequency, for example producing a signal having a frequency within a targeting bandwidth of 15% or less of the target frequency, for example 10% or less. While various configurations of a helicon antenna may be used to suitably ionize the lithium vapour, in this particular example the helicon antenna comprises a half-turn helical antenna comprising two angularly spaced apart half-turn antenna portions (angularly spaced apart with reference to a central axis of the antenna and guide tube, which in this example corresponds to the longitudinal portion 108 of the lithium pathway 102). The angular separation of the two antenna portions is 180°. A particular sizing for the antenna may be selected based on known design principles for helicon antennas. The antenna turn may be configured to excite the mode which allows for most efficient coupling into the electrons in the plasma column. In the specific example described herein, the helicon antenna has a longitudinal dimension (length) of 0.25m, and a diameter of 0,11m. The plasma generation module 120 comprises a second magnetic array 126 disposed around the housing 126, and configured to generate a second magnetic field having a longitudinal extent within the housing 126. As noted above, in some examples the magnetic array around the housing 126 of the plasma generation module may be the first magnetic array along the lithium pathway (e.g., if no magnetic array is provided in association with the vaporisation module), or there may be a single magnetic array extending over multiple modules, however for the purposes of the present example the numbering of the respective magnetic arrays is such the magnetic array 126 associated with the plasma generation module is the “second" magnetic array. The second magnetic field has a second magnetic field strength (also referred to herein as a generation module magnetic field strength) as will be further defined below. In various examples, the second magnetic array 126 may be an electromagnetic array or a permanent magnet array (e.g., consisting only of permanent magnets, with no electromagnets). In this particular example, the second magnetic array 126 is a permanent magnet array. The provision of the second magnetic field serves to contain the vapor and generated 5 plasma to form a beam around the lithium pathway 102 (e.g., around the longitudinal portion 108 of the lithium pathway). Further, in this example the provision of the second magnetic field interacts with the radiofrequency waves radiated from the radiofrequency antenna 124 to establish helicon waves within the plasma generation device. Helicon waves have helical phase fronts which are suitable for penetrating through the plasma, 10 and are thereby suitable for efficiently generating dense plasmas. Although a range of operating conditions of a signal generator can be selected for.the plasma generation module, a suitable signal generator for the example helicon antenna described above is a 1kW 13.56 MHz radiofrequency generator. The generator may be provided with an automatic matching network (AMN) system configured to manage 15 power transfer between the radiofrequency generator and the antenna (e.g., by impedance matching), as is known in the art. The impedance of the plasma is dependent on the nature of the vapor being ionized and the resultant of the plasma. For example, the AMN system may be configured to change an impedance of the radiofrequency generator (e.g., a 50 ohm impedance) to match a relatively lower impedance of the 20 plasma. The inventors consider a helicon plasma source to provide advantages in the context of permitting a relative high density plasma with a relatively low operating power of the radiofrequency generator, and with a relatively low magnetic field strength. While many radiofrequency sources operate in the GHz electron frequency range, a helicon source 25 operates around the hybrid wave frequency, in the order of 10s (tens) of MHz. At such frequencies, helicon waves are caused to arise in the plasma, and it is known that helicon waves are suitable for generating relatively dense plasmas at relatively low temperatures. In contrast to other radiofrequency plasma sources, a helicon plasma . source can utilise a relatively low magnetic field strength (e.g., between 0.01T to 1T as 30 will be discussed in further detail below). Further, the inventors have determined that particular advantages are provided in the context of a lithium plasma in contrast to other plasma sources, since the helicon antenna need not contact the plasma and there is no requirement for any electrodes to contact the plasma. This is advantageous considering the reactivity of lithium. While other plasma sources may be used, the inventors consider there to be particular advantages associated with a helicon plasma source, and the avoidance of various aspects of other plasma sources that may make them less suitable. For example, other plasma source types include (i) a photo-ionised metallic plasma source (ionisation of a 5 metallic vapour by a laser); (ii) a radiofrequency metallic vapour plasma source (using radiofrequency waves at the electron cyclotron frequency to heat electrons); (iii) a vacuum-arc discharge source, (iv) a thermionic vacuum arc source (TVA); and (v) a DC discharge plasma source. By way of example of potentially less advantageous aspects of such sources, a vacuum-10 arc discharge source produces a highly rotational plasma, which may be less suitable for use in conjunction with a drift-orbit separation device, considering that a drift-orbit separation devices operates to introduce a gradual directional drift between species. Such sources involve electrode contact with the plasma, which is also the case for thermionic vacuum arc plasma sources. Further, while a photo-ionised metallic plasma 15 source may be suitable for creating high density plasmas, the inventors determined that such sources create relatively low diameter beams, which may be less suitable for use in conjunction with a drift-orbit separation device as discussed elsewhere herein. Among these plasma sources, a radiofrequency metallic vapour source and DC discharge source (types (ii) and (v) above) avoid some of the aspects discussed immediately 20 above, but may generate a lower density plasma than a helicon plasma source. The inventors determined that a helicron plasma source is suitable for generating high density plasmas, and is particularly suitable for use In conjunction with a drift-orbit separation device as described elsewhere herein, for example in view of the size of the generated beam (e.g., in the range of 0.02-0.2m), the non-directive nature of the plasma, 25 a relatively lower power input and a relatively low magnetic field strength (permitting use of permanent magnets), which may be configured to match that of a plasma heating module and drift-orbit separation module. Figure 5 schematically shows a cross-sectional cut-away view of the plasma heating module 130 along the longitudinal portion 108 of the lithium pathway 102, according to 30 an example implementation. The plasma heating module 130 is configured to receive the lithium plasma from the plasma generation module 120, and to selectively heat the lithium plasma. Although similar in the overall schematic configuration to the plasma generation module, the example plasma heating module 130 discussed below is an ion cyclotron resonance heating (ICRH) device which operates based on a different physical principle to the example helicon plasma source 120 of Figure 4 (as will be discussed below). The plasma heating module 130 comprises a housing 132 configured to define a respective part of the sealed internal volume of the apparatus 100. In this example, the housing is tubular (e.g., cylindrical), and is disposed between adjacent connecting tubes 104 as described above. The housing 132 is open at both ends for fluid communication with the remainder of the sealed internal volume of the apparatus 100 (in this example via the connecting tubes 104). In other examples, the housing 132 may be directly coupled to one or more housings of adjacent modules, without such connecting tubes, or a common housing may extend between multiple modules as discussed elsewhere herein. Optionally the guide tube 160 as described above with respect to Figures 3 and 4 extends through the plasma generation module as shown Figure 5, and in this example is coaxially aligned with the longitudinal portion 108 of the lithium pathway, and with the housing 132. As shown in Figure 5, an antenna 134 is disposed around an outer surface of the guide tube 160 and within the housing 132. The antenna 134 is configured to generate radiofrequency waves tuned to a target frequency for selective heating of 6Li. However, unlike the antenna 124 of the example helicon plasma source discussed above, the antenna 134 of the plasma heating module 130 is configured to generate radiofrequency waves tuned to the ion cyclotron frequency of 6Li that prevails in the associated magnetic field (discussed below). While various configurations of a radiofrequency antenna may be used, in this particular example the antenna comprises a half-turn helical antenna comprising two angularly spaced apart half-turn antennas (angularly spaced apart with reference to a central axis of the antenna and guide tube, which in this example corresponds to the longitudinal portion 108 of the lithium pathway 102). The angular separation of the two antenna portions is 180°. A particular sizing for the antenna may be selected based on known design principles for radiofrequency antennas. In this example, the antenna is sized appropriately to radiate radiofrequency waves corresponding to the ion cyclotron frequency of 6Li as discussed above. In the specific example described herein, the antenna has a longitudinal dimension (length) of 0.2m, and a diameter of 0.11m. The orientation of the antenna (e.g. left or right turning with respect to the direction of the prevailing magnetic field) may be the same as the upstream antenna of the plasma source, or may be the opposite. In this example, the helicon antenna has a left turning orientation which is considered to have advantages for radiofrequency coupling with electrons, whereas the antenna for the plasma heating module has a right-turning configuration which is considered to have advantages for coupling with ions. Other orientations are envisaged, and the example implementation is optional. The plasma heating module 130 may comprise a third magnetic array 136 disposed around the housing 136 as shown tn Figure 5, and configured to generate a third magnetic field having a longitudinal extent within the housing 136. As discussed elsewhere herein a magnetic array such as the fourth magnetic array may be provided by a multi-module magnetic array extending over multiple modules of the apparatus. The third magnetic field has a third magnetic field strength (also referred to herein as a heating module magnetic field strength) as will be further defined below. In various examples, the third magnetic array 136 may be an electromagnetic array or a permanent magnet array (e.g., consisting only of permanent magnets, with no electromagnets). In this particular example, the third magnetic array 136 is a permanent magnet array. The provision of the third magnetic field serves to contain the plasma in a beam around the lithium pathway (e.g., around the longitudinal portion of the lithium pathway). Further, in this example the third magnetic field influences the ion cyclotron frequency of the lithium ions (e.g., 6Li and 7Li Ions), at which they gyrate (e.g. orbit) as discussed elsewhere herein. The relationship between magnetic field strength, ion cyclotron frequency, charge and gyro-radius is established in the technical field, as discussed elsewhere herein. A radiofrequency generator associated is controlled to produce an AC current in the antenna at the respective ion cyclotron frequency. This results in inductive coupling between an oscillating magnetic field caused by the radiofrequency waves, thereby inducing an electric field and consequently local currents in the respective ions of the plasma, and consequently selective heating of the respective ions. Although a range of operating conditions of a signal generator can be selected for the plasma heating module, a suitable signal generator for the example antenna described above may be configured to operate at a power of between 0.1 kW and 30kW. This power may depend on the efficiency at which the radiofrequency waves are coupled to the plasma for heating. As above, a suitable operating frequency can be selected in dependence on the strength of the magnetic field, to match the ion cyclotron frequency of 6Li. An example frequency is 400kHz, based on a magnetic field strength of 0.157T (as compared with an ion cyclotron frequency of 343kHz for 7Li at the same magnetic field strength). It will be appreciated that the frequency may vary in dependence on the prevailing magnetic field strength in order to correspond to the ion cyclotron frequency for 6Li. The generator may be provided with an automatic matching network (AMN) system configured to manage power transfer between the radiofrequency generator and the antenna (e.g., by impedance matching), as is known in the art. For example, the AMN system may be configured to change an impedance of the radiofrequency generator (e.g., a 50 ohm impedance) to match a relatively lower impedance of the plasma. The signal generator may be a single frequency generator (e.g., configured to produce a stable, unmodulated signal at a specific frequency), which may otherwise be referred to or known as a continuous wave generator. Otherwise, the signal generator may be a variable signal generator (which may be referred to or known as a function generator or arbitrary waveform generator (AWG)) capable to output a signal at variable frequencies, but configured (e.g., controlled) for operation with the plasma heating module to target a target frequency corresponding to the ion cyclotron frequency of 6Li, for example producing a signal having a frequency within a targeting bandwidth of 15% or less of the target frequency (the ion cyclotron frequency), for example 10% or less. To aid selective heating of 6Li (i.e., with minimal direct heating of 7Li), it is desirable to maintain any non-uniformity of the magnetic field relatively low, so that the selectivity of the heating is based on the mass of the ion species as opposed to variations in the magnetic field. In the case of 6Li and 7Li, this corresponds to the non-uniformity of the magnetic field being less than 1 / 6, but in practice it is generally desirable to maintain the non-uniformity far lower, such as less than 5%, or less than 1%. As noted elsewhere herein, in ICRH isotope separation techniques, it is also desirable to avoid collisions during selective heating that may disrupt perpendicular and parallel energies of toe ions with respect to their Larmor orbits (i.e., orbits at the respective gyroradius). However, when using ICRH for selective heating and not relying on it for ion separation, a greater incidence of collisions is permissible (since a further module is being used tor isotope separations - the drift-orbit separation module as discussed below). Further, the inventors have determined that particular performance advantages in selective heating are achieved in the context of lithium plasma. This corresponds to lithium being the lightest metallic element, and as such the relative difference in isotope mass is relatively large, therefore affording a relatively high degree of selectivity in the coupling between the resonant magnetic field and the respective ion cyclotron frequency. As already noted, the particular dimensions of the plasma heating module and associated antenna may be varied and selected based on such factors as the magnetic field strength, plasma beam size and throughput. However, the inventors have determined that a lower bound length corresponds to the completion of several Larmor orbits in order to be accelerated by the AC field, and generally increases with reducing magnetic field. For example, a length of 0.02m or more may be appropriate for a magnetic field strength of 0.1T, but a lower length may be appropriate for higher 5 magnetic field strengths (e.g. 0.1m or more for a magnetic field strength of 0.2T). The length may be 0.05-1 m, for example 0.05-0.8m, 0.1-0.8m, or 0.2-0.8m, such as approximately 0.5m. Figure 6 schematically shows a cross-sectional plan view of the example drift-orbit separation module 140, extending along a curvilinear portion 109 of the lithium pathway 10 102. The drift-orbit separation module is configured to receive the selectively heated lithium plasma from the plasma heating module 130, and to convey the plasma along the curvilinear portion 109 of the lithium pathway 102 to an optional collector 150. The drift-orbit separation module 140 may comprise a curvilinear tubular housing 142 configured to define a respective part of the sealed internal volume of the apparatus 100, 15 and which defines the curvilinear portion 109 of the lithium pathway 102 along a centre thereof. In this example, the curvilinear housing 142 is arcuate about an axis 143, and is shown with an angular extent of 180° (although a greater or lesser angular extent is envisaged herein). When installed so that the lithium pathway 102 is horizontal, the axis 143 is vertical, although as discussed elsewhere herein the housing 142 may be helical 20 (in particular to accommodate a greater angular extent about the axis 143). In this example, the housing 142 is coupled to the plasma heating module 130 via a connecting tube, but in other examples it may be directly coupled to the housing of the plasma heating module 130, or a common housing may extend between multiple modules as discussed elsewhere herein. 25 As shown in Figure 6, the guide Kibe 160 extending through one or more upstream modules may terminate upstream of the drift-orbit separation device (and is indicated in dashed lines in Figure 6 for this reason). The guide tube 160 may terminate upstream of the drift-orbit separation module along the longitudinal portion 108 of the lithium pathway, or may terminate at a boundary of the drift-orbit separation device. The guide 30 tube 160 may be formed in two or more discrete parts along the longitudinal portion of the lithium pathway, for example with longitudinal gaps between the parts. Such longitudinal gaps may facilitate the provision of a gate valve between such parts, for example between the lithium vaporisation module 110 and the plasma generation module 120, and / or between the plasma generation module 120 and the plasma heating 35 module 130. When the guide tube 160 terminates upstream of the drift-orbit separation device, the guide tube 160 is configured to discharge the heated plasma beam into the space within the housing 142 of the drift-orbit separation device. The example drift-orbit separation device 140 comprises a fourth magnetic array 146 indicated highly schematically in Figure 6 by opposing pairs of magnetic elements angularly distributed around the angular extent of the housing 142. It will be appreciated that the magnetic elements may be provided at a less open angular spacing and that the illustrated arrangement is schematic only. Further, as discussed elsewhere herein a magnetic array such as the fourth magnetic array may be provided by a multi-module magnetic array extending over multiple modules of the apparatus. The fourth magnetic array is configured to generate a fourth magnetic field having a coextensive angular extent with the housing 142. The fourth magnetic array has a fourth magnetic field strength (also referred to herein as a drift-orbit magnetic field strength) as will be further defined below. In various examples, the fourth magnetic array 146 may be an electromagnetic array or a permanent magnet array (e.g., consisting only of permanent magnets, with no electromagnets). In this particular example, the fourth magnet array 146 is a permanent magnet array, comprising discrete rings of magnetic elements to form a solenoid-like curve, or a crescent Helmholtz array. For example, the field strength may be between 0.05T and 1T, for example 0.05T to 0.5T. The scientific principle of the drift-orbit separation device and the relative drift between species is discussed elsewhere herein. The size of the plasma heating module and of the downstream drift-orbit separation device may depend on a velocity of the plasma along the lithium pathway, as this influences the path length over which the respective heating and separation effects occur. The respective velocity may be dependent on the temperature of the plasma, throughput, density and the operating principle of the plasma generation module. Accordingly, the inventors have determined that it is advantageous, when using an ICRH plasma heating module and a drift-orbit separation device, to provide a plasma source that permits a relatively cool plasma with a relatively low directive flow (i.e., flow along the direction of the lithium pathway 102). In view of this, the use of a helicon plasma source is considered to be particularly effective in combination with ICHR plasma heating and drift-orbit separation technologies, as compared with other plasma generation technologies. Nevertheless, it is envisaged that other plasma sources and plasma heating modules may be used. Figure 6 also schematically shows the optional collection module 150. The collection module 150 may be configured to permit collection of enriched 6Li corresponding to enriched regions of the plasma beam at the downstream end of the drift-orbit separation device. Such enriched regions will be further discussed below with respect to simulation results. A wide variety of suitable collection technologies could be used. At its simples, the collection module 150 may be a plate disposed at the downstream end of the drift-orbit separation device. Enriched lithium from enriched regions of the plasma beam will impact at corresponding enriched regions of the plate, which may be separately collected. The plate may be an actively cooled metallic plate (e.g. provided with an external fluid-cooling system). In a more advanced implementation, the plate may have separable plate segments at contiguous locations, with one or more plate segments corresponding to an enriched region. Accordingly, plate segments corresponding to the enriched region may be separately removed and / or separately processed to collect enriched 6Li. Still further options for collection may be provided, including collection modules based on removable / replaceable elements for batch removal during use (e.g., collection plates or similar elements which may be removed in use without interrupting the vacuum in the wider apparatus), or continuous removal. As discussed above, the apparatus 100 may provide a sealed internal volume defined by the housings of the respective modules and any intervening connecting tubes, and the apparatus may be operated with the volume at vacuum or very low pressure conditions. Vacuum pumps (for example turbomolecular pumps and / or scroll pumps) may be provided to maintain the vacuum or low pressure conditions in the sealed internal volume. The vacuum pumps may conveniently be installed at the connecting tubes or ports (e.g. maintenance ports) at other locations along the apparatus. Vacuum pumps may be coupled to ports at any of the housings of the modules, for example in the plasma source module or the plasma heating module. A suitable pressure level may be between 1O'7 mBar to 1mBar. The controller 180 as shown in Figure 1 is representative of one or more controllers optionally provided to control the various modules of the apparatus 100. In this example, a controller may be configured to control the vaporisation device 114, for example to operate the device at a target power corresponding to vaporisation of lithium at a target rate. The target power for the associated target rate may be predetermined (e.g., set based on empirical operation). The target rate may correspond to a target particle flux, mass flux or density (particle density). A controller may be configured to monitor an amount of lithium source in the vaporisation device 114 and / or to control resupply (which may be batch or continuous resupply) of the lithium source. As described elsewhere herein, one or more controllers may be configured to control radiofrequency generators for the respective antennas 124, 134 of the plasma generation module 120 and the plasma heating module 130. In this particular example, each of the magnetic arrays comprises (e.g. consists of) permanent magnet elements, and as such there may be no controller configured to control activation of electromagnetic elements of such magnetic arrays. However, in other examples one or more respective controllers may be provided for activating and or controlling a magnetic field of one or more magnetic arrays comprising electromagnets. While the above example refers to respective magnetic arrays associated with the respective modules, it is also envisaged that a magnetic array may extend over multiple modules, for example along a plasma generation module and a plasma heating module (e.g., along a linear portion of the lithium pathway), or along a plasma generation module, plasma heating module, and drift-orbit separation module (e.g., along both a linear and curvilinear portion of the lithium pathway). Such a magnetic array (which may be referred to as a multi-module magnetic array) may comprise a plurality of magnetic elements (e.g. annular magnetic elements as shown in Figures 4, 5 and 6 discussed above, and in Figure 7 and 8 discussed below)), which may be distributed along the respective portion of the lithium pathway, for example at a constant or substantially constant spacing along the lithium pathway. Such a magnetic array may be configured to provide a uniform magnetic field strength along the respective portion of the lithium pathway. Discussion elsewhere herein relating to the magnetic field strength prevailing at the respective modules as caused by the respective magnetic arrays also applies equivalently to refer to the magnetic field strength at the respective modules as caused by respective portions of such a multi-module magnetic array). A spacing of the magnetic elements may be such that any connecting tube and / or maintenance ports provided between respective modules are provided in a gap between adjacent magnetic elements. Further, it is also envisaged that the example connecting tubes and any associated maintenance ports may not be present and / or may not be provided between adjacent modules. For example, maintenance ports may instead be provided at one or more location corresponding to the modules, such as at a longitudinal location corresponding to the plasma generation module (e.g., for providing a radio frequency feed to an antenna of the module) and / or at a longitudinal location corresponding to the plasma heating module (e.g., for providing a radio frequency feed to an antenna of the module). This may provide for the connecting tubes between modules to be relatively short and / or omitted. Further, while the collection module 150 may comprise a (wholly or partially) removable collection plate not actively controlled, in variant examples a controller may be configured 5 to control the collection module 150, for example to cause removal and / or replacement of collection plates or the like, or to control a continuous collection process. ■ One or more controllers may be provided to operate one or more vacuum pumps (e.g., turbomolecular pumps and / or scroll pumps as described above), and may do so based on monitoring of vacuum (e.g., pressure) conditions within the sealed internal volume of 10 the apparatus 100. The above description generally refers to the individual modules in isolation, noting various features relating to their mutual compatibility and suitability in the context of lithium plasma processing. In addition to those features noted elsewhere, the combination of the described modules provides advantages in enabling low power 15 operation for a relatively high throughput use. In particular, by using drift-orbit separation instead of ICR separation for separation of the isotopes, a relatively higher throughput can be achieved as the inventors have found that drift-orbit separation is less prone to adverse collision affects associated with high-density operation in ICRH separation devices. Further, by using drift-orbit separation in conjunction with ICR and a helicon 20 plasma source, relatively low magnetic field strengths can be adopted throughout the system. In particular, the inventors consider it to be advantageous to maintain a relatively consistent magnetic field strength to avoid adverse effects on the plasma beam as it progresses through the apparatus, such as variations in forces and the size of the plasma beam as it transitions between regions of different magnet field strengths. While t 25 it is expected that such variations may be suitably accommodated, it may also be advantageous to minimize such effects. A helicon plasma source is particularly suitable for operation at relatively low magnetic field strength, and this permits the same throughout the remainder of the apparatus, to the extent that permanent magnets may be used in any or all of the modules, significantly reducing operational complexity and 30 energy resources. Yet further, the plasma source module, plasma heating module and drift-orbit separation module are each suitable to receive and convey plasma beams of similar size, and the upstream modules (e.g. the helicon plasma source module and the ICR heating module) perform their functions while maintaining a relatively low velocity flow and without introducing adverse rotational or turbulent flow regimes. Figures 7 and 8 show perspective and perspective cutaway views of an example implementation of the apparatus 100 as described above. The cutaway plane in Figure 8 is along the lithium pathway as described above, and in a horizontal plane in the orientation shown. The apparatus 100 of Figures 7 and 8 includes the same components as discussed above but showing more implementation detail from a perspective view, for example showing the particular form of the various ports and connections. The illustrated components include the lithium vaporisation module 110, which in Figures 7 and 8 is shown with multiple viewing and access ports 111 on the housing 112. For simplicity, the internal detail of the vaporisation device (e.g., the effusion cell as discussed above) is not shown). Instead, Figure 8 schematically shows a pathway 702 extending from an interior of tie lithium vaporisation module and further through the downstream modules. The pathway 702 corresponds to the and includes the longitudinal portion and the curvilinear portion as described above. The illustrated components further include: - the plasma generation module 120 with the housing 122, and antenna 124 around the guide tube 160; - the plasma heating module 130 with the housing 132, and antenna 134 around the guide tube; - the drift-orbit separation module 140 including the curvilinear housing 142. The illustrated components further indude the first, second, third and fourth magnetic arrays 116,126,136,146 disposed around the respective housings 112,122,132,142 (although as noted above, in other examples there may be no magnetic array associated with the vaporisation module 110, or one or more magnetic arrays may extend over multiple modules). Further, connection tubes 104 provided with maintenance ports 106 are provided between the plasma generation module 120 and the plasma heating module 130, and between the plasma heating module 130 and the drift-orbit separation module 140. The second and third magnetic arrays 126, 136 and the respective housings 122,132 are shown in dashed lines in Figure 7 to reveal the antennas 124,145 which are provided within the housings and surrounding the guide tube 160. As shown in Figure 8, the lithium pathway 102 extends through the guide tube, which extends linearly around the longitudinal portion 108 of the lithium pathway, is open at one end to receive vaporised lithium from the lithium vaporisation module 110, and is open at an opposing end to discharge heated lithium plasma to the interior of the housing 142 of the drift-orbit separation module 140. Suitable materials for the magnetic arrays discussed herein include neodymium permanent magnet rings. Such rings may have an inner dimension corresponding to the respective diameters of the housings of the modules, for example ranging from 0.3m to 0.8m diameter. They may have any suitable outer diameter, for example up to 1 m. Such rings may have any suitable axial length, for example in a range from 0.02m to 0.2m, for example 0.02m to 0.1m. Suitable materials for antennas discussed herein include copper (e.g. a material composition comprising of a copper or copper alloy, or substantially consisting of copper). As noted elsewhere herein, the antennas can be sized appropriately according to known principles, for example having a length of in the range 0.1-0.5m, and a diameter of 0.1-0.8m, for example 0.1-0.5m. Simulations Non-linear transient simulations of plasma behaviour in an example drift-orbit separation device as discussed above were conducted to evaluate the competing effects of drift-orbit separation, collisions and self-induced electric fields. The simulations were implemented by evaluating a system of equations relating to the relevant physics phenomena. The system of equations was defined to evaluate at least t (i)-(xi) below: (i) the transport of multiple ion and electron species (®Li, 7Li, electrons): (ii) self-induced electric fields that arise due to ion transport in the curvilinear drift-orbit magnetic field; (iii) plasma vorticity; (iv) internal pressures of the ion and electron species; (v) ion species temperature and electron temperature (simulated as isothermal in the drift-orbit separation module); (vi) complex collisional diffusion of species, including diffusive (inter-species collisional diffusion) and hyper diffusive (intra-species collisional diffusion) terms in the equations for ion species density; (vii) parallel current; (viii) ion gyro-radius; (ix) ion and electron gyro-viscous terms; (x) plasma resistivity; (xi) ion and electron velocity; The simulations were developed starting from the MIHESEL model developed by Poulson et al as described in Ref 1 (A. Poulsen, “PhD Thesis: Turbulent Transport in a multi-ion drift fluid model”, Technical University of Denmark, 2020") and Ref 2 (A. 5 Poulsen, J Juul Rasmussen, M. Wiesenberger and V, Naulin, “Collisional multispecies drift fluid model", Physics of Plasmas, vol 27(3), Article 032305. DOk https: / / doi.Org / 10,10631.5140522). The simulations were written within the BOUT++ framework for plasma simulation in curvilinear magnetic fields. BOUT++ is an open source framework developed at the 10 University of York (UK) in collaboration with other partners. The simulations were implemented as a 3D representation, with the spatial domain including the two dimensions perpendicular to the magnetic field with time-stepping of the domain to represent the evolution of the plasma along and a dimension parallel to the magnetic field. 15 The perpendicular directions indude an x-dimension corresponding to the radial direction of the curvilinear drift-orbit separation module (256 grid points), and a Z-dimension perpendicular to the direction of the magnetic field and to the x-dimension (512 grid points), representative of a domain which has a dimension of 0.9m in the x-dimenslon and 1.8m in the z direction. The physical boundary of the housing is not directly 20 modelled, and instead the domain is made large enough such that interactions with the physical boundary can be neglected. The location of the plasma beam as a source in the domain is at 0.3m in the x-dimension and 0.5 in the z-dimension, which is offset from the centre of the domain to reflect that drift-orbit will be positive along the z-dimension, and ExB drift is typically positive along the x-dimension. 25 The simulations are defined to reflect a radius of curvature of the magnetic field of 1m over a 180° arcuate extent (i.e., having a parallel length of 3.14m). The transient model includes 200 timesteps, each timestep representative of 40 ion cyclotron periods. In all simulations described herein, the particle fraction of eLi in the plasma source is 30 0.075. Further parameters of the simulations are varied between simulation cases, including the magnetic field strength, the size of the plasma beam, the density of the plasma beam, the ion and species temperature in the drift-orbit separation module (representative of heating being conducted in the upstream plasma heating module). Yield Evaluation Results for the simulations discussed herein include contour plots of 6Li and 7Li concentration, and yield of enriched 6Li. The yield of enriched 6Li is determined from the simulation result in a plane corresponding to the plane of the collector 150 (e.g., the end of the lithium pathway 102). For each grid point in the respective domain, the number of particles of each species (6Li and 7Li) is determined. For each grid point, an enrichment ratio is calculated as the proportion of 6Li particles relative to the total of the 6Li and 7Li particles. A target enrichment ratio is selected, for example 50% or 90%. A contour within the domain demarcating the target enrichment is then determined (i.e. within that domain, the enrichment is at least 50% or at least 90%). A sum of all 6Li and a sum of all 7Li particles within that contour are then determined, in order to determine an integrated enrichment ratio (i.e., the proportion of 6Li contours within the contour relative to the total of the 6Li and 7Li partides within the contour) and a total yield of material at that ratio. As will be appreciated, the integrated enrichment ratio will typically be larger than the target enrichment ratio. The process is iterated at different target ratios until the integrated enrichment ratio matches the target enrichment ratio. The respective yield of lithium is the total amount of lithium (i.e., of both 8Li and 7Li) within the respective contour. The simulation post-processing was established to report this yield as an annual yield in kg, assuming operation of the apparatus 12 hours per day at the respective conditions. This simply reflects a consistent multiplication of a yield per unit time as determined from the simulation, across all simulation cases. Simulation Cases The simulation cases reported below correspond to operation of the apparatus at different conditions. A baseline simulation case (simulation case 1) was established to correspond to low collision operation, as may be achievable with alternative separation technologies. As discussed elsewhere herein, low collision operation corresponds to operation at a relatively low throughput. It has also been proposed that low collision operation can be achieved by maintaining a modest selective heating between the two species, thereby avoiding differential parallel acceleration of the species (i.e. along the magnetic field lines) that may promote collisions. 35 . Accordingly, the baseline simulation case (simulation case 1) was implemented by setting a plasma density at a level considered to correspond to a relatively low rate of collisions, as may be implemented with ICR separation technologies (as discussed above), instead of drift-orbit separation. The plasma beam input to simulation case 1 5 has a density of 1 e 16 particles / m3. In simulation case 1, the temperature of 7Li and the electron temperature is set at leV, representative of a low temperature plasma in which only 6Li is selectively heated, as received from the plasma heating module. The temperature of ®Li ions is set at 15eV. As shown in Table 1 below, the annual yield at a 50% enrichment is approximately 10 0.0128kg. Simulation case 2 corresponds to simulation case 1, but operating at a significantly higher throughput made feasible by the apparatus proposed herein. The density is 1000x times greater, at 1e19 particles / m3. The annual yield may be expected to scale with the increased throughput (i.e. by a factor 15 of 1000). However, as shown in Table 1 below, the annual yield is approximately 4kg, which is less than one third of the yield from simulation case 1 if scaled by the same factor (approximately 31%). Without being bound by theory, this may be considered to support a prevailing technical prejudice that higher throughput operation leads to collisions (e.g., considered to adversely impact separation technologies that influence 20 the orbit of particles about the gyro-radius, such as ICR separation and drift-orbit separation as discussed elsewhere herein). Nevertheless, the simulations have shown that operation at elevated density is viable. Further, the Inventors have determined that the performance can be surprisingly increased to mitigate the apparent reduction in yield when operating at higher densities. 25 In this regard, a number of parameters have been explored to evaluate trends, including the magnetic field strength, the size of the beam, the electron temperature, 7Li ion temperature, 6Li temperature. The inventors have determined that increasing the 6Li temperature relative to the 7Li temperature (which may be similar to the background electron temperature) improves the yield as compared with a scaled yield at lower 30 densities, at corresponding ion temperatures. In particular, simulation cases 3 and 4 correspond to simulation cases 1 and 2, but with the 6Li temperature is set to 65eV. As shown in table 1, at this 6Li temperature the yield at high density is significantly improved. This yield, expressed as a proportion of the scaled yield from the corresponding low density simulation case (case 3) is significantly higher than the same comparison at lower 6Li temperatures. The high density yield is approximately 72% of the scaled low density yield at the elevated 6Li temperature, as compared with only approximately 31% at the relatively lower 6Li temperature. Table 1. Simulation Case 1 Simulation Case 2 Simulation Case 3 Simulation Case 4 Magnetic Field Strength 0.1T Beam diameter 0.1m (circular profile) Electron temperature 1eV 7LI temperature lev 6Li temperature 15eV 15eV 65eV 65eV Plasma Density 1e16 particles / m3 1e19 particles / m3 1e16 particles / m3 1e19 particles / m3 Yield at 50% enrichment 0.0128kg 4kg 0.029 21 Scaled low density yield (by factor of 1000) 12.8kg n / a 29kg n / a Ratio - high density yield / scaled low density yield 31% 72% Without wishing to be bound by theory, the inventors consider that the relatively greater temperature,difference between the lithium ion species may promote a stronger drift orbit separation (e.g., a higher drift velocity) that drives a greater spatial separation between the species early that may avoid collisions along the extent of the drift-orbit separation, 10 Additionally or alternatively, it is considered that the stronger drift-orbit separation effect for 6Li may reduce a coupling between the species, and any drift associated with induced electric fields (known as ExB drift, as discussed elsewhere herein). In particular, it may ' be that the drift orbit separation effect can become established as a dominant aspect of particle motion in a drift-orbit separation module even at relatively high throughput, 15 overcoming competing effects as may be caused by particle collisions and any turbulent interaction of particles owing to ExB drift (also known as E-cross-B-drift, which is associated with movement of charged particles in the presence of both an electric field E (e.g. self-induced by opposing drift of ions and electrons) and a magnetic field B). Figures 9-11 show contour plots of 6Li and 7Li concentration for simulation cases 1, 2 and 4 respectively. For each drawing, the upper plot shows 6Li density and the lower plot shows 7Li density. The legend of each plot shows five contour levels, which envelope the data shown in the plot itself, such that the three contour lines shown.in the plot itself correspond to the middle three contour levels in the legend. As shown in Figure 9, the 6Li plot shows a tail of enriched 6Li extending from a centre of the beam along the z-direction (downwardly in the plane), which is not present in the 7Li plot. The tail is caused by the drift-orbit separation effect for simulation case 1, taking eLi spatially away from 7U to provides relatively large region of enriched 6Li for collection. The inventors have found that 7Li tends to be more influenced by turbulent radial motion arising from electrostatic potential and / or ExB drift effects, whereas the motion of the heated 6Li is dominated by the drift-orbit separation effect, forming the tail as shown in Figure 9. Figure 10 provides the same plots for the higher density operating point of simulation case 2. This shows a moderate tail effect. The relatively weaker yield discussed above (as compared with a scaled yield from simulation case 1) appears to correspond to more instability in the beam. It is observed that the 7Li distribution is influenced by more turbulent dynamics, which may result from more collisions in at the higher density operating point. The relatively weaker yield may be driven by the 7Li being more turbulently distributed, such that a size of an enriched 6Li region produced (e.g. a tail associated with the drift-orbit separation effect) is more limited. Figure 11 provides comparable plots for the high density, high 6Li temperature operating point of simulation case 4. This shows the dominance of the drift-orbit separation effect for 6Li, with a less turbulent interaction in 7Li. This appears to drive the much improved yield. As discussed above, a collection module 150 of the apparatus may be configured to permit collection of enriched 6Li corresponding to an enriched region of a plasma beam. As noted in the above discussion, the drift-orbit effect causes drift of 6Li relative to 7Li, corresponding to the formation of a tail of enriched 6LI in a tail. The nature of the tail means that the enriched region may be directionally concentrated, rather than uniformly disposed in an annular region. The tail extends away from a central region of the beam in the drift direction. As such, the collection module 150 may be configured to delimit a central region (e.g. of the beam and / or of the collection module) from an enriched 6Li region (e.g., of the beam and / or of the collection module). As defined elsewhere herein, the enriched 6Li region corresponding to the tail and as shown in Figures 9-11 may be characterised in various ways and have a corresponding geometric extent. For example, as described elsewhere herein, the enriched 6Li region may have an acute angular extent of between 5-40° (and other ranges as disclosed herein). It may have a lateral extent corresponding to a lateral width of the beam upon receipt into the drift-orbit separation module. Figure 12 is a plot of yield (at 50% and 90% enrichment) for a set of simulation cases corresponding to simulation case 1 but at varying 6Li temperature. This therefore also includes the result for simulation case 3. Figure 13 is a similar plot of yield (at 50% and 90% enrichment) for a further set of simulation cases corresponding to simulation case 2 but at varying 6Li temperature. This therefore also includes the result for simulation case 4. By comparing Figures 12 and Figures 13, it can be seen that the temperature difference between the species has a relatively greater proportional influence on improving yield for the higher density simulation cases, as compared with the lower density simulation cases. Without wishing to be bound by theory, this is considered to reflect that the greater temperature difference mitigates against collisional diffusion, which is less prevalent at the lower density operating point (corresponding to a relatively weaker improvement with increasing temperature). This trend is also revealed by the numerical results in Table 1 above. Such advantages arise in the particular context of lithium owing to the nature of the stable isotopes (6Li and 7Li), and the relatively large temperature difference that can be exploited between them. In particular, while 7Li can be maintained at a relatively low temperature (e.g. 1eV), the temperature of 6Li ions can be greatly increased towards 70eV, which is the second ionization energy for lithium. Heating of particles to the second ionization would change the ion cyclotron frequency and may adversely impact the drift orbit effect. The possibility for a large temperature difference drives a significantly larger gyro-radius for 6Li and therefore a greater drift-orbit separation effect suitable for becoming a dominant trend in the plasma dynamics, overcoming collisional, turbulence and ExB couplings that may otherwise prevent drift-orbit separation from enabling a region of enriched 6Li for collection. Such effects may not be present for other species (e.g. other elements and ion pairs), and may reflect surprising success of the proposed use of drift-orbit separation for a metallic element, possibly associated with the low mass number of lithium element, possibly associated with the low mass number of lithium ions and the proportionally large difference in mass and temperature than can be achieved between them. Further simulations were conducted and demonstrated over ranges of various other operating conditions, including magnetic field strength. Figure 14 shows results for 50% and 90% enrichment yields for a set of cases corresponding to simulation case 4, but at a varying magnetic field strength of 0.1T to 0.5T, It is also envisaged to operate at higher magnetic field strengths (e.g. up to 1T). Further simulations were conducted at a range of plasma beam sizes, including over a range from approximately 0.05m diameter (e.g. 0.045m diameter) to 0.18m diameter and beyond, with a consistent trend of increasing yield of enriched 6Li (both at 50% and 90% enrichment). A range for the plasma beam dimension (e.g., diameter) as envisaged herein is 0,03m-0.3m. Further ranges as disclosed elsewhere herein are also envisaged. As discussed above, the combination of plasma generation module, plasma heating module and drift-orbit separation device for the apparatus permits operation with relatively low magnetic field strengths, such that permanent magnets may be used. In an example implementation, the relatively low magnetic field strength associated with the drift-orbit separation device (e.g., between 0.1T to 1T and other ranges as discussed herein) may advantageously be matched by other magnetic arrays of the apparatus. In particular, when using a helicon plasma generation module and an ICRH plasma heating module, each of those modules also provide suitable performance with a relatively low magnetic field strength (e.g., as compared with alternative plasma sources and plasma heating technologies). As such, the respective magnetic arrays can be implemented with similar magnetic field strengths to reduce discontinuities in the magnetic field along the lithium pathway, which may otherwise adversely affect containment and maintenance of the plasma beam. A difference in magnetic field strength between the respective modules may be less than 10%, for example less than 5%, less than 2% or less than 1%. Although the examples discussed herein relate to plasma beams having a generally circular cross-sectional profile, it is also envisaged to operate with plasma beams having different cross-sectional profile as provided to the drift-orbit separation module, for example an elliptical profile having a greater extent in a direction corresponding to the radial direction of the curvilinear portion of the lithium pathway, and a lesser extent in a direction perpendicular to the radial direction and the magnetic field direction. Magnetic field strengths as defined herein may be measured by a Hall effect magnetic field monitor. The expression “drift-orbit magnetic field” is intended to denote the magnetic field at the arcuate path of the drift-orbit separation device. The expression is not intended to denote, in and of itself, any particular features of the magnetic field other than its association with the drift-orbit separation device. A drift-orbit magnetic field may nevertheless have any combination of the features described herein with respect to the drift-orbit magnetic field. The controllers) described herein may comprise a processor. The controller and / or the processor may comprise any suitable circuity to cause performance of the methods or control processes described herein, including with respect to any relevant drawings. The controller or processor may comprise: at least one application specific integrated circuit (ASIC); and / or at least one field programmable gate array (FPGA); and / or single or multiprocessor architectures; and / or sequential (Von Neumanriyparallel architectures; and / or at least one programmable logic controllers (PLCs); and / or at least one microprocessor; and / or at least one microcontroller; and / or a central processing unit (CPU), to perform the methods and or stated functions for which the controller or processor is configured. The controller may comprise, or the processor may comprise or be in communication with one or more memories that store that data described herein, and / or that store machine readable instructions (e.g. software) for performing the processes and functions described herein (e.g. determinations of parameters and execution of control routines). The memory may be any suitable non-transitory computer readable storage medium, data storage device or devices, and may comprise a hard disk and / or solid-state memory (such as flash memory). In some examples, the computer readable instructions may be transferred to the memory via a wireless signal or via a wired signal. The memory may be permanent non-removable memory or may be removable memory (such as a universal serial bus (USB) flash drive). The memory may store a computer program comprising computer readable instructions that, when read by a processor or control I er, causes performance of the methods described herein, and / or as illustrated in the drawings. The computer program may be software or firmware, or be a combination of software and firmware. Various examples have been described, each of which comprise one or more combinations of features. It will be appreciated by those skilled in the art that, except where clearly mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.

Claims

1. A method of enriching 6Li, comprising:generating and discharging a beam of lithium plasma comprising 6Li and TLi, using a helicon plasma source;■ selectively heating 6Li within the beam of lithium plasma in a plasma heating module; andconveying the beam of lithium plasma comprising selectively-heated 6Li along an arcuate path within a drift-orbit magnetic field to drive separation of 6Li from 7Li along the arcuate path in a drift direction normal to the arcuate path and define an enriched 6Li region of the beam for collection.

2. The method of claim 1, wherein at a location between the plasma heating module and a drift-orbit separation module that defines the arcuate path, the beam of lithium plasma has:a partide flux of 1018-1022 particles / m2s; and / ora mass flux of 10-8 -1kg / s; and / ora density of at least 1016 particles / m3; and / ora density of at least 1019 particles / m3.

3. The method of claim 1 or 2, wherein at a location between the plasma heating module and a drift-orbit separation module that defines the arcuate path, the beam of lithium plasma has a cross-sectional profile having:a major dimension of 0.025m - 0.2m; and / or an area of 0.5e-3 m2 - 30e-3 m2.

4. The method of any one of the preceding claims, wherein the ®Li is selectively heated by ion cyclotron resonance heating in the plasma heating module in the presence of a heating module magnetic field having a heating module magnetic field strength; andwherein the plasma heating module generates radio-frequency waves tuned to a target frequency for selective heating of 6Li that corresponds to the ion cyclotron frequency of 6Li for the heating module magnetic field strength, using a radio-frequency generator coupled to a radio-frequency antenna.

5. The method of any one of the preceding claims, wherein the selective heating in the plasma heating module causes heating of 6Li within the plasma beam by a temperature difference of at least 30eV, for example at least 50eV, at least 60eV, 30-65eV or 30-60eV.

6. The method of any one of the preceding claims, wherein the drift-orbit magnetic field is provided by permanent magnets and has a magnetic field strength of 0.15T-0.25T;wherein, at a location between the plasma heating module and a drift-orbit separation module that defines the arcuate path, the beam of lithium plasma has:a cross-sectional profile having a major dimension of 0.04m - 0.15m; and a density of at least 10ie particles / m3 or a mass flux of at least 1.7 kg / s.

7. The method of any one of the preceding claims, wherein the separation of 6Li from 7Li along ttie arcuate path is confined about the drift direction so that, in a crosssection of the beam at a downstream end of the arcuate path, an angular distribution of 6Li around an annular region of the beam relative to a beam centre is non-uniform and concentrated in the enriched 6Li region.

8. Apparatus for 6Li enrichment, comprising:a helicon plasma source configured to generate a beam of lithium plasma;,a plasma heating module configured to receive the beam of lithium plasma from the helicon plasma source, and configured to selectively heat 6Li within the lithium plasma; anda drift-orbit separation module configured to receive the beam of lithium plasma from the plasma heating module along an arcuate path, wherein the drift-orbit separation module comprises a magnetic array configured to generate a drift-orbit magnetic field around the arcuate path to drive separation of 6Li from 7Li along the arcuate path in a drift direction normal to the arcuate path, to define an enriched 8Li region of the beam for collection.

9. The apparatus of claim 8, configured so that at a location between the plasma heating module and the drift-orbit separation module, the beam of lithium plasma has:a particle flux of 1018-1022 partides / m2s; and / ora mass flux of 10* - KT4 kg / s; and / ora density of at least 1016 particles / m3; and / ora density of at least 1019 partides / m3.10, The apparatus of claim 8 or 9, configured so that, at a location between the plasma heating module and the drift-orbit separation module, the beam of lithium plasma has a cross-sectional profile having:a major dimension of 0.025m - 0.2m; and / or an area of 0.5e-3 m2 - 30e-3 m2.

11. The apparatus of any one of claims 8-10, comprising a guide tube extending through the plasma heating module and configured to discharge the beam of lithium plasma to the drift-orbit separation module at a discharge end, wherein the discharge end has a cross-sectional profile having a major dimension of 0.03-0.3m, and / or an area of 0.0025-0.3mz.

12. The apparatus of any one of claims 8-11, wherein the magnetic array of the drift-orbit separation module is configured so that the drift-orbit magnetic field has a magnetic field strength of Q.1T-1T along the curvilinear path.

13. The apparatus of any one of claims 8-12, wherein the magnetic array of the driftorbit separation module comprises permanent magnets which are configured to generate the drift-orbit magnetic field.

14. The apparatus of any one of claims 8-13, wherein the plasma source comprises a magnetic array configured to generate a plasma source magnetic field, optionally wherein the magnetic array of the plasma source is configured so that a magnitude of a strength difference between the plasma source magnetic field strength relative to the drift-orbit magnetic field strength is 10% or less; and / orwherein the plasma heating module comprises a magnetic array configured to generate a heating module magnetic field, optionally wherein the magnetic array of the plasma heating module is configured so that a magnitude of a strength difference between the heating module magnetic field relative to the drift-orbit magnetic field is 10% or less.

15. The apparatus of any one of claims 8-14, wherein the plasma heating module is an ion cyclotron resistance heating module comprising a radio-frequency antenna configured to selectively heat 6Li within the beam of lithium plasma.

16. The apparatus of claim 15, wherein the plasma heating module comprises a magnetic array configured to generate a heating module magnetic field having a heating module magnetic field strength;wherein the plasma heating module further comprises a radio-frequency generator coupled to the radio-frequency antenna and configured to generate radiofrequency waves tuned to a target frequency for selective heating of 6Li, wherein the target frequency corresponds to the ion cyclotron frequency of 6Li for the heating module magnetic field strength.

17. The apparatus of any one of claims 8-16, wherein the helicon plasma source comprises a helicon antenna configured to emit radiofrequency (RF) waves to generate plasma within a plasma containment, and a magnetic array around the plasma containment to confine the plasma within the plasma containment and define a path for discharging the beam of lithium plasma from the plasma containment.

18. The apparatus of any one of the claims 8-17, configured so that selective heating in the plasma heating module corresponds to heating of ®Li within the plasma beam by a temperature difference of at least 30eV, for example at least 50eV, at least 60eV, 30-65eV or 30-60eV.

19. The apparatus of any one of claims 8-18, wherein the drift-orbit separation module comprises a magnetic array of permanent magnets configured to provide the drift-orbit magnetic field having a magnetic field strength of 0.15T-0.25T;wherein the apparatus is configured so that, at a location between the plasma heating module and the drift-orbit separation module, the beam of lithium plasma has:a cross-sectional profile having a major dimension of 0.04m - 0.15m; and a density of at least 1018 particles / m3 or a mass flux of at least 1.7 kg / s.

20. The apparatus of any one of claims 8-19, comprising a collector module disposed at an arcuate end of the drift-orbit separation device opposite to the plasma heating module, wherein the collector module is configured to delimit a central region and an enriched 6Li region for separate collection of enriched 6Li, wherein:the central region is disposed around an intersection of the arcuate path and the35drift-orbit separation module;the enriched 6Li region comprises an annular region or annular segment disposed around the central region;optionally wherein the enriched eU region comprises an annular segment to capture a tail of 6Li drifted from a central region along a drift direction normal to the 5 arcuate path.

21. The method of any one of claims 1 -7, performed using the apparatus of any one of claims 8-20.Intellectual PropertyQ^flGOoD No:Claims searched:GB2415684.61-21Examiner: Mr Tristan BallardDate of search: 16 May 2025Patents Act 1977: Search Report under Section 17Documents considered to be relevant:Category Relevant to claims Identity of document and passage or figure of particular reference A - JPH10109019 A (WATANABE)Categories: X Document indicating lack of novelty or inventive step A Document indicating technological background and / or state of the art. Y Document indicating lack of inventive step if combined with one or more other documents of same category. P Document published on or after the declared priority date but before the filing date of this invention. & Member of the same patent family E Patent document published on or after, but with priority date earlier than, the filing date of this application.Field of Search:Search of GB, EP, WO &US patent documents classified in the following areas of the UKCWorldwide search of patent documents classified in the following areas of the IPC_____________BOID; HOU_____________________________________________The following online and other databases have been used in the preparation of this search reportSEARCH-PATENT, SEARCH-NPL

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