Method and apparatus for operating a liquid metal ion source or liquid metal electron source, as well as liquid metal ion source or liquid metal electron source

Abstract

Liquid metal ion beam system (1) or liquid metal electron beam system, comprising: - a conductive emitter electrode (2), - a conductive extractor electrode (3) opposite the emitter electrode (2), - a liquid metal reservoir (4) fluidically connected to the emitter electrode (2) to transport liquid metal to the emitter electrode (2), - a control unit (5) configured to apply a periodically varying operating voltage between the emitter electrode (2) and the extractor electrode (3), wherein an operating voltage threshold is specified which defines a voltage level above which non-ionized or only slightly ionized liquid metal at the emitter electrode (2) is extracted by emitted ions or electrons accelerated towards the extractor electrode (3), wherein the control unit (5) is configured to periodically vary the operating voltage between a subthreshold voltage,to vary the voltage that is below the operating voltage threshold and the voltage that is above the operating voltage threshold.

Description

Technical field The invention relates to liquid metal ion sources, in particular for electric propulsion systems in spacecraft, and ion beam tools in focused ion beam systems, as well as liquid metal electron sources. Furthermore, the present invention relates to methods for operating such liquid metal ion sources or liquid metal electron sources with reduced liquid metal consumption. Technical background Liquid metal-ion ion sources have been used for electric space propulsion systems, as described by M. Tajmar et al., “Liquid metal-ion source development for space propulsion at ARC,” Ultramicroscopy, Vol. 109, No. 5, pp. 442-446, 2009, and as ion beam tools in focused ion beam systems, as described by J. Gierak, “Focused ion beam technology and ultimate applications,” Semiconductor Science and Technology, Vol. 24, No. 4, p. 043001, 2009, for many decades. Liquid metal ion sources and liquid metal electron sources typically feature an emitter in the form of a pointed or porous needle or a capillary protruding from a reservoir containing liquid metal. An extractor, for example, in the form of a perforated electrode, is positioned at a distance from this. A metal with a low melting point, such as gallium or indium, is often used as the propellant and is fed to the emitter. The emitter is heated to a temperature at which the metallic propellant is liquid. When a high voltage is applied between the emitter and extractor, a liquid metal cone forms at the emitter tip, with an atomic point extending towards the extractor. There, the electric field is so strong that, depending on the polarity, ions are emitted when operating as a liquid metal ion source, or electrons are emitted when operating as a liquid metal electron source. When operating as a liquid metal ion source, the emitter is supplied with a positive voltage relative to the extractor. Up to a critical current threshold of a few µA, a pure ion beam is generated. Beyond this threshold, slightly charged microdroplets are emitted in addition to the ion stream, detaching from the liquid metal cone. These microdroplets form because the transport of the liquid metal to the tip of the emitter is not fast enough, causing the atomic tip of the liquid metal cone to repeatedly collapse. The microdroplets are only slightly electrically charged and therefore do not contribute significantly to generating thrust when operating in a propulsion system. The efficiency of the liquid metal ion source as a propulsion system decreases, and fuel consumption increases considerably. The resulting ion beam is therefore pulsating. The pulsation frequency depends on the emitter geometry and the viscosity of the liquid metal and, above the critical current, is typically in the high kHz and low MHz range. The use of liquid metal II electron sources is advantageous when high electron currents are required. In a liquid metal II electron source, a negative voltage is applied between the emitter and the extractor. A liquid metal cone forms, and the atomic tip directly generates an electron current. Continuous operation is not possible with this method; the electrons are always emitted in pulses because they ionize nearby neutral atoms, which are attracted to the negative emitter tip due to their positive charge. This bombardment leads to intense local heating of the tip, causing even more electrons to be emitted. This creates an avalanche effect, resulting in an explosive increase in the electron current in the ampere range before the entire liquid metal II tip evaporates and the process restarts.Even with this operating mode, liquid metal consumption is relatively high. Furthermore, the emission of metal droplets is a major disadvantage for many applications, as only electrons are needed and the metal vapor coats the surrounding surfaces. While liquid metal ion sources for focused ion beam applications are typically operated at very low currents below the critical current threshold, propulsion systems for spacecraft use currents above the critical current threshold to achieve significant thrust. In this case, droplet formation and the associated additional propulsion mass loss are accepted. Droplet formation necessitates extensive protective measures (e.g., labyrinth shielding) and limits the lifespan. The best solution to date is the use of a very pointed needle emitter, which offers an efficiency advantage over a capillary emitter, as reported by M. Tajmar, “Influence of Taylor cone size on droplet generation in an indium liquid metal ion source,” Appl. Phys. A Mater. Sci. Process, vol. 81, no. 7, pp. 1447–1450, 2005. Liquid metal electron sources are currently a niche product. The individual current pulse is high, and the repetition frequency is in the millisecond range, which complicates the practical application of such electron sources, as alternative solutions generate more uniform electron currents without metal droplets. From EP 0 202 685 A1, a liquid metal ion source is known, comprising an ion emitter tip to which an ion source liquid metal material is supplied, a withdrawal electrode for withdrawing an ion beam from the emitter tip, and a first device for applying a DC voltage between the emitter tip and the withdrawal electrode. The DC voltage is set to a value close to the threshold voltage for withdrawing an ion beam. A second device is provided which superimposes voltage pulses on the DC voltage to effect the withdrawal of a pulsating ion beam. German patent application DE 3688808T2 discloses a liquid metal ion source comprising an ion emitter tip, a holding device for holding a liquid metal ion source material for feeding to the emitter tip, a withdrawal electrode for withdrawing an ion beam from the emitter tip, and a first device for applying a DC voltage between the emitter tip and the withdrawal electrode. The DC voltage is set to a value close to the threshold voltage (Vth) for withdrawing an ion beam. A second device is provided that superimposes voltage pulses on the DC voltage to effect the withdrawal of a pulsating ion beam. Patent application US 2009 / 0121160 A1 discloses a charged particle beam device comprising an emitter unit with an emitter tip; a voltage supply unit designed to provide a stable voltage to generate a stable extraction field at the emitter tip; a pulsed voltage supply element designed to provide a pulsed voltage to generate a pulsed extraction field over the stable extraction field; a measuring unit for measuring an emitter characteristic; and a control unit designed to receive a signal from the measuring unit and to control the pulsed voltage supply element. Document US2010 / 0251690 A1 discloses a field emission propulsion (FEEP) system comprising: a FEEP thruster with at least one emitter and an extraction electrode, a power supply to provide an extractor voltage applied between the at least one emitter and the extractor electrode, wherein the power supply can be operated in a regulated current mode in which the extractor voltage is controlled to regulate an ion current flowing from the at least one emitter at a target current level. Document US 2003 / 0230961 A1 discloses a drift ion source comprising a channel with an open end and a closed end, wherein the channel includes an inlet port for an ionizable gas; a first magnetic pole located at the open end of the channel and extending from there in a first direction; a second magnetic pole located at the open end of the channel and extending from there in a second direction, the first direction being opposite to the second direction; an anode located in the channel; a primary magnetic field line located between the first magnetic pole and the second magnetic pole, wherein the primary magnetic field line has a mirror field greater than 2. It is therefore an object of the present invention to operate a liquid metal ion source or a liquid metal electron source in such a way that the consumption of the liquid metal used is reduced. Disclosure of the invention This problem is solved by the method for operating a liquid metal ion source or liquid metal electron source according to claim 1 and a liquid metal ion beam system or liquid metal electron beam system according to the dependent claim. Further details are specified in the dependent claims. According to a first aspect, a liquid metal ion beam system or a liquid metal electron beam system is provided, comprising: - a conductive emitter electrode, - a conductive extractor electrode opposite the emitter electrode, - a liquid metal reservoir fluidically connected to the emitter electrode to transport liquid metal to the emitter electrode, - a control unit configured to apply a periodically varying operating voltage between the emitter electrode and the extractor electrode. According to another aspect, a method for operating a liquid metal ion beam system or liquid metal electron beam system is provided, comprising: - a conductive emitter electrode, - a conductive extractor electrode opposite the emitter electrode, - a liquid metal reservoir fluidically connected to the emitter electrode to transport liquid metal to the emitter electrode, with a periodically varying operating voltage being applied between the emitter electrode and the extractor electrode. Furthermore, an operating voltage threshold is specified, which defines a voltage level from which non-ionized or only slightly ionized liquid metal is released from the emitter electrode by emitted ions or electrons accelerated towards the extractor electrode, wherein the control unit is designed to vary a periodically varying operating voltage between a sub-threshold voltage, which is below the operating voltage threshold in magnitude, and a supra-threshold voltage, which is above the operating voltage threshold in magnitude. The liquid metal ion source or liquid metal electron source comprises an emitter electrode and an extractor electrode. These are operated by applying a periodically varying operating voltage between the emitter and extractor electrodes, causing ions or electrons to be emitted from the emitter electrode. A liquid metal is supplied to the emitter electrode, which, during operation, collects in the area of ​​the emitter electrode facing the extractor electrode and serves there to generate ions or electrons. Depending on the material of the liquid metal and the geometry of the emitter, there is an operating voltage threshold above which droplet formation occurs, i.e., through the generation of liquid metal ions, droplets of liquid metal are detached from the emitter, or through an avalanche effect, liquid metal is evaporated at the emitters. Droplet formation only occurs after a certain period of time following the exceedance of the operating voltage threshold. Therefore, by using pulsed operation with alternating voltages between the over-threshold and under-threshold voltages, droplet formation can be avoided even when the operating voltage threshold is exceeded. The operating frequency of the voltage variation is set such that droplet formation at the emitter electrode is prevented. In this process, the extractor can be connected to a reference potential, and a pulsed voltage from a pulsed voltage source can be applied to the emitter. An advantage of this approach is its low complexity, as only a high-voltage source (pulsed voltage source) is required. However, the pulsed voltage requires high voltage pulses, which are not always easy to generate. By additionally using needle emitters, the time required to build up a liquid metal cone or droplet that forms around the tip of the emitter can be reduced. Alternatively, the extractor can be connected to a reference potential and the emitter to a constant operating potential, resulting in a voltage difference below the operating voltage threshold. This causes a liquid metal cone to build up on the emitter, although no emission occurs. By applying, i.e., connecting in series, a pulsed potential source, which therefore only needs to provide a lower pulsed potential, pulsed operation of the ion or electron source can be achieved. Alternatively, the extractor can be connected to a high, constant operating potential and the emitter to a reference potential, resulting in a voltage difference below the operating voltage threshold. The pulse voltage source is applied between the reference potential and the emitter and provides a periodic voltage swing that varies the operating voltage between the emitter and extractor between the first and second voltage differences. The pulse voltage source does not require high-voltage insulation and only a small voltage swing for the pulses. The energy of the resulting ion or electron beam is relatively low for operation as an electron source, but this is advantageous because only negative charges are needed for charge balance. By selecting a suitable pulse frequency for the pulse voltage source that prevents droplet formation, the consumption of liquid metal for operating the liquid metal ion or liquid metal electron source can be significantly reduced. The pulse frequency can be fixed, particularly within a frequency range of 100 kHz to 100 MHz for a liquid metal ion source and between 1 MHz and 1 GHz for a liquid metal electron source. Alternatively, the pulse frequency can be set through controlled operation, specifically by reducing the applied potential difference below an operating voltage when the ion or electron current exceeds a predefined threshold or when the gradient of the ion or electron current exceeds a predetermined gradient threshold. According to one embodiment, the operating voltage can be generated by a constant high voltage source to provide a constant high voltage and a pulse voltage source to provide a periodically varying pulse voltage, which are connected in series with the arrangement of the emitter electrode and the extractor electrode, wherein the sum of the voltages of the constant high voltage source and the pulse voltage source yields the periodically varying operating voltage. It may be provided that the pulse voltage source is connected to a reference potential, in particular a ground potential. Furthermore, the emitter electrode can be designed as a tip electrode or as a capillary electrode. The control unit can be configured to adjust the periodically varying operating voltage through regulated operation. The operating voltage is reduced below the operating voltage threshold when the current flow through the emitter-extractor electrode arrangement exceeds a predetermined current threshold and / or a current gradient rises above a predetermined gradient threshold. Conversely, the operating voltage is raised above the operating voltage threshold when the current flow through the emitter-extractor electrode arrangement falls below a predetermined current threshold and / or a current gradient falls below a predetermined gradient threshold. The predetermined current threshold and gradient threshold are selected to define the threshold at which droplet formation begins. According to one embodiment, the liquid metal ion beam system can be operated with a pulse frequency of the operating voltage of greater than 100 kHz and less than 100 MHz. Furthermore, in the liquid metal electron beam system, the pulse frequency of the operating voltage can be greater than 1 MHz and less than 1 GHz. According to another aspect, a system is provided with the above liquid metal ion beam system and the above liquid metal electron beam system, with the operating voltages referenced to the same reference potential. In this way, electrostatic charging of an ungrounded vehicle, especially a spacecraft, can be avoided, since charge equalization between the amount of emitted positive and negative charge can be achieved. In particular, the liquid metal ion beam system and the liquid metal electron beam system can be operated with identical pulse frequencies, especially in phase shift, to reduce alternating load on an electrical energy storage device. Furthermore, a single liquid metal ion beam system, which can serve as both an ion and an electron source, would, when used as a propulsion system for a spacecraft, make it possible to balance the charge by alternating between ion and electron sources, thus avoiding charging of the spacecraft or eliminating the need for a separate electron source. Brief description of the drawings The embodiments are explained in more detail below with reference to the accompanying drawings. These show: Fig. 1 a schematic representation of a liquid metal ion beam system; Figs. 2a to 2c different variants for generating the pulsed operating voltage for the liquid metal ion source and liquid metal electron source, respectively. Description of embodiments Fig. 1 shows a schematic representation of a liquid metal ion beam system 1 with a conductive emitter electrode 2, which can be in the form of a conductive tip electrode (as shown) or alternatively a capillary electrode, and a conductive extractor electrode 3 opposite the emitter electrode 2 for accelerating charge carriers. The extractor 3 can be configured as a pinhole aperture. Near the emitter electrode 2, a liquid metal reservoir 4 is arranged, which is fluidically connected to the emitter electrode 2 region to transport liquid metal to the emitter electrode 2. A very pointed liquid metal cone forms at the tip of the emitter electrode 2, from which metal ions are released during operation and accelerated towards the extractor electrode 3. The material from these released metal ions is replaced from the liquid metal reservoir 4. A control unit 5 is provided which applies a positive operating voltage VE between emitter electrode 2 and extractor electrode 3. The operating voltage VE is of a magnitude that causes ions I to be released from emitter electrode 2 and accelerated towards extractor electrode 3. This results in a current in the µA range. If the current rises above a threshold current, slightly charged microdroplets T can be released and separated from the tip of the emitter electrode 2 in addition to the ion current, thereby significantly increasing the consumption of liquid metal without these droplets T contributing significantly to the ion beam. The arrangement shown in Fig. 1 can also be operated as a liquid metal electron beam system if the control unit 5 applies a negative operating voltage VE between emitter electrode 2 and extractor electrode 3. The operating voltage VE has a value that causes electrons to be emitted at the emitter electrode 2. When operating as an electron source, a liquid metal cone forms at the tip of the electrode, directly generating the electron current. The electrons are always emitted in pulses because they ionize nearby neutral atoms, which, due to their positive charge, are attracted to the negatively charged tip of the emitter electrode. This bombardment leads to intense local heating of the tip and a resulting increase in electron emission. This avalanche effect results in a very high electron current, causing the liquid metal cone at the tip of emitter electrode 2 to vaporize. This process leads to a consumption of liquid metal and significant abrasion of emitter electrode 2. To avoid these effects, the liquid metal ion beam system or the liquid metal electron beam system is now to be operated with a pulsed operating voltage VE. The operating voltage VE, which is applied between the emitter electrode 2 and the extractor electrode 3, is periodically varied between a subthreshold voltage, which is below an operating voltage threshold Vthr, and a superthreshold voltage, which is above an operating voltage threshold Vthr. The operating voltage threshold corresponds to the voltage threshold above which droplet formation or detachment of liquid metal occurs. The pulse frequency of the operating voltage VE is chosen to avoid the effects mentioned above, i.e., droplet formation when operating as an ion source and evaporation when operating as an electron source. The pulse frequency can be fixed and, in particular, can be greater than 100 kHz and less than 100 MHz for liquid metal ion sources and greater than 1 MHz and less than 1 GHz for liquid metal electron sources. The pulse shape can be rectangular, sinusoidal, sawtooth-shaped, or any other shape. In principle, several variants are conceivable for generating the periodically varying operating voltage VE. As shown in Fig. 2a, the extractor electrode 3 can be connected to a reference or ground potential B. The emitter electrode 2 is connected to a supply voltage source 6, which is periodically varied and directly generates the supply voltage VE. Voltage pulses are shown in the voltage-time diagram. The complexity of such an arrangement is relatively low, as only the supply voltage source 6 is required. However, this source must provide high voltage pulses with very high voltages, which are very difficult to generate. The subthreshold voltage can have a potential that corresponds to the reference potential B or to a potential between the reference potential B and the supply voltage threshold Vthr.The emitter electrode 2 should also be designed as a tip electrode, since the time to build up a liquid metal cone depends on the geometry of the electrode and, in the case of emitter electrodes designed as capillaries, usually requires more time than is provided by the pulse width of the operating voltage VE (duration of the application of the threshold voltage). The embodiment shown in Fig. 2b depicts the extractor electrode 3 connected to the reference potential (ground potential) B. A constant high voltage HV is applied to the emitter electrode 2 by means of a constant high voltage source 7. The voltage level of this HV is below the operating voltage threshold, below which no ion or electron current is emitted. Additionally, a pulse voltage source 8 is connected in series. This pulsed voltage raises or lowers the operating voltage VE applied to the emitter electrode 2 above or below the operating voltage threshold Vthr. Thus, the high voltage source 7 and the pulse voltage source 8 constitute the operating voltage source of Fig. 2a. By applying the sub-threshold voltage in the region of the operating voltage threshold Vthr, particularly in a range between 70 and 90% of the operating voltage threshold, the formation of a liquid metal cone is enabled, but no emission of ions or electrons is caused.By pulsedly switching on the pulse voltage source 8, which only needs to provide a lower voltage, the liquid metal ion source or liquid metal electron source can now be operated with lower liquid metal consumption. In the embodiment shown in Fig. 2c, the extractor electrode 3 can be operated at a high constant potential HP below the operating voltage threshold Vthrin of the opposite polarity to the emitter electrode 2. A periodic pulse voltage is generated by means of a pulse voltage source 8 and applied to the emitter electrode 2. In contrast to the previous embodiment, this does not require high-voltage insulation and only a relatively small voltage swing to raise the total potential between the emitter electrode 2 and the extractor electrode 3 above the operating voltage threshold Vthrin. The operating voltage source 6 or the pulse voltage source 8 is designed to raise the total voltage between emitter electrode 2 and extractor electrode 3 from the subthreshold voltage above the operating voltage threshold Vthrin to a surthreshold voltage in pulsed operation and then back down below it. The resulting frequency can be fixed or set by controlled operation. For a liquid metal ion source, a frequency between 100 kHz and 100 MHz should be selected, and for a liquid metal electron source, a frequency between 1 MHz and 1 GHz. In controlled operation, the voltage can be reduced below the operating voltage threshold Vthrd when the ion or electron current exceeds a predetermined current threshold or the current gradient rises above a predetermined gradient threshold. After the operating voltage VE has been reduced to the subthreshold voltage, the overthreshold voltage can be reapplied or the operating voltage VE increased again after a dead time to restart the cycle.

Claims

Liquid metal ion beam system (1) or liquid metal electron beam system, comprising: - a conductive emitter electrode (2), - a conductive extractor electrode (3) opposite the emitter electrode (2), - a liquid metal reservoir (4) fluidically connected to the emitter electrode (2) to transport liquid metal to the emitter electrode (2), - a control unit (5) configured to apply a periodically varying operating voltage between the emitter electrode (2) and the extractor electrode (3), wherein an operating voltage threshold is specified which defines a voltage level above which non-ionized or only slightly ionized liquid metal at the emitter electrode (2) is extracted by emitted ions or electrons accelerated towards the extractor electrode (3), wherein the control unit (5) is configured to periodically vary the operating voltage between a subthreshold voltage,to vary the voltage that is below the operating voltage threshold and the voltage that is above the operating voltage threshold. Liquid metal ion beam system (1) or liquid metal electron beam system according to claim 1, wherein the operating voltage is generated by a constant high voltage source (7) for providing a constant high voltage and a pulse voltage source (8) for providing a periodically varying pulse voltage, which are connected in series with the arrangement of the emitter electrode (2) and the extractor electrode (3), wherein the sum of the voltages of the constant high voltage source (7) and the pulse voltage source (8) gives the periodically varying operating voltage. Liquid metal ion beam system (1) or liquid metal electron beam system according to claim 2, wherein the pulse voltage source (8) is connected to a reference potential, in particular a ground potential. Liquid metal ion beam system (1) or liquid metal electron beam system according to one of claims 1 to 3, wherein the emitter electrode (2) is designed as a tip electrode or as a capillary electrode. Liquid metal ion beam system (1) or liquid metal electron beam system according to one of claims 1 to 4, wherein the control unit (5) is configured to adjust the periodically varying operating voltage by means of a regulated operation, wherein the operating voltage is reduced below the operating voltage threshold when the current flow through the arrangement of emitter electrode (2) and extractor electrode (3) exceeds a current above a predetermined current threshold and / or a current gradient increases above a predetermined gradient threshold, and is raised above the operating voltage threshold when the current flow through the arrangement of emitter electrode (2) and extractor electrode (3) falls below a current above a predetermined current threshold and / or a current gradient falls below a predetermined gradient threshold. Liquid metal ion beam system (1) according to one of claims 1 to 4, wherein the pulse frequency of the operating voltage is greater than 100 kHz and less than 100 MHz. Liquid metal electron beam system according to one of claims 1 to 4, wherein the pulse frequency of the operating voltage is greater than 1 MHz and less than 1 GHz. Method for operating a liquid metal ion beam system (1) or liquid metal electron beam system, comprising: - a conductive emitter electrode (2), - a conductive extractor electrode (3) opposite the emitter electrode (2), - a liquid metal reservoir (4) fluidically connected to the emitter electrode to transport liquid metal to the emitter electrode (2), wherein a periodically varying operating voltage is applied between the emitter electrode (2) and the extractor electrode (3), wherein an operating voltage threshold is specified which defines a voltage magnitude above which liquid metal that is not or only slightly ionized at the emitter electrode (2) is extracted by emitted ions or electrons accelerated towards the extractor electrode (3), wherein the periodically varying operating voltage ranges between a subthreshold voltage, which is less than the operating voltage threshold in magnitude, and a suprathreshold voltage.the value which is above the operating voltage threshold is varied. System comprising a liquid metal ion beam system (1) according to one of claims 1 to 6 and a liquid metal electron beam system according to one of claims 1 to 5 and 7, wherein the operating voltages are referenced to the same reference potential. System according to claim 9, wherein the liquid metal ion beam system (1) and the liquid metal electron beam system are operated with identical pulse frequencies, in particular in phase shift, to reduce alternating load on an electrical energy storage device.

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