Ion source

Abstract

The present invention relates to an ion source (1), in particular an ion thruster for propelling a spacecraft, comprising a reservoir (2) for a propellant (3), a heater (4) for heating the propellant (3), an emitter (6) having one or more projections (7) for emitting ions (3') of the propellant (3), wherein the one or more projections (7) are in fluid communication with the reservoir (2), an extractor (8) facing the emitter (6) for extracting ions (3') of the propellant (3) from the emitter (6) and for accelerating the extracted ions (3') in a direction of emission (R), wherein the heater (4) comprises a projection heating unit (5) and is configured to liquefy solid propellant (3) at the one or more projections (7) by the projection heating unit (5) before the solid propellant (3) in the reservoir (2) is liquefied.

Description

[0001] Ion Source

[0002] The present invention relates to an ion source, in partic¬ ular an ion thruster for propelling a spacecraft, comprising a reservoir for a propellant that has a solid state and a liquid state and can be liquefied from the solid state into the liquid state in which the propellant has a lower density than in the solid state, a heater for heating the propellant, an emitter having one or more proj ections for emitting ions of the propel¬ lant when the propellant is in its liquid state, wherein the one or more proj ections are in fluid communication with the reservoir, and an extractor facing the emitter for extracting ions of the propellant from the emitter when the propellant is in its liquid state and for accelerating the extracted ions in a direction of emission.

[0003] Ion sources are used, e . g. , for ion implantation or for creating focussed ion beams in semiconductor industry, in metal finishing, in material science and / or analysis, or in ion thrusters for the propulsion of spacecraft . In a liquid metal ion source ("LMIS") , the propellant is a metal (usually caesi¬ um, indium, mercury, etc) . In so-called colloid or electrospray ion sources, the propellant is typically a molten salt or the like . In either case, the propellant is usually heated to liq¬ uefy into its liquid state in the reservoir by the heater and received therefrom by the emitter . From the emitter, ions of the liquefied propellant are electrically extracted and accel¬ erated by the extractor to form a directed beam of ions, which in case of an ion thruster provides the thrust .

[0004] To achieve a strong electric field between the emitter and the extractor, which is necessary for ion extraction, the emit¬ ter'' s one or more proj ections are typically in the shape of cones, pyramids, triangular prisms, ridges, blades, needles or the like, that are sharp-tipped or sharp-edged to utilize the field-concentrating effect of the tip or edge . Applying the electric field to such a proj ection causes the formation of a so-called Taylor cone on top of the tip or edge of each of the emitter'' s proj ections, which further enhances the field-concen¬ trating effect .

[0005] For transporting liquid propellant from the reservoir to each proj ection of the emitter, passive forces, like capillary effects produced by capillary ducts penetrating the emitter (as described, e . g. , in AT 500 412 Al or US 4 328 667 B) or by a porous emitter (as described, e . g. , in US 2016 / 0297549 Al or EP 3 724 497 Al ) and / or by adhesion effects on the wetting surface of the emitter' s proj ections (as described, e . g. , in US 2009 / 114838 Al or US 2011 / 192968 Al ) , are usually employed in LMIS . However, the distance over which the liquid metal can be transported by these passive forces is limited by, i . a . , in¬ sufficient adhesive forces between the liquid propellant and the emitter or excessive cohesive forces within the propellant .

[0006] Under certain conditions, e . g. , when the ion emission and the heating of the reservoir are paused, the propellant will solidify, i . e . freeze, and, having a higher density in its sol¬ id state compared to its liquid state, will contract in volume . Hence, upon solidification the propellant retracts from the proj ections of the emitter due to contraction forces in the reservoir having the larger propellant volume and cohesive forces within the propellant . In such cases of retraction of propellant, the above-mentioned passive forces might not be sufficient to restart the propellant supply to the emitter when the ion source is reheated. As a result, the ion source may de¬ grade after having been paused. Countermeasures such as apply¬ ing external forces, e . g. pressurising the reservoir from a separate pressure reservoir or by means of mechanical pumps or pistons, are often not desirable for safety, reliability and complexity reasons, particularly in spacecraft .

[0007] It is an obj ect of the present invention to provide a safe, reliable and efficient liquid metal ion source .

[0008] This obj ect is achieved with an ion source specified at the outset, which is distinguished in that the heater comprises a proj ection heating unit for feeding energy into the one or more proj ections, wherein the heater is configured to, when the propellant is in its solid state both in the reservoir and at the one or more projections, liquefy the propellant at the one or more proj ections by the proj ection heating unit before the propellant in the reservoir is partially of fully liquefied.

[0009] By melting the propellant at the one or more proj ections first, the liquefying propellant the volume of which increases due to the lower density can only expand towards the respective tops of the one or more proj ections and compensates for the amount of propellant that has retracted therefrom during previ¬ ous solidification. An expansion towards the reservoir is blocked by the still solid propellant . After at least some of the propellant inside the reservoir is also liquefied, the above-mentioned passive forces suffice to restart the supply of liquid propellant to each proj ection of the emitter . Conse¬ quently, failures of the ion source due to a lack of liquid propellant at the one or more proj ections of the emitter can reliably be prevented. Hence, the present ion source is more reliable without requiring, e . g. , pressurised reservoirs and / or mechanical pumps or pistons for forced propellant supply and is, therefore, also safer .

[0010] The heater further comprises a reservoir heating unit for feeding energy into the reservoir to heat the propellant in the reservoir . The heater and, thus, the ion source comprises two distinct heating units, i . e . , the reservoir heating unit for heating the propellant in the reservoir and the proj ection heating unit for heating the propellant at the one or more pro¬ j ections . While the reservoir heating unit may initially pre¬ heat the solid propellant in the reservoir to a temperature be¬ low its melting point, i . e . , where the propellant still remains in its solid state, the proj ection heating unit is configured to heat the proj ections to a temperature above the melting point of the propellant and thereby liquefy the propellant at the one or more projections, i . e . on and / or in the proj ec- tion / s . Thereafter, the reservoir heating unit can start or support liquefying the propellant in the reservoir .

[0011] In a first advantageous embodiment, the proj ection heating unit comprises one or more heating wires for feeding the energy into the one or more proj ections by irradiating them with ther¬ mal radiation. Heating wires are a particularly straightforward means of generating thermal radiation and, due to their flexi¬ ble nature, can be adapted to any geometry of the emitter . Moreover, the thermal radiation fed into the one or more pro¬ j ections is proportional to an electrical power supplied to the heating wires . Hence, by adjusting the electrical power the rate of liquefaction and, thus, the ratio of liquefied propel¬ lant to solid propellant can easily be controlled.

[0012] In a first favourable variant of this embodiment, the pro¬ j ection heating unit further comprises an annular concave mir¬ ror surrounding the one or more proj ections, in the concavity of which the one or more heating wires are arranged for concen¬ trating the thermal radiation onto the one or more proj ections . By concentrating the thermal radiation, a higher percentage thereof irradiates the one or more proj ections, which allows for lower heat transfer latency and higher energy efficiency of the proj ection heating unit . The annular mirror is particularly effective when the emitter has a single proj ection or is crown¬ shaped having an annular base from which the one or more pro¬ j ections protrude in axial direction.

[0013] While the shape of the annular concave mirror can be cho¬ sen from a wide variety, it is particularly favourable when the concavity of the annular concave mirror is elliptical in cross section, such that the annular concave mirror has a first and a second circular focal line, along one of which the one or more heating wires are arranged and along the other one of which the one or more proj ections are arranged. The annular mirror with a concavity of elliptical axial cross section, i . e . , in the form of segment of an ellipse, creates a first and a second sequence of focal points along the first and the second focal line, re- spectively. The elliptical concavity allows for a more accurate concentration of the thermal radiation onto the one or more proj ections . Due to the focused concentration, higher local temperatures can be attained at the one or more proj ections and the rate of local liquefaction and the efficiency and control¬ lability of the liquefaction are further increased.

[0014] In an additional or alternative second favourable variant of the first embodiment, the one or more heating wires are sub¬ stantially point-shaped and the proj ection heating unit further comprises one or more cup-shaped mirrors, in the cup of each of which a respective one of the one or more heating wires is ar¬ ranged for concentrating the thermal radiation onto a respec¬ tive one of the one or more proj ections . Thereby, an even high¬ er share of the thermal energy emitted by the respective heat¬ ing wire is two-dimensionally concentrated onto the respective proj ection. This results in an even lower heat transfer latency and a further enhanced energy efficiency of the proj ection heating unit compared to the annular concave mirror. Moreover, individual cup-shaped mirrors can adapt to a great variety of emitter structures, e . g. , wherein the emitter base is polyhe¬ dral and the one or more proj ections are arranged in a line or an array and are needle- or ridge-shaped or have any other shape . Thereby ensuring uniform heating in a particularly effi¬ cient way.

[0015] While the shape of the cup / s of the one or more cup-shaped mirrors can be chosen from a wide variety, it is particularly favourable when the one or more proj ections are needle-shaped and each one of the one or more cup-shaped mirrors is an ellip¬ soidal mirror and has a first and a second focal point, wherein at each first focal point a respective one of the one or more heating wires is arranged and at each second focal point a re¬ spective one of the one or more needle-shaped proj ections is arranged. By using ellipsoidal mirrors which concentrate most of the thermal radiation of each heating wire to a respective single point, i . e . , the respective focal point, it can be pre- cisely chosen where the thermal energy is fed into each respec¬ tive proj ection. Hence, the rate and direction of the melting front can be predetermined specifically for each proj ection. This also allows for emitters of any form and proj ection ar¬ rangement .

[0016] In a third favourable variant of the first embodiment, the extractor additionally or alternatively has an annular groove surrounding the one or more proj ections, in which groove the one or more heating wires are arranged for concentrating the thermal radiation onto the one or more proj ections . In this variant, the thermal radiation does not only heat the one or more proj ections but also the extractor to a considerable ex¬ tent . Some of the extracted ions (or possible weakly charged droplets of undesirably sprayed liquid propellant) may deposit on the extractor forming deposits thereon. The deposits are prone to accretion towards the emitter (known as "clogging") eventually resulting in a short-circuit between extractor and emitter leading to a complete failure of the ion source . By heating the extractor, the effects of such deposits can be mit¬ igated by melting them and, e . g. , wicking them away from the emitter .

[0017] In a modification of the aforementioned variant, the one or more heating wires are substantially point-shaped and the extractor has one or more recesses in each of which a respec¬ tive one of the one or more heating wires is arranged for con¬ centrating the thermal radiation onto the one or more proj ec¬ tions . Thereby, the one or more heating wires can be arranged in a way corresponding to the emitter'' s and / or extractor' s ge¬ ometry to facilitate uniform heating in particular in case of non-circular proj ection arrangement, e . g. , when the emitter base is polyhedral and the one or more proj ections are arranged in a line or an array.

[0018] In a further beneficial variant, the extractor itself is one of said one or more heating wires . Thereby, no separate heating wires are required, which constitutes a particularly simple and space-saving variant . Moreover further heating wires may be applied when necessary.

[0019] In a second advantageous embodiment, the proj ection heat¬ ing unit additionally or alternatively comprises an annular parabolic mirror for feeding the energy into the one or more proj ections by concentrating essentially parallel rays of inci¬ dent thermal radiation onto the one or more proj ections . In this case the incident thermal radiation can be provided by an external source, e . g. the sun, minimising the energy consump¬ tion of the proj ection heating unit . This is particularly bene¬ ficial when the ion source is a thruster for a spacecraft the energy resources of which are limited.

[0020] In a modification of the second advantageous embodiment, the one or more proj ections are needle-shaped and the proj ec¬ tion heating unit comprises one or more cup-shaped parabolic mirrors for feeding the energy into the one or more proj ections by concentrating essentially parallel rays of incident thermal radiation onto one of the one or more proj ections, respective¬ ly. Using individual cup-shaped parabolic mirrors for concen¬ trating, e . g. , sunlight, most of the incident thermal radiation is two-dimensionally concentrated onto the one or more needle- shaped proj ections such that higher local temperatures can be attained compared to the annular parabolic mirror . Moreover, individual cup-shaped parabolic mirrors can be adapted to a great variety of emitter structures, e . g. , when the emitter base is polyhedral and the one or more proj ections are arranged in a line or an array, thereby facilitating uniform heating in a particularly efficient way.

[0021] In any of the abovementioned embodiments, the proj ection heating unit is advantageously offset from the emitter opposite to the direction of emission. This allows the proj ection heat¬ ing unit to be space-savingly mounted, in particular around a bottle neck of the reservoir when it is bottle-shaped. Alterna¬ tively, the proj ection heating unit is beneficially offset from the emitter in the direction of emission. This increases the flexibility in designing the ion source, in particular when the reservoir is a cylinder or a cuboid. When the emitter and the proj ection heating unit are each rotationally-symmetric about a central axis the proj ection heating unit may then be axially offset along this axis opposite to or in the direction of emis¬ sion .

[0022] The ion source may optionally comprise a focusing elec¬ trode which decreases divergence of the accelerated ions, thereby producing a more focused ion beam emission. Particular¬ ly in this case and when the proj ection heating unit is offset from the emitter in the direction of emission, it is beneficial when the focusing electrode is arranged between the proj ection heating unit and the one or more proj ections and is transmis¬ sive to the thermal radiation. Hence, both the proj ection heat¬ ing unit and the focusing electrode can be positioned closer to the central axis and / or the proj ection heating unit closer to the emitter proj ections, thereby allowing for a particularly compact ion source design.

[0023] In a third advantageous embodiment, the proj ection heating unit additionally or alternatively comprises one or more laser sources, each of which is directed at a respective one of the one or more proj ections for feeding the energy into the one or more proj ections by irradiating them with laser light . Laser light is a high energy radiation beam of very small width of, e . g. , a few micrometres . Hence, the laser sources, e . g. laser diodes, can be mounted rather distant from the emitter projec¬ tions which increases the flexibility in designing the ion source . Moreover, the wavelength of the laser light can be spe¬ cifically chosen to exploit the absorption profiles of the pro¬ pellant and / or the emitter proj ections . Thereby, the proj ection heating unit can selectively heat the propellant and / or the emitter and is particularly energy efficient .

[0024] In an additional or alternative fourth advantageous embod¬ iment, the one or more proj ections and / or the propellant are electrically conductive, the proj ection heating unit comprises an AC-current generator and, connected thereto, one or more electromagnetic coils, each of which surrounds at least one of the one or more proj ections for feeding the energy into the one or more proj ections by electromagnetic induction. Thereby, the propellant can be liquefied energy efficiently and under low latency .

[0025] In a further additional or alternative fifth advantageous embodiment, the proj ection heating unit comprises an AC-current generator and, connected thereto, an electromagnetic coil adj a¬ cent to the extractor to heat the extractor by electromagnetic induction, for feeding the energy into the one or more proj ec¬ tions by irradiating them with thermal radiation from the heat¬ ed extractor .

[0026] The invention shall now be described in detail on the ba¬ sis of exemplary embodiments thereof with reference to the ac¬ companying drawings, in which:

[0027] Fig. 1 shows a first embodiment of an ion source according to the invention in a longitudinal section;

[0028] Fig. 2 shows a variant of an emitter and a proj ection heating unit of the embodiment of the ion source of Fig. 1 in a top view;

[0029] Figs . 3a, 3b, 4a, 4b, 5a, 5b, 6 and 7 show further exem¬ plary variants and embodiments of the ion source according to the invention, each in a fragmentary longitudinal section.

[0030] Fig. 1 shows an ion source 1 which comprises a reservoir 2 for a propellant 3. The propellant 3 has a solid state and can be liquefied from its solid state into a liquid state by heat¬ ing and, of course, solidifies from its liquid state into its solid state when cooling. In the liquid state, the propellant 3 has a lower density than in its solid state . In order to lique¬ fy the propellant 3, the ion source 1 comprises a heater 4. The heater 4 comprises a proj ection heating unit 5 and a reservoir heating unit 5 ’ which, when activated, heat and liquefy the propellant 3 as will be described in detail further down. The ion source 1 also has an emitter 6 with one or more proj ections 7 for emitting ions 3 ’ of the propellant 3 when the propellant 3 is in its liquid state . The one or more proj ec¬ tions 7 are in fluid communication with the reservoir 2 and are thereby receiving liquefied propellant 3 from the reservoir 2 during operation of the ion source 1.

[0031] For extracting ions 3 ’ of the propellant 3 from the emit¬ ter 6 when the propellant 3 is in its liquid state and for ac¬ celerating the extracted ions 3 ’ in a direction of emission R, the ion source 1 comprises an extractor 8 which faces the emit¬ ter 6. A strong electric field is applied to the emitter 6 and the extractor 8 by connecting a voltage source of, e . g. , a few kilovolts (kV) .

[0032] In the example of Fig. 1, the emitter 6 is crown-shaped, having an annular base 9 (Fig. 3a) that is mounted on a rod 10 extending through the reservoir 2. From the annular base 9 of the emitter 6 one or more (here : seven visible needle-shaped) proj ections 7 protrude on that side of the emitter 6 directing away from the reservoir 2. Each proj ection 7 has the shape of a cone, a pyramid, a triangular prism, a ridge, a blade, (in this example) a needle, or the like and has a sharp tip or edge . In other embodiments, the emitter 6 may be directly connected to the reservoir 2 without the support of the rod 10 and / or have a different shape, e . g. , without a base 9, with a base 9 that is polyhedral and / or one or more proj ections 7 that are arranged thereon in a line or an array etc .

[0033] Applying the strong electric field to the sharp tip or edge of each proj ection 7 causes the formation of a so-called Taylor cone on top of each proj ection 7. At the apex of the Taylor cone, the ions 3 ’ are extracted and accelerated by the strong electric field between the extractor 8 and the emitter 6, thereby creating an ion beam 11 that is propagating in the direction of emission R away from the emitter 6 and - when the extractor 8 is mounted, e . g. , in a housing 12 of the ion source 1 as in this example - away from the ion source 1. The ion source 1 can be used, e . g. , for ion implantation or for creating focused ion beams in semi-conductor industry, in metal finishing, in material science and / or analysis, and particularly as an ion thruster for propelling spacecraft . Moreover, the ion source 1 may comprise or be connected to fur¬ ther elements, e . g. a controller, a power supply, the voltage source, a neutralizer etc . , as known in the art .

[0034] The propellant 3 in the embodiment of Fig. 1 is a metal, e . g. caesium, indium, mercury, etc . In such a "liquid metal ion source" ("LMIS") , neutral atoms of the metal propellant 3, when in its liquid state, f ield-evaporate at the apex of the Taylor cone and negative electrons tunnel back to the surface, leaving positively charged ions 3 ’ of the liquid metal propellant 3 for extraction. Hence, the propellant 3 is both ionised, extracted and accelerated by one and the same electric field between the extractor 8 and the emitter 6. In ion sources 1 of the "col¬ loid" or "electrospray" type, on the other hand, the propellant 3 is, e . g. , a salt which is ionized in its liquefied state such that positively or negatively charged ions are extracted from the emitter 6 and accelerated by the electric field.

[0035] For receiving the liquefied propellant 3 from the reser¬ voir 2 at the emitter 6 and transporting it to the one or more proj ections 7, passive forces (here : capillary effects) are used. To this end, the emitter 6 is, e . g. , penetrated by capil¬ lary ducts (not shown) or (in the present example) made of po¬ rous material . The proj ections 7 are likewise penetrated by ca¬ pillary ducts or made of porous material and / or they have a wetting surface to which the liquefied propellant 3 adheres .

[0036] When operation of the ion source 1 is not required, heat¬ ing of the reservoir 2 and emission of ions 3 ’ may be paused. The propellant 3 then freezes, i . e . solidifies, both at the emitter 6 and in the reservoir 2. During the solidification, the propellant 3 retracts from the emitter 6 towards the reser¬ voir 2 due to its higher density in the solid state, due to strong contraction forces inside the reservoir 2 having the larger propellant volume and due to cohesive forces within the propellant 3. However, excessive retraction of propellant 3 from the one or more proj ections 7 impedes a subsequent for¬ mation of a Taylor cone when restarting the ion source 1. In order to compensate for such retraction when heating the pro¬ pellant 3 from its solid state upon restarting, the heater 4 is configured to liquefy the propellant 3 by the proj ection heat¬ ing unit 5 at the one or more proj ections 7 before the propel¬ lant 3 in the reservoir 2 is liquefied. To this end, the pro¬ j ection heating unit 5 feeds energy into the one or more pro¬ j ections 7 to heat the propellant 3 in the one or more proj ec¬ tions 7 .

[0037] In the example of Fig. 1, the heater 4 further comprises a reservoir heating unit 5 ’ for feeding energy into the reservoir 2, thereby supporting the heating of the propellant 3 in the reservoir 2. The reservoir heating unit 5 ’ in this example is a resistive heating unit abutting against the outer walls of the reservoir 2 to feed energy (here : in the form of heat) into the reservoir 2 (here : by conduction) . Alternatively, the reservoir heating unit 5 ’ may be spaced apart from and irradiate the res¬ ervoir 2 or may be a different type of heating unit, e . g. , an inductive heating unit, a laser source or the like .

[0038] By additionally feeding energy into the reservoir 2 when the propellant 3 is in its solid state, the propellant 3 in the reservoir 2 is heated, e . g. , to a predefined temperature which is initially below the melting point of the propellant 3. By thermal conduction also the emitter 6 with its one or more pro¬ j ections 7 and the propellant 3 therein or thereon are heated. However, the heater 4 will always liquefy the propellant 3 at the one or more projections 7 prior to the propellant 3 in the reservoir 2 by means of the proj ection heating unit 5, e . g. as exemplified below.

[0039] In the embodiment shown in Fig. 1, the proj ection heating unit 5 comprises one or more heating wires (here : one annular heating wire) 13, which optionally surround the emitter 6 as shown, for feeding the energy into the one or more proj ections 7 (here : seven visible needle-shaped proj ections 7 ) . One or more heating wires 13 may additionally or alternatively be po¬ sitioned between the proj ections 7, in particular in a central gap of the emitter 6 when it is crown-shaped or has different shape with one or more gap / s between several proj ections 7. When activated, the heating wires 13 generate thermal radiation 14 irradiating the proj ection / s 7, such that the temperature at the proj ection / s 7 rises . When surpassing the melting point, the propellant 3 at the one or more proj ections 7, i . e . the propellant 3 in and / or on the proj ection / s 7, is liquefied without the propellant 3 in the reservoir 2 melting yet . During liquefaction, the volume of the propellant 3 expands due to its lower density in its liquid state compared to its solid state . An expansion towards the reservoir 2 is blocked by propellant 3 that is still solid (here : both in the emitter base 9 and in the reservoir 2 ) . Hence, the liquefied propellant 3 can only expand towards the top / s of the proj ection / s 7 compensating for propellant 3 previously retracted therefrom during the solidi¬ fication .

[0040] With increasing energy (here : the thermal radiation 14 ) fed into the proj ection / s 7, the melting front of propellant 3 gradually moves towards the reservoir 2 and an increasing amount of the propellant 3 liquefies . This provides further ex¬ pansion of liquefied propellant 3 towards the proj ection / s 7. After a while, when sufficient liquid propellant 3 has expanded to the proj ection / s 7, the propellant 3 at the rest of the emitter 6 and in the reservoir 2 is liquefied by means of the proj ection heating unit 5 and / or the reservoir heating unit 5 ’ . Eventually, all of the propellant 3 is liquefied both at the emitter 6 and in the reservoir 2. When sufficient (or all) of the propellant 3 in the reservoir 2 is liquefied, the extrac¬ tion and acceleration of the ions 3 ’ may be started and the proj ection heating unit 5 may optionally be deactivated while the reservoir heating unit 5 ’ keeps all the propellant 3 in its liquid state as required. This procedure of heating is per¬ formed with any type or combination of proj ection heating unit / s 5 described below.

[0041] The heating wires 13 can have a great variety of shapes, e . g. , be long wires that meander between or surround the one or more proj ections 7 in a zigzag or circular (Fig. 1) shape . As shown in the example of Fig. 1, the proj ection heating unit 5 optionally comprises an annular concave mirror 15, e . g. , a ring having a mirror 15 at its inner face, the mirror 15 being both curved in the circumferential direction of the ring surrounding the proj ection / s 7 and concave in the axial direction of the ring forming a concavity 16. In the concavity 16, the one or more heating wires (here : one heating wire) 13 is / are arranged. In this arrangement, the concavity 16 of the annular concave mirror 15 at least to some extent concentrates the thermal ra¬ diation 14 of the heating wire / s 13 onto the proj ection / s 7.

[0042] When viewed in the axial cross section illustrated in Fig. 1 the concavity 16, i . e . its line of intersection, may have a variety of shapes, e . g. , be angular, polygonal (not shown) or curved (Fig. 1 ) . Optionally, the concavity 16 of the annular concave mirror 15 is elliptical in axial cross section, 1. e . , a segment of an ellipse . Similar to an ellipse that has two focal points, the annular concave mirror 15 with an ellip¬ tical axial cross section has two focal lines created by re¬ spective first and second sequences of focal points . By arrang¬ ing the one or more heating wires 13 along one of the focal lines and the one or more proj ections 7 along the other focal line, the thermal radiation 14 is concentrated onto the one or more proj ections 7. When the emitter 6 has only one needle- shaped proj ection 7, said other focal line may optionally coin¬ cide to a single point on this proj ection 7.

[0043] In a second variant of the first embodiment, shown in Fig.

[0044] 2, the proj ection heating unit 5 additionally or alternatively comprises one or more, in particular two or more, separate cup¬ shaped, i . e . two-dimensionally concave, mirrors 17. In the cup 18, i . e . in the two-dimensional concavity, of each of the cup¬ shaped mirrors 17 of this embodiment, a respective one of the one or more heating wires 13 is arranged such that the thermal radiation 14 is concentrated two-dimensionally onto the one or more proj ections 7 . To this end, the one or more heating wires 13 are substantially point-shaped. The cup-shaped mirrors 17, when more than one, may be spaced apart from each other (not shown) or, as shown in this example, joined together forming a segmented mirror (here : a ring of cup-shaped mirror segments) .

[0045] The cups 18 of the one or more cup-shaped mirrors 17 may have a variety of shapes when seen in any cross section, e . g. , be angular or curved. Optionally, each one of the cup-shaped mirrors 17 is an ellipsoidal mirror, i . e . a segment of an el¬ lipsoid, such that it has a first and a second focal point . When arranging a respective one of the one or more heating wires 13 at each first focal point and a respective one of the one or more proj ections 7, which in this case may be needle- shaped, at each second focal point, the thermal radiation 14 of the one or more heating wires 13 is concentrated to the respec¬ tive one of the one or more proj ections 7.

[0046] Figs . 3a and 3b show two subvariants of a third variant of the first embodiment, wherein in each of the subvariants the extractor 8 has an annular groove 19 surrounding the one or more proj ections 7. In the annular groove 19, the one or more heating wires (here : one heating wire) 13 is / are arranged, such that the thermal radiation 14 is to some extent concentrated onto the one or more proj ections 7. To this end, the annular groove 19 may have a variety of shapes, e . g. , when viewed in the axial cross section illustrated in Fig. 3a and 3b, be angu¬ lar (not shown) , polygonal (Fig. 3a) or curved (Fig. 3b) .

[0047] In a modification of the third variant, the extractor 8 has one more recesses instead of the annular groove 19 shown in Figs . 3a and 3b . In each recess, a respective one of the one or more heating wires 13 is arranged such that the thermal radia¬ tion 14 is to some extend concentrated onto the one or more proj ections 7. The one or more heating wires 13 are substan¬ tially point-shaped and the respective opening of each of the one or more recesses is directed towards the one more proj ec¬ tions 7 .

[0048] Within the framework of the present disclosures, "point¬ shaped" denotes a heating wire 13 that is short or wound such that its envelope' s diameter is substantially smaller than the one or more cup-shaped mirrors 17 of the example shown in Fig.

[0049] 2 or than the one or more recesses, respectively, e . g. , rather in the size of the diameter of the one or more proj ections 7.

[0050] In a further variant which is not shown, the extractor 8 itself is one of said one or more heating wires 13, which is synonymous to one of the one or more heating wired 13 forming the extractor 8 .

[0051] Figs . 4a and 4b show two modifications of a second embodi¬ ment, in each of which modifications the proj ection heating unit 5 comprises an annular parabolic mirror 20 which concen¬ trates essentially parallel rays of incident thermal radiation 21 onto the one or more proj ections 7, thereby feeding the en¬ ergy into the one or more proj ections 7. The source of the par¬ allel rays of incident thermal radiation 21 may either be an internal source, e . g. , a thermal radiator 22 having a wide beam of thermal radiation 21 (Fig. 4a) or an external source (Fig.

[0052] 4b) , in particular the sun, in which latter case the parabolic mirror 20 actually forms the proj ection heating unit 5. When the incident thermal radiation 21 originates from an external source either the extractor 8 or the housing 12 (here : the ex¬ tractor 8 ) may have one or more apertures 23 allowing for a passage of the incident thermal radiation 21.

[0053] Instead of or in addition to the single annular parabolic mirror 20 shown in Fig. 4a and 4b, the proj ection heating unit 5 may comprise one or more separate cup-shaped parabolic mir¬ rors (not shown) , which feed the energy into the one or more proj ections 7 by concentrating the essentially parallel rays of incident thermal radiation 21 onto the one or more proj ections 7 which, in this variant, are optionally needle-shaped. The one or more cup-shaped parabolic mirrors can be either spaced apart from each other or joined together to form a ring of mirror segments .

[0054] In any of the aforementioned embodiments, the mirrors 15, 17, 20, 21 may optionally be formed by respective Fresnel re¬ flectors .

[0055] In each of the examples shown, the proj ection heating unit 5 and the emitter 6 are each rotationally-symmetric about a central axis A (Fig. 1) . However, the proj ection heating unit 5 and / or the emitter 6 may be of different shape and / or differ¬ ently arranged, e . g. , be linear or square when seen from the top etc . or have an irregular shape . Moreover, in each of the examples of Figs . 1, 2, 3a, 3b, 4a and 4b, the proj ection heat¬ ing unit 5 is offset from the emitter 6 in a direction opposite to the direction of emission R, i . e . , in the views of Figs . 4a and 4b below the tops of the proj ections 7.

[0056] In contrast, Figs . 5a and 5b show examples in which the proj ection heating unit 5 is offset from the emitter 6 in the direction of emission R. In this example, the proj ection heat¬ ing unit 5 and the emitter 6 are also each rotationally symmet¬ ric about the axis A.

[0057] The ion sources 1 shown in the examples of Figs . 5a and 5b additionally comprise optional focusing electrodes 24, 25, which, in these exemplary cases, are arranged in axial direc¬ tion between the projection heating unit 5 and the one or more proj ections 7. Focusing electrodes 24, 25 are electrodes used to decrease the divergence of the ion beam 11 as known in the art, thereby "focusing" the accelerated ions 3 ’ . Moreover, the focusing electrodes 24, 25 may be composed in different ways .

[0058] In the variant of Fig. 5a, the focusing electrode 24 is solid and the thermal radiation 14 passes by, i . e . the proj ec¬ tion heating unit 5 is arranged such that the traj ectory of the thermal radiation 14 passes by the focusing electrode 24. In the alternative variant of Fig. 5b, on the other hand, the fo- cusing electrode 25 is composed of an electrically chargeable wire mesh or the like that is at least largely transmissive to thermal radiation 14, such that the proj ection heating unit 5 can be arranged closer to the one or more proj ections 7 as the thermal radiation 14 passes through the focusing electrode 25, i . e . , through the gaps in the wire mesh. In this variant, a longitudinal extension H of the arrangement of the focusing electrode 25 and the proj ection heating unit 5 can be substan¬ tially reduced compared to the variant shown in Fig. 5a .

[0059] In a third embodiment shown in Fig. 6, the proj ection heating unit 5 additionally or alternatively comprises one or more laser sources (here : one laser source) 26. Each one of which is directed at a respective one of the one or more pro¬ j ections 7 (here : one proj ection 7 ) for feeding the energy into the one or more projections 7 by irradiating them with laser light 27. Alternatively, several laser sources 26 may be di¬ rected at a single proj ection 7 or rather the laser light 27 of a single laser source 26 may be split by means of beam split¬ ters and / or diffractive elements and directed at multiple pro¬ j ections 7.

[0060] In a fourth embodiment according to Fig. 7, the proj ection heating unit 5 comprises an AC current generator 28 and, con¬ nected thereto, one or more electromagnetic coils 29. Each electromagnetic coil (here : one electromagnetic coil) 29 has one or more windings and surrounds at least one of the one or more proj ections 7 (here : one proj ection 7 ) for feeding the en¬ ergy into the one or more proj ections 7 by electromagnetic in¬ duction. To this end, the proj ection / s 7 and / or the propellant 3 are electrically conductive .

[0061] In a fifth advantageous embodiment which is not shown, the proj ection heating unit 5 comprises the AC-current generator and, connected thereto, an electromagnetic coil adj acent to the extractor 8 to heat the extractor 8 by electromagnetic induc¬ tion. Thereby, the energy is fed into the one or more proj ec- tions 7 by irradiating them with thermal radiation from the heated extractor 8 .

[0062] The invention is not restricted to the specific embodi¬ ments described in detail herein, but encompasses all variants, combinations and modifications thereof that fall within the framework of the appended claims, e . g. , by combining two or more of the described embodiments and variants of proj ection heating units 5 and their arrangements in a single ion source 1 .

Claims

Claims :1 . An ion source , in particular an ion thruster for pro¬ pelling a spacecraft , comprisinga reservoir ( 2 ) for a propellant ( 3 ) that has a solid state and a liquid state and can be liquefied from the solid state into the liquid state in which the propellant ( 3 ) has a lower density than in the solid state ,a heater ( 4 ) for heating the propellant ( 3 ) ,an emitter ( 6 ) having one or more proj ections ( 7 ) for emitting ions ( 3 ’ ) of the propellant ( 3 ) when the propellant ( 3 ) is in its liquid state , wherein the one or more proj ections ( 7 ) are in fluid communication with the reservoir ( 2 ) , and an extractor ( 8 ) facing the emitter ( 6 ) for extracting ions ( 3 ’ ) of the propellant ( 3 ) from the emitter ( 6 ) when the propellant ( 3 ) is in its liquid state and for accelerating the extracted ions ( 3 ’ ) in a direction of emission (R) , wherein the heater ( 4 ) comprises a reservoir heating unit ( 5 ’ ) for feeding energy into the reservoir ( 2 ) to heat the propellant ( 3 ) in the reservoir ( 2 ) and a proj ection heating unit ( 5 ) for feeding energy into the one or more proj ections ( 7 ) to heat the propellant ( 3 ) in the one or more proj ections ( 7 ) , characterised in thatthe heater ( 4 ) is configured to , when the propellant ( 3 ) is in its solid state both in the reservoir ( 2 ) and at the one or more proj ections ( 7 ) , liquefy the propellant ( 3 ) at the one or more proj ections ( 7 ) by the proj ection heating unit ( 5 ) be¬ fore the propellant ( 3 ) in the reservoir ( 2 ) is liquefied such that the liquefying propellant ( 3 ) can only expand towards the respective tops of the one or more proj ections ( 7 ) .2 . The ion source according to claim 1 , wherein the pro¬ j ection heating unit ( 5 ) comprises one or more heating wires ( 13 ) for feeding the energy into the one or more proj ections ( 7 ) by irradiating them with thermal radiation ( 14 ) .3 . The ion source according to claim 2 , wherein the pro¬ j ection heating unit ( 5 ) further comprises an annular concave mirror ( 15 ) surrounding the one or more proj ections ( 7 ) , in the concavity ( 16 ) of which the one or more heating wires ( 13 ) are arranged for concentrating the thermal radiation ( 14 ) onto the one or more proj ections ( 7 ) .4 . The ion source according to claim 3 , wherein the con¬ cavity ( 16 ) of the annular concave mirror ( 15 ) is elliptical in cross section, such that the annular concave mirror ( 15 ) has a first and a second circular focal line , along one of which the one or more heating wires ( 13 ) are arranged and along the other one of which the one or more proj ections ( 7 ) are arranged .5 . The ion source according to any one of claims 2 to 4 , wherein the one or more heating wires ( 13 ) are substantially point-shaped and the proj ection heating unit ( 5 ) further com¬ prises one or more cup-shaped mirrors ( 17 ) , in the cup ( 18 ) of each of which a respective one of the one or more heating wires ( 13 ) is arranged for concentrating the thermal radiation ( 14 ) onto a respective one of the one or more proj ections ( 7 ) .6 . The ion source according to claim 5 , wherein the one or more proj ections ( 7 ) are needle-shaped and each of the one or more cup-shaped mirrors ( 17 ) is an ellipsoidal mirror and has a first and a second focal point , wherein at each first fo¬ cal point a respective one of the one or more heating wires ( 13 ) is arranged and at each second focal point a respective one of the one or more needle-shaped proj ections ( 7 ) is ar¬ ranged .7 . The ion source according to any one of claims 2 to 6 , wherein the extractor ( 8 ) is one of said one or more heating wires ( 13 ) .8 . The ion source according to any one of claims 2 to 7 , wherein the extractor ( 8 ) has an annular groove ( 19 ) surround¬ ing the one or more proj ections ( 7 ) , in which groove ( 19 ) the one or more heating wires ( 13 ) are arranged for concentratingthe thermal radiation ( 14 ) onto the one or more proj ections ( 7 ) .9 . The ion source according to any one of claims 2 to 7 , wherein the one or more heating wires ( 13 ) are substantially point-shaped and the extractor ( 8 ) has one or more recesses in each of which a respective one of the one or more heating wires ( 13 ) is arranged for concentrating the thermal radiation ( 14 ) onto the one or more proj ections ( 7 ) .10 . The ion source according to any one of claims 1 to 9 , wherein the proj ection heating unit ( 5 ) comprises an annular parabolic mirror ( 20 ) for feeding the energy into the one or more proj ections ( 7 ) by concentrating essentially parallel rays of incident thermal radiation ( 21 ) onto the one or more proj ec¬ tions ( 7 ) .11 . The ion source according to any one of claims 1 to 9 , wherein the one or more proj ections ( 7 ) are needle-shaped and the proj ection heating unit ( 5 ) comprises one or more cup¬ shaped parabolic mirrors for feeding the energy into the one or more proj ections ( 7 ) by concentrating essentially parallel rays of incident thermal radiation ( 21 ) onto one of the one or more proj ections ( 7 ) , respectively .12 . The ion source according to any one of claims 2 to 11 , wherein the proj ection heating unit ( 5 ) is of fset from the emitter ( 6 ) opposite to the direction of emission (R) .13 . The ion source according to any one of claims 2 to 11 , wherein the proj ection heating unit ( 5 ) is of fset from the emitter ( 6 ) in the direction of emission (R) .14 . The ion source according to claim 13 , wherein the ion source ( 1 ) further comprises a focusing electrode ( 24 , 25 ) for focusing the accelerated ions ( 3 ’ ) , which focusing electrode ( 24 , 25 ) is arranged between the proj ection heating unit ( 5 ) and the one or more proj ections ( 7 ) and is transmissive to the thermal radiation ( 14 ) .15 . The ion source according to any one of claims 1 to 14 , wherein the proj ection heating unit ( 5 ) comprises one ormore laser sources ( 26 ) , each of which is directed at a respec¬ tive one of the one or more proj ections ( 7 ) for feeding the en¬ ergy into the one or more proj ections ( 7 ) by irradiating them with laser light ( 27 ) .16 . The ion source according to any one of claims 1 to 15 , wherein the one or more proj ections ( 7 ) and / or the propel¬ lant ( 3 ) are electrically conductive and the proj ection heating unit ( 5 ) comprises an AC-current generator ( 28 ) and, connected thereto , one or more electromagnetic coils ( 29 ) , each of which surrounds at least one of the one or more proj ections ( 7 ) for feeding the energy into the one or more proj ections ( 7 ) by electromagnetic induction .17 . The ion source according to any one of claims 1 to 16 , wherein the proj ection heating unit ( 5 ) comprises an AC- current generator and, connected thereto , an electromagnetic coil adj acent to the extractor ( 8 ) to heat the extractor ( 8 ) by electromagnetic induction, for feeding the energy into the one or more proj ections ( 7 ) by irradiating them with thermal radia¬ tion ( 14 ) from the heated extractor ( 8 ) .

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