Ion source and method for operating ion source

Abstract

Disclosed is an ion source, in particular an ion propeller for propelling a spacecraft, comprising: a reservoir (2) for a liquid metal (3); an emitter (4) having a central axis (C) and in fluid communication with the reservoir (2) for emitting ions (5) of the liquid metal (3) drawn from the reservoir (2); an annular extractor (6) coaxial with the emitter (4) and facing the emitter (4) for accelerating the separation of the ions (5); a voltage source (7) for effecting the emission and acceleration; and a chassis (10) on which the reservoir (2), the emitter (4) and the extractor (6) are mounted, wherein the extractor (6) has a structure (14) on a side (15) facing the emitter (4) configured to absorb the liquid metal (3) by capillary forces. Also disclosed is a method (21) for operating an ion source (1).

Description

Technical Field

[0001] The present invention relates to an ion source, in particular to an ion thruster for propelling a spacecraft, and a method for operating an ion source. The ion source includes a reservoir for liquid metal; an emitter having a central axis and fluidly connected to the reservoir for extracting liquid metal from the reservoir and emitting ions of the liquid metal; an annular extractor coaxial with the emitter and facing the emitter for accelerating the ions away from the emitter; a voltage source electrically connected to the emitter and the extractor for effecting said emission and acceleration; and a chassis mounting the reservoir, the emitter, and the extractor. Background Art

[0002] Ion sources are used, for example, for ion implantation or for generating focused ion beams in the semiconductor industry, in metal processing, in materials science and / or analysis, or as an ion thruster for propelling a spacecraft. In an ion source that emits liquid metal and is also referred to as a "liquid metal ion source" ("LMIS"), the liquid metal (usually cesium, indium, gallium, mercury, or bismuth) is heated to a liquid state in a reservoir and is drawn out of the reservoir by an emitter, where ions of the liquid metal are extracted electrically from the emitter and accelerated by an extractor to form a directed metal ion beam. The emitter of an LMIS has one or more emission sites, typically protrusions in the shape of a cone, pyramid, triangular prism, etc. To achieve a high electric field strength at the emission site, which is the basis for ion emission, these protrusions usually have sharp tips or sharp edges to utilize the field concentration effect at the tip or edge. Applying an electric field to such a sharp tip or edge causes the formation of a so-called Taylor cone on top of the tip or edge of the emitter protrusion; due to field emission negative electrons tunneling back to the surface to change the originally neutral atoms into positively charged ions, neutral atoms of the liquid metal at the apex of the Taylor cone evaporate from the surface. The ions thus generated are accelerated by the electric field.

[0003] This principle of generating positive ions and accelerating them by the same electric field is used to generate a directed ion beam and to generate thrust in an ion thruster. Such an ion thruster is therefore also referred to as a field emission electric propulsion (FEEP) system. The ion beam and, where applicable, the thrust can be precisely controlled by the electric field strength. Since moving parts are avoided, LMIS is beneficial due to its low complexity, low weight, and its high reliability, durability, and efficiency.

[0004] To transport the liquid metal from the reservoir to the sharp tip or edge of each protrusion of the emitter, driving forces, such as capillary effects generated by capillary channels penetrating the emitter or by a porous emitter and / or adhesion effects on the wetted surface of the protrusions of the emitter, are typically used in an LMIS.

[0005] In addition to the directed ion beam, ion sources of the above type also produce an unwanted, substantially omnidirectional spray of neutral or weakly charged droplets of liquid metal during operation. Although the spray is insignificant for the ion source operation itself, the droplets gradually deposit on the surface surrounding the emitter. Since the extraction electrode faces the emitter and attracts positively charged particles, the extractor is very much affected by this deposition. Over time, the deposits on the extractor gradually accumulate in the direction of the emitter in such a way that the deposited metal eventually comes into contact with the emitter. This situation is often referred to as "clogging" and can cause catastrophic failure of the ion source, especially in the case of ion thrusters in spacecraft, where such failure is permanent. Summary of the Invention

[0006] It is an object of the present invention to provide an LMIS which is more reliable by preventing or at least significantly delaying clogging.

[0007] According to a first aspect of the present invention, this object is achieved by the ion source as described above, characterized in that the extractor, on the side facing the emitter, has a structure configured to absorb liquid metal by means of capillary force.

[0008] In such an ion source, the liquid metal that undesirably accumulates on the extractor diffuses away from the emitter and is absorbed by the structure of the extractor. For this purpose, the structure, at least the part thereof that may come into contact with the liquid metal, is wettable by the corresponding liquid metal, for example made of a wettable material or coated with a wettable material. Thus, clogging is largely prevented.

[0009] In a first advantageous embodiment, the absorption structure consists of a plurality of fins spaced apart from each other to form a plurality of (capillary) gaps therebetween. Thus, the liquid metal is absorbed in the corresponding gaps between adjacent fins, and the fin design of the extractor does not impair the function and normal operation of the extractor and the ion source.

[0010] The plurality of fins and thus the plurality of gaps can extend away from the emitter and its central axis in any manner, for example forming a spiral or a similar shape when viewed in the direction of the central axis. In an advantageous variant, the plurality of gaps extend radially away from the central axis. This helps the rapid absorption of the liquid metal and its diffusion away from the emitter.

[0011] It is particularly advantageous that the width of each gap is constant as the distance from the central axis increases. Thus, the capillary effect is constant.

[0012] Each fin can, for example, be orthogonal or inclined with respect to the central axis. In a beneficial variant, each fin is parallel to the central axis. This helps to achieve a large amount of clearance between every two adjacent fins; the shape complexity of each fin is low and each fin is easy to manufacture.

[0013] In an alternative second embodiment of the ion source, the absorption structure consists of a plurality of (capillary) pores in the extractor. These pores can either result from using a porous material to manufacture the extractor or from creating pores in the extractor by, for example, drilling, etching, etc. The liquid metal is absorbed in the pores and thus diffuses away from the emitter.

[0014] In a third alternative embodiment, the absorption structure consists of a plurality of (capillary) grooves on the surface of the extractor. Although the capillary effect of the grooves is generally weaker than that of the pores or fin-clearances, it may be easier to create the grooves on the surface by, for example, engraving or etching.

[0015] Like the individual fins and the individual clearances, the plurality of grooves can extend away from the emitter and its central axis in any manner, for example, forming a spiral or a similar shape when viewed in the direction of the central axis. In a beneficial variant, the plurality of grooves extend radially away from the central axis.

[0016] In a particularly advantageous variant, the plurality of grooves bifurcate as the distance from the central axis increases. Thus, the amount of liquid metal that can be absorbed by the extractor increases.

[0017] To further increase the amount of liquid metal that can be absorbed, it is beneficial for the extractor to increase its extension in the axial direction as the distance from the central axis increases. The absorption amount thus increases, especially in embodiments of finned or porous extractors; this effect is independent of whether the clearance width between adjacent fins is constant, widens, or narrows as the distance from the central axis increases.

[0018] To avoid electron emission from the extractor towards the emitter, it is beneficial for the extractor to be rounded on the side facing the emitter when viewed transversely to the central axis. This prevents undesired field concentration due to sharp tips or edges on the side of the extractor facing the emitter.

[0019] In an advantageous embodiment, the emitter has a plurality of emission sites with sharp tips or sharp edges, which are arranged axially symmetrically and project parallel to the central axis. This type of emitter results in an ion source that allows a higher ion emission than an emitter with a single emission site. Still, precise control of the ion emission can be easily achieved by adjusting the electric field between the emitter and the extractor with the help of a voltage source.

[0020] To avoid or reverse the solidification of the liquid metal deposited on the extractor, the extractor can be heated. In an advantageous embodiment, the voltage source is switchable between an operating mode and a cleaning mode, in which, in the operating mode, the voltage source applies a first extractor potential to the extractor and a first emitter potential to the emitter, and in the cleaning mode, the voltage source applies a second extractor potential to the extractor and a second emitter potential to the emitter, where the first extractor potential is higher than the second extractor potential, and the first emitter potential is higher than the second emitter potential, each potential being with respect to the potential of the chassis. In the operating mode, ions of the liquid metal are substantially emitted and accelerated away from the emitter, while in the cleaning mode, the emitted ions are largely attracted to the extractor, such that the extractor and any (originally liquid) metal solidified thereon are heated by the colliding ions. In another advantageous embodiment, the ion source alternatively or additionally includes a heater, preferably an electrical heater, for heating the extractor in the cleaning mode. Wherein, the extractor can be either directly thermally connected to the heater or indirectly heated via thermal radiation from the emitter heated by the heater. When the metal is heated above its liquefaction temperature, the capillary force of the structure of the extractor comes into play. The liquefied metal accumulated on the side of the extractor facing the emitter is absorbed into the structure and thus diffuses away from the emitter.

[0021] The cleaning mode can be enabled, for example, after a predetermined duration of operation of the ion source or when a specific deposition limit of the solidified metal on the extractor is reached.

[0022] In a second aspect, the present invention provides a method for operating an ion source of the above type, the method comprising:

[0023] First, in the operating mode, applying a first extractor potential to the extractor and a first emitter potential to the emitter, and

[0024] Second, in the cleaning mode, applying a second extractor potential to the extractor and a second emitter potential to the emitter,

[0025] where the first extractor potential is higher than the second extractor potential, and the first emitter potential is higher than the second emitter potential, each potential being with respect to the potential of the chassis of the ion source.

[0026] Regarding the advantages of the method and other embodiments, reference is made to the statements about the ion source above.

[0027] In a beneficial variant of the method, the first extractor potential is between -10 kV and 0 V, preferably -5 kV, the first emitter potential is between +10 kV and +20 kV, preferably +15 kV, the second extractor potential is between -25 kV and -15 kV, preferably -20 kV, and the second emitter potential is between -5 kV and +5 kV, preferably 0 V, each potential being relative to the potential of the chassis. Thereby, both efficient operation of the ion source and effective cleaning of the extractor are achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The present invention will now be described in detail with reference to the accompanying drawings based on exemplary embodiments, which show:

[0029] Figures 1a and 1b are a top view (Figure 1a) of an ion source according to the prior art and a longitudinal sectional view (Figure 1b) thereof along line A-A of Figure 1a;

[0030] Figure 2 is a top view of the emitter and extractor of the ion source of Figures 1a and 1b, where the emitted liquid metal has been deposited on the extractor;

[0031] Figure 3 is a top view of the emitter and extractor of a first embodiment of an ion source according to the present invention, the extractor having multiple fins and gaps therebetween;

[0032] Figure 4 is Figure 3 a cross-sectional perspective view of the extractor;

[0033] Figure 5 is a top view of the emitter and extractor of a second embodiment of an ion source according to the present invention, the extractor having pores;

[0034] Figure 6 is Figure 5 a cross-sectional perspective view of the extractor;

[0035] Figure 7 is a top view of the emitter and extractor of a third embodiment of an ion source according to the present invention, the extractor having a grooved surface;

[0036] Figure 8 is a flowchart of a method for operating an ion source for Figure 3 , 5 or 7; and

[0037] Figure 9 is Figure 3 a display in the same figure of detail B of the extractor in different liquid metal absorption states in DETAILED DESCRIPTION

[0038] Figures 1a and 1b show a conventional ion source 1', which has a reservoir 2 containing a metal 3 in its liquid state, such as liquid cesium, indium, gallium, mercury, bismuth, etc. The ion source 1' includes an emitter 4 having a central axis C and being in fluid communication with the reservoir 2. The emitter 4 draws the liquid metal 3 from the reservoir 2 in order to emit ions 5 of the liquid metal 3. In order to emit the ions 5 of the liquid metal 3 and accelerate the ions 5 away from the emitter 3, the ion source 1' includes an annular extractor 6' arranged coaxially with the emitter 4 and (here: with its inner circumference) facing the emitter 4.

[0039] A voltage source 7' is electrically connected to the emitter 4 and the extractor 6', and applies a high voltage of several kilovolts (kV) between the emitter 4 and the extractor 6', thereby generating a strong electric field around it.

[0040] The emitter 4 may have one or more emission sites 8, each of which is a protrusion having a sharp tip or edge in the shape of a cone, pyramid, triangular prism, needle or the like, protruding away from the reservoir 2 parallel to the central axis C. Applying a strong electric field to the sharp tip or edge forms a so-called Taylor cone at the top of the tip or edge of each emission site 8. At the apex of the Taylor cone, the ions 5 are emitted and accelerated by the strong electric field between the extractor 6' and the emitter 4, generating an ion beam 9 directed away from the emitter 4 and the ion source 1'.

[0041] The ion source 1' also includes a chassis 10 mounting the reservoir 2, the emitter 4 and the extractor 6'. The voltage source 7' may be mounted on the chassis 10 or separately.

[0042] This type of ion source 1' is called a liquid metal ion source (“LMIS”). The LMIS is used, for example, for ion implantation or for generating a focused ion beam in the semiconductor industry, metal processing, materials science and / or analysis, or as an ion thruster for propelling a spacecraft. When the ion source 1' in Figures 1a and 1b is used as an ion thruster, it may be equipped with an optional neutralizer 11, i.e., an electron source.

[0043] FIG. 2 shows the emitter 4 and the extractor 6' of the ion source 1' in FIG. 1a after a period of operation. Although most of the liquid metal 3 is accelerated away from the emitter 4 by a strong electric field, the ion source 1' also produces an undesirable, substantially omnidirectional spray of neutral or weakly charged droplets of the liquid metal 3, which deposits on the surface surrounding the emitter 4. Since the extractor 6' faces the emitter 4 and is typically negatively charged during operation, it attracts positively charged particles and is very affected by the deposit 12 of the liquid metal 3. The liquid metal 3 typically solidifies on the extractor 6' and gradually accumulates on the extractor 6' as a metal deposit 12 in the direction of the emitter 4. Eventually, the metal deposit 12 contacts the emitter 4 (as shown by the arrow 13 in FIG. 2), resulting in catastrophic failure of the ion source 1'.

[0044] Based on Figures 3 to 9 the example of, a different ion source 1 is proposed herein, which helps to avoid or at least significantly delay this situation. The same or substantially the same elements have the same reference numerals as in FIGS. 1a, 1b, and 2; similar elements that differ only in specific features are distinguished by adding an apostrophe after the reference numerals in FIGS. 1a, 1b, and 2. However, the similar functions and details described above are not repeated below.

[0045] The ion source 1 differs from the ion source 1' especially in that the annular extractor 6 has a structure 14 configured to absorb the liquid metal 3 by means of capillary force on the side 15 facing the emitter 4.

[0046] In Figure 3 and Figure 4 the first embodiment shown, the structure 14 is composed of a plurality of fins 16 spaced apart from each other and thus forming a plurality of capillary gaps 17 therebetween, i.e., each gap 17 is located between two adjacent fins 16. The liquid metal 3 deposited on the extractor 6 is absorbed in the respective gaps 17 and thus diffuses away from the side 15 of the extractor 6 facing the emitter 4 and further away from the emitter 4. For this purpose, each fin 16 or at least its surface within each gap 17 is made of or coated with a material that can be wetted by the liquid metal 3, and the width d of each gap 17 is selected to promote the capillary effect they produce. For example, in the case of using liquid indium as the liquid metal 3, the gap width d can be selected in the range of 1 millimeter (mm) or less, for example, about 0.4 mm. Optionally, the gap width d can be estimated based on the rising height h that the liquid metal 3 is to achieve to overcome gravity according to the following where: g is the gravitational constant, γ is the surface tension coefficient of the liquid metal 3, and ρ is the density of the liquid metal 3.

[0047] In Figure 3 the embodiment, the respective gaps 17 extend radially away from the central axis C of the emitter 4. Alternatively, each gap 17 may be inclined with respect to the radial direction x of the extractor 6 r such that all the gaps 17 form a spiral shape with curved or straight spiral arms around the central axis C.

[0048] It goes without saying that since each gap 17 is merely the result of the separation between two adjacent fins 16, the shape of each gap 17 is defined by the respective fins 16 and their shape and mutual arrangement.

[0049] As the distance D from the central axis C increases, the width d of each gap 17 may also increase to increase the amount of liquid metal 3 that can be absorbed. Alternatively, the width d of each gap 17 may decrease as the distance D from the central axis C increases to increase the capillary force exerted on the liquid metal 3. In Figure 3 another alternative shown, the width d of each gap 17 remains constant as the distance D from the central axis C increases. In this case, when observed along the axial direction x of the extractor 6 a the respective fins 16 are wedge-shaped, i.e., they become thicker as the distance D from the central axis C increases.

[0050] Figure 4 An optional example is shown where the extension e of each fin 16 in the axial direction x a increases as the distance D from the central axis C increases. That is, the extension e1 at a distance D1 from the central axis C is less than the extension e2 at a larger distance D2. Alternatively, as the distance from the central axis C increases, each fin 16 has a constant, decreasing or even varying extension e in the axial direction x a thereon.

[0051] Furthermore, the respective fins 16 may be orthogonal to the central axis C. In this case, each fin 16 is a disk with a central hole, and these disks are stacked and spaced apart to form gaps 17 therebetween. Alternatively, the respective fins 16 may be inclined with respect to the central axis C, forming spiral gaps 17 therebetween. However, in this example, each fin 16 is parallel to the central axis C.

[0052] In Figure 5 and Figure 6In the second embodiment of the extractor 6 shown, the above-described structure 14 is constituted by a plurality of capillary pores 18 in the extractor 6. That is, the porous extractor 6 of this embodiment works like a sponge, absorbing the liquid metal 3 by means of the capillary force of its respective pores 18. In addition, the extractor 6 of the second embodiment may have any shape as described above for the extractor 6 in the first embodiment. For example, it may optionally have an extension e that increases as the distance D from the central axis C increases in the axial direction x a as shown Figure 6 .

[0053] In a third alternative embodiment according to Figure 7 , the above-described structure 14 of the extractor 6 is constituted by a plurality of capillary grooves 19 on the surface 20 of the extractor 6. Each groove 19 may have a cross-section such as semi-circular, rectangular, triangular, etc., and is engraved, cast, etched, or otherwise formed on the surface 20 of the extractor 6. Each groove 19 may extend radially away from the central axis C of the emitter 4 as Figure 7 shown. Alternatively, each groove 19 may be inclined with respect to the radial direction, forming a spiral shape with curved or straight spiral arms around the central axis C. In another alternative, each groove 19 may meander away from the emitter 4.

[0054] In Figure 7 a variant of each groove 19 depicted in the lower half, each groove 19 is only straight; in an alternative variant depicted in the upper half of Figure 7 , each groove 19 bifurcates (bifurcates or multi-furcates) as the distance D from the central axis C increases.

[0055] In addition to the different structures 14, the extractor 6 of the third embodiment may have any shape as described above for the respective extractors 6 in the first and second embodiments.

[0056] When viewed transversely to the central axis C, the extractor 6 may optionally be rounded on its side 15 facing the emitter 4, as Figure 4 and Figure 6 shown. However, alternatively, the extractor 6 may have a different, particularly non-rounded shape.

[0057] As briefly pointed out above, the emitter 4 may have one or more emission sites 8 with sharp tips or sharp edges, which are arranged axially symmetrically with respect to the central axis C and project parallel to the central axis. In the examples of Figure 3 and Figure 5 , the emitter 4 is coronal, having a plurality of emission sites 8 arranged in a circle around the central axis C. However, different arrangements of the emission sites 8 are possible, such as in an array, etc. Figure 7Shows an example where the emitter 4 has a single central emission site 8. In any case, the extractor 6 has an annular shape corresponding to the arrangement of the emission site 8 to cooperate with the coaxial emitter 4 to generate a strong electric field.

[0058] Figure 8 and Figure 9 Shows a method 21 for operating the ion source 1. According to Figure 8 , the voltage source 7 can first be switched to the operating mode 22. In the operating mode 22, ions 5 of the liquid metal 3 are emitted and accelerated away from the emitter 4. For this purpose, the voltage source 7 applies a first extractor potential to the extractor 6 and a first emitter potential to the emitter 4. The first extractor potential is typically between -10 kV and 0 V relative to the potential of the chassis 10, and the first emitter potential is typically between +10 kV and +20 kV (also relative to the potential of the chassis 10). In an exemplary embodiment, the first extractor potential is between -3 kV and -8 kV, for example -5 kV, and the first emitter potential is between +12 kV and +17 kV, for example +15 kV, each potential being relative to the potential of the chassis 10.

[0059] When the voltage source 7 has been in the operating mode 22 for a predetermined duration, or when excessive metal deposits 12 are detected on the side 15 of the extractor 6 facing the emitter 4 (e.g., by machine vision), or when any (other) predetermined criterion crt1 is reached, see decision box 23, the voltage source 7 can be switched to the cleaning mode 24 (branch "Y" in box 23); otherwise, the voltage source 7 can remain in the operating mode 22 (branch "N" in box 23 and loop 25).

[0060] In the cleaning mode 24, the ions 5 emitted by the emitter 5 are mainly attracted by the extractor 6 rather than being accelerated away from the ion source 1. As a result, the extractor 6 heats up due to ion bombardment, i.e., due to the ions 5 being attracted and hitting the extractor 6 while liquefying the solidified metal deposits 12 on the extractor 6. For this purpose, the voltage source 7 applies a second extractor potential to the extractor 6 and a second emitter potential to the emitter 4. The first extractor potential is higher than the second extractor potential, and the first emitter potential is higher than the second emitter potential, each potential being relative to the potential of the chassis 10. Relative to the potential of the chassis 10, the second extractor potential is typically between -25 kV and -15 kV, for example -20 kV, and the second emitter potential is between -5 kV and +5 kV, for example 0 V.

[0061] The effects of the operating mode 22 and the cleaning mode 24 on the metal deposits 12 are in Figure 9Shown (for an embodiment of an extractor having a plurality of fins 16 and gaps 17 therebetween), different stages of metal deposition / accumulation in different regions 261, 262, …, 265 of the extractor 6 are presented in the same figure. It should be understood that these different stages do not typically occur simultaneously.

[0062] The start of metal deposition 12 on the side 15 of each fin 16 of the extractor 6 facing the emitter 4 is shown in region 261. This typically occurs during operation of the ion source 1 in operating mode 22.

[0063] The accumulated metal deposits 12 in region 262 are already quite substantial and the metal has solidified.

[0064] Region 263 shows the metal deposits 12 in region 262 which have been liquefied into droplets of liquid metal 3 on the side 15 of the extractor fins 16 in cleaning mode 24.

[0065] Droplets of liquid metal 3 on adjacent fins 16, when large enough, will coalesce as shown in region 264.

[0066] Due to the capillary effect, the coalesced droplets of liquid metal 3 will be absorbed by the respective gaps 17 and spread away from the emitter 4 as shown in region 265 so as to clean the side 15 of the extractor 6 facing the emitter 4.

[0067] After the side 15 of the extractor 6 facing the emitter 4 has been cleaned, for example after a predetermined duration, or when sufficient cleaning is detected, or when a (different) predetermined criterion crt2 is reached ( Figure 8 ), see decision box 27, the voltage source 7 can be switched back to operating mode 22 (branch “Y” in box 27 and loop 28); otherwise, the cleaning mode 24 will continue (branch “N” in box 27 and loop 29).

[0068] It should be understood that the same method 21 of operating the ion source 1 can equally be applied to extractors 6 having different structures 14, in particular the pores 18 or grooves 19 described above. Additionally, the extractor 6 can alternatively or additionally be heated in cleaning mode 24 by a heater such as an electric heater, where the extractor 6 can be either directly thermally connected to such a heater or indirectly heated by the thermal radiation of the correspondingly heated emitter 4. Further, when neither operation nor cleaning of the ion source 1 is required, the voltage source 7 can be idle or switched off.

[0069] The present invention is not limited to the specific embodiments described in detail herein, but encompasses those variations, combinations and modifications that fall within the scope of the appended claims.

Claims

1. An ion source, in particular an ion thruster for propelling a spacecraft, comprising: A reservoir (2) for liquid metal (3); An emitter (4) having a central axis (C) and being in fluid communication with the reservoir (2), for extracting liquid metal (3) from the reservoir (2) and emitting ions (5) of the liquid metal (3); An annular extractor (6) coaxial with and facing the emitter (4), for accelerating the ions (5) away from the emitter (4); A voltage source (7) electrically connected to the emitter (4) and the extractor (6), for effecting the emission and acceleration; And A chassis (10) mounting the reservoir (2), the emitter (4) and the extractor (6); Characterized in that The extractor (6) has a structure (14) configured to absorb liquid metal (3) by means of capillary force on a side (15) facing the emitter (4).

2. The ion source according to claim 1, wherein the structure (14) is constituted by a plurality of fins (16) spaced apart from each other to form a plurality of gaps (17) therebetween.

3. The ion source according to claim 2, wherein the plurality of gaps (17) extend radially away from the central axis (C).

4. The ion source according to claim 2 or 3, wherein the width (d) of each gap (17) is constant as the distance (D) from the central axis (C) increases.

5. The ion source according to any one of claims 2 to 4, wherein each fin (16) is parallel to the central axis (C).

6. The ion source according to claim 1, wherein the structure (14) is constituted by a plurality of pores (18) in the extractor (6).

7. The ion source according to claim 1, wherein the structure (14) is constituted by a plurality of grooves (19) on the surface (20) of the extractor (6).

8. The ion source according to claim 7, wherein the plurality of grooves (19) extend radially away from the central axis (C).

9. The ion source according to claim 7 or 8, wherein the plurality of grooves (19) bifurcate as the distance (D) from the central axis (C) increases.

10. The ion source according to any one of claims 1 to 9, wherein the extension (e) of the extractor (6) in the axial direction (x a ) increases as the distance (D) from the central axis (C) increases.

11. The ion source according to any one of claims 1 to 10, wherein when viewed transversely to the central axis (C), the extractor (6) is rounded on the side (15) facing the emitter (4).

12. The ion source according to any one of claims 1 to 11, wherein the emitter (4) has emission sites (8) with a plurality of sharp tips or sharp edges, which are arranged axially symmetrically and protrude parallel to the central axis (C).

13. The ion source according to any one of claims 1 to 12, wherein the voltage source (7) is switchable between an operating mode (22) and a cleaning mode (24), in which the voltage source applies a first extractor potential to the extractor (6) and a first emitter potential to the emitter (4) in the operating mode (22), and applies a second extractor potential to the extractor (6) and a second emitter potential to the emitter (4) in the cleaning mode (24), wherein the first extractor potential is higher than the second extractor potential, and the first emitter potential is higher than the second emitter potential, each potential being relative to the potential of the chassis (10).

14. The ion source according to any one of claims 1 to 13, further comprising a heater, preferably an electric heater, for heating the extractor (6) in the cleaning mode (24) / the cleaning mode (24).

15. A method for operating an ion source (1) according to any one of claims 1 to 14, comprising: First, in an operating mode (22), a first extractor potential is applied to the extractor (6) and a first emitter potential is applied to the emitter (4); And Second, in a cleaning mode (24), a second extractor potential is applied to the extractor (6) and a second emitter potential is applied to the emitter (4), wherein the first extractor potential is higher than the second extractor potential, and the first emitter potential is higher than the second emitter potential, each potential being relative to the potential of the chassis (10) of the ion source (1).

16. The method according to claim 15, wherein the first extractor potential is between -10 kV and 0 V, preferably -5 kV, the first emitter potential is between +10 kV and +20 kV, preferably +15 kV, the second extractor potential is between -25 kV and -15 kV, preferably -20 kV, and the second emitter potential is between -5 kV and +5 kV, preferably 0 V, each potential being relative to the potential of the chassis (10).

Images