Search

EP-4741656-A1 - ION SOURCE

EP4741656A1EP 4741656 A1EP4741656 A1EP 4741656A1EP-4741656-A1

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.

Inventors

  • KREJCI, David
  • Vasiljevich, Ivanhoe
  • LITTLE, Bryan

Assignees

  • ENPULSION GmbH

Dates

Publication Date
20260513
Application Date
20241112

Claims (18)

  1. An ion source, in particular an ion thruster for propelling a spacecraft, comprising a 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 projections (7) for emitting ions (3') of the propellant (3) when the propellant (3) is in its liquid state, wherein the one or more projections (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), characterised in that the heater (4) comprises a projection heating unit (5) for feeding energy into the one or more projections (7) to heat the propellant (3) in the one or more projections (7), wherein the 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 projections (7), liquefy the propellant (3) at the one or more projections (7) by the projection heating unit (5) before the propellant (3) in the reservoir (2) is liquefied.
  2. The ion source according to claim 1, wherein the heater (4) further comprises a reservoir heating unit (5') for feeding energy into the reservoir (2) to heat the propellant (3) in the reservoir (2).
  3. The ion source according to claim 1 or 2, wherein the projection heating unit (5) comprises one or more heating wires (13) for feeding the energy into the one or more projections (7) by irradiating them with thermal radiation (14).
  4. The ion source according to claim 3, wherein the projection heating unit (5) further comprises an annular concave mirror (15) surrounding the one or more projections (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 projections (7).
  5. The ion source according to claim 4, wherein the concavity (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 projections (7) are arranged.
  6. The ion source according to any one of claims 3 to 5, wherein the one or more heating wires (13) are substantially point-shaped and the projection heating unit (5) further comprises 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 projections (7).
  7. The ion source according to claim 6, wherein the one or more projections (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 focal 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 projections (7) is arranged.
  8. The ion source according to any one of claims 3 to 7, wherein the extractor (8) is one of said one or more heating wires (13).
  9. The ion source according to any one of claims 3 to 8, wherein the extractor (8) has an annular groove (19) surrounding the one or more projections (7), in which groove (19) the one or more heating wires (13) are arranged for concentrating the thermal radiation (14) onto the one or more projections (7) .
  10. The ion source according to any one of claims 3 to 8, 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 projections (7).
  11. The ion source according to any one of claims 1 to 10, wherein the projection heating unit (5) comprises an annular parabolic mirror (20) for feeding the energy into the one or more projections (7) by concentrating essentially parallel rays of incident thermal radiation (21) onto the one or more projections (7).
  12. The ion source according to any one of claims 1 to 10, wherein the one or more projections (7) are needle-shaped and the projection heating unit (5) comprises one or more cup-shaped parabolic mirrors for feeding the energy into the one or more projections (7) by concentrating essentially parallel rays of incident thermal radiation (21) onto one of the one or more projections (7), respectively.
  13. The ion source according to any one of claims 3 to 12, wherein the projection heating unit (5) is offset from the emitter (6) opposite to the direction of emission (R).
  14. The ion source according to any one of claims 3 to 12, wherein the projection heating unit (5) is offset from the emitter (6) in the direction of emission (R).
  15. The ion source according to claim 14, 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 projection heating unit (5) and the one or more projections (7) and is transmissible to the thermal radiation (14).
  16. The ion source according to any one of claims 1 to 15, wherein the projection heating unit (5) comprises one or more laser sources (26), each of which is directed at a respective one of the one or more projections (7) for feeding the energy into the one or more projections (7) by irradiating them with laser light (27).
  17. The ion source according to any one of claims 1 to 16, wherein the one or more projections (7) and/or the propellant (3) are electrically conductive and the projection 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 projections (7) for feeding the energy into the one or more projections (7) by electromagnetic induction.
  18. The ion source according to any one of claims 1 to 17, wherein the projection heating unit (5) comprises an AC-current generator and, connected thereto, an electromagnetic coil adjacent to the extractor (8) to heat the extractor (8) by electromagnetic induction, for feeding the energy into the one or more projections (7) by irradiating them with thermal radiation (14) from the heated extractor (8).

Description

The present invention relates to an ion source, in particular 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 projections for emitting ions of the propellant when the propellant is in its liquid state, wherein the one or more projections 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. 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 caesium, 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 liquefy 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 accelerated by the extractor to form a directed beam of ions, which in case of an ion thruster provides the thrust. To achieve a strong electric field between the emitter and the extractor, which is necessary for ion extraction, the emitter's one or more projections 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 projection causes the formation of a so-called Taylor cone on top of the tip or edge of each of the emitter's projections, which further enhances the field-concentrating effect. For transporting liquid propellant from the reservoir to each projection of the emitter, passive forces, like capillary effects produced by capillary ducts penetrating the emitter (as described, e.g., in AT 500 412 A1 or US 4 328 667 B) or by a porous emitter (as described, e.g., in US 2016/0297549 A1 or EP 3 724 497 A1) and/or by adhesion effects on the wetting surface of the emitter's projections (as described, e.g., in US 2009/114838 A1 or US 2011/192968 A1), 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., insufficient adhesive forces between the liquid propellant and the emitter or excessive cohesive forces within the propellant. 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 solid state compared to its liquid state, will contract in volume. Hence, upon solidification the propellant retracts from the projections 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 degrade after having been paused. Countermeasures such as applying 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. It is an object of the present invention to provide a safe, reliable and efficient liquid metal ion source. This object is achieved with an ion source specified at the outset, which is distinguished in that the heater comprises a projection heating unit for feeding energy into the one or more projections, 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 projections by the projection heating unit before the propellant in the reservoir is partially of fully liquefied. By melting the propellant at the one or more projections 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 projections and compensates for the amount of propellant that has retracted therefrom during previous 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