Ion thruster

Abstract

The present invention relates to an ion thruster for propulsion of spacecrafts, including: a reservoir for a propellant, an emitter for emitting ions of the propellant, the emitter having one or more projections of porous material and a base with a first side supporting said projections and a second side connected to the reservoir, and an extractor facing the emitter for extracting and accelerating the ions from the emitter, wherein the base is impermeable to the propellant at least on said first side and has pores or channels for providing flow of propellant from the reservoir to said projections.

Claims

1. An ion thruster for propulsion of spacecrafts, comprising: a reservoir for a propellant, an emitter for emitting ions of the propellant, the emitter having one or more projections of porous material and a base with a first side supporting said one or more projections and a second side connected to the reservoir, and an extractor facing the emitter for extracting and accelerating the ions from the emitter, wherein the base has pores or channels for providing a flow of the propellant from the reservoir to said one or more projections, and further wherein said first side of the base is coated with a coating which is impermeable to the propellant and wherein the one or more projections protrude through the coating.

2. The ion thruster according to claim 1, wherein the pores or channels of the base are covered with a material that is wettable by the propellant.

3. The ion thruster according to claim 1, wherein the coating extends over an adjacent portion of each projection of the one or more projections.

4. The ion thruster according to claim 1, wherein the coating extends over an adjacent portion of the reservoir.

5. The ion thruster according to claim 1, wherein the coating is repellent to the propellant.

6. The ion thruster according to claim 1, wherein the coating is made of an epoxy resin.

7. The ion thruster according to claim 1, wherein the base and the one or more projections are made of porous tungsten.

8. The ion thruster according to claim 1, wherein the one or more projections are needle-shaped.

9. The ion thruster according to claim 1, wherein the one or more projections on the emitter comprise a plurality of projections arranged in a circle on said first side.

10. The ion thruster according to claim 1, wherein the reservoir comprises an internal propellant guide leading to said second side of the base.

11. An ion thruster for propulsion of spacecrafts, comprising: a reservoir for a propellant, an emitter for emitting ions of the propellant, the emitter having one or more projections of porous material and a base with a first side supporting said one or more projections and a second side connected to the reservoir, and an extractor facing the emitter for extracting and accelerating the ions from the emitter, wherein the base is impermeable to the propellant at least on said first side and has pores or channels for providing a flow of the propellant from the reservoir to said one or more projections, wherein said first side of the base is coated with a coating impermeable to the propellant, and wherein the coating is made of an epoxy resin.

12. An ion thruster for propulsion of spacecrafts, comprising: a reservoir for a propellant, an emitter for emitting ions of the propellant, the emitter having one or more projections of porous material and a base with a first side supporting said one or more projections and a second side connected to the reservoir, and an extractor facing the emitter for extracting and accelerating the ions from the emitter, wherein the base is impermeable to the propellant at least on said first side and has pores or channels for providing a flow of the propellant from the reservoir to said one or more projections, wherein said first side of the base is coated with a coating impermeable to the propellant, and wherein the base and the one or more projections are made of porous tungsten.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The disclosed subject matter shall now be explained in more detail below on the basis of an exemplary embodiment thereof with reference to the accompanying drawings, in which:

(2) FIGS. 1a and 1b show an example of an ion thruster according to the disclosed subject matter in a top view (FIG. 1a) and in a detail of a longitudinal section along line A-A of FIG. 1a (FIG. 1b), respectively;

(3) FIGS. 2a and 2b show a porous emitter projection of the ion thruster of FIGS. 1a and 1b in a longitudinal section (FIG. 2a) and a detail C of FIG. 2a (FIG. 2b);

(4) FIGS. 3a to 3d schematically show three embodiments of the emitter of the ion thruster of FIGS. 1a and 1b, in respective longitudinal sections (FIGS. 3a to 3c) and a detail D of FIG. 3a (FIG. 3d); and

(5) FIG. 4 shows an embodiment of a guiding structure in a propellant reservoir of the ion thruster of FIGS. 1a and 1b in a perspective view.

DETAILED DESCRIPTION

(6) FIGS. 1a and 1b show an ion thruster 1 for propulsion of spacecrafts, particularly satellites. The ion thruster 1 comprises a reservoir 2—herein also called tank—for a propellant 3 (FIGS. 2a and 2b). The ion thruster 1 further comprises an emitter 4 for emitting ions 3.sup.+ of the propellant 3 and an extractor 5 facing the emitter 4 for extracting and accelerating the ions 3.sup.+ from the emitter 4.

(7) The ion thruster 1 of FIGS. 1a and 1b is of field-emission electric propulsion (FEEP) type. Ion thrusters 1 of this type use liquid metal as propellant 3, e.g. caesium, indium, gallium or mercury, which is ionized by field-emission as will be explained in greater detail below. The extractor 5 then extracts and accelerates the generated (here: positive) ions 3.sup.+ of the propellant 3 for propulsion of the spacecraft. Moreover, the ion thruster 1 also optionally comprises one or more (here: two) electron sources 10 (also known in the art as “neutralizers”) to the sides of the emitter 4 for balancing a charging of the ion thruster 1—and thus of the spacecraft—due to emission of positively charged ions 3.sup.+.

(8) Alternatively, the ion thruster 1 may be of colloid type using ionic liquid, e.g. room temperature molten salts, as propellant 3. In this case, the electron sources 10 may not be necessary, as colloid thrusters usually change polarity periodically so that a continued self-charging of the ion thruster 1 and the spacecraft does not occur. In a further alternative, the ion thruster 1 can use gas, e.g. xenon, as propellant 3, which is again ionized by extracting electrons from the atoms.

(9) The emitter 4 has one or more projections 11 and a base 12. The base 12 has a first side 12.sub.1 supporting said projections 11 and a second side 12.sub.2 connected to the reservoir 2. Each projection 11 can have the shape of a cone, a pyramid, a triangular prism, or the like and has a sharp tip 11′ or edge (FIGS. 2a to 2c), respectively, opposite the base 12. Particularly, each projection could be needle-shaped, i.e. a narrow, pointed cone. Herein, the projections 11 are also referred to as sharp emitter structures or needles.

(10) The emitter 4 shown in FIG. 1b has a multitude of needle-shaped projections 11, which are arranged in a circle (FIG. 1a) on said first side 121 of the base 12. The base 12 itself is ring-shaped. Thereby, a crown-shaped emitter 4 is formed. Moreover, the extractor 5 has a single aperture P for emission of ions 3.sup.+ of the propellant 3 from all projection 11 of the crown-shaped emitter 4. It shall be understood, however, that other shapes of bases 12 and other shapes and arrangements of projections 11 for the emitter 4 and respective extractors 5 may alternatively be chosen. For example, extractors 5 may have a separate aperture for each projection 11 for extracting and accelerating the of ions 3.sup.+ from this very projection 11.

(11) FIG. 2a shows a projection 11 of the present ion thruster 1, which is made of porous material, e.g., porous tungsten, for transporting propellant 3 to the tip 11′ of the projection 11 via capillary forces. Between the projection 11 of the emitter 4 and the extractor 5, a strong electric field in the range of a few kilovolts (kV) is applied by means of electrodes E.sup.+, E.sup.−. By applying the electric field, a so-called Taylor cone T is formed on the tip 11′ of the projection 11.

(12) In FEEP ion thrusters 1 neutral atoms of the liquid metal evaporate from the surface. In the strong electric field at the tip 11′ of the Tailor cone T, one or more electrons tunnel back to the surface of the projection 11 due to field-emission, changing the formerly neutral atom to a positively charged ion 3.sup.+. In case of colloid ion thrusters 1 with ionic propellant 3, this ionization is not necessary.

(13) As shown in FIG. 2b, a further consequence of the strong electric field is that a jet J is formed on the apex of the Tailor cone T, from which the ions 3.sup.+ of the propellant 3 are extracted and then accelerated by the extractor 5 generating thrust. Due to the precision at which the voltage between the needle 3 and the extraction electrode E.sup.− can be controlled, the generated thrust can be controlled with high accuracy.

(14) Summing up, in case of FEEP the metallic propellant 3 in the tank 2 is heated above the liquefaction temperature, and capillary forces, by a combination of surface tension, (pore) geometry and wettability of the surface of the reservoir 2 and the emitter 4, feed the propellant 3 from the propellant reservoir 2 towards the emitter 4, and further towards the tips 11′ of the sharp emitter structures 11. A high voltage is applied to the liquid propellant 3 with respect to a counter electrode E.sup.−, surpassing the threshold of ionization locally at the induced liquid instabilities formed by electrical stresses at the tips 11′ of the sharp emitter structures 11. Propellant 3 is therefore extracted, and replenished by capillary forces from downstream.

(15) FIGS. 3a to 3c show three embodiments of the emitter 4 for use in the ion thruster 1. In all three embodiments, however, the base 12 is impermeable to the propellant 3 at least on said first side 12.sub.1 thereof as will be explained in detail further down. Thereby, a seeping of propellant 3 through the base 12—at least through said first side 12.sub.1 thereof—and a subsequent accumulation of propellant 3 around each projection 11 and/or between two neighboring projections 11 is inhibited. At the same time, the base 12 itself has pores 13 or channels 14 for providing flow of propellant 3 from the reservoir 2 to said projections 11; therefore, the pores 13 or channels 14 connect the reservoir 2 to the projections 11.

(16) In the first embodiment (FIG. 3a) of said three embodiments (FIGS. 3a to 3c), the entire base 12 is made of a material which is impermeable to the propellant 3. For providing flow of propellant 3 from the reservoir 2 to the projections 11, the base 12 in this case has—open or porous—channels 14. The channels 14, when necessary, are optionally covered with a material that is wettable by the propellant 3 for easing the flow of propellant 3 by means of capillary forces.

(17) It is understood, that in a variation of this embodiment, just a part of the base 12, i.e. the first side 12.sub.1, can be made of a material impermeable to the propellant 3, while the rest, e.g. the interior, of the base 12 could be permeable (and wettable) by the propellant 3.

(18) In the second embodiment (FIG. 3b), said first side 12.sub.1 of the base 12 is coated with a coating 15 which is impermeable to the propellant 3. The base 12 may optionally be of the same porous material as the projections 11, in which case the pores 13 are blocked by the coating 15 on said first side 12.sub.1. The base 12 can be unitary with the projections 11 as in the example of FIG. 3b, or separate therefrom and connected, e.g., glued, additively manufactured or welded, thereto.

(19) In the third embodiment (FIG. 3c), which can also be seen as a variation of the aforementioned second embodiment (FIG. 3b), the propellant-impermeable coating 15 extends from the first side 12.sub.1 of the base 12 over a portion 16 of each projection 11, which portion 16 is adjacent to said first side 12.sub.1. Hence, the coating 15 covers the lower base, i.e. the adjacent portion 16, of the projections 11 and the gap between neighboring projections 11, i.e. said first side 12.sub.1. Thereby, also seeping of propellant 3 through said lower base of the projections 11 is prevented.

(20) The maximum height H of the coating 15 of said portion 16 of the projection 11 is determined by the necessary flow of propellant 3 and particularly depends on the cross section of the projection 11 and its properties in respect to the propellant 3, which in turn depend on environmental conditions such as temperature: For a projection 11 with a cross section A, whose porous properties are in a manner that a fraction pf*A is available for liquid transport of the propellant 3 with temperature dependent density ρ, and which is used for emitting a current I of charged particles of an average charge-to-mass ratio e/m and a volume flow rate per unit surface area q, the average local flow velocity v at the height of the termination of the coating 15 is given by

(21) $\begin{array}{cc}v=\frac{I}{\rho A.Math.\mathrm{pf}}\frac{m}{\mathrm{eq}}& \left(\mathrm{eq}.4\right)\end{array}$

(22) For a projection 11 in the form of a cone, the average local flow velocity v can be described dependent on the height h measured from the base 12 towards the tip 11′ of the cone, which is described by the angle φ and radius at the base R, by

(23) $\begin{array}{cc}v=\frac{I}{\rho {\pi \left(R-h\tan \phi \right)}^2.Math.\mathrm{pf}}\frac{m}{\mathrm{eq}}& \left(\mathrm{eq}.5\right)\end{array}$

(24) For a liquid with temperature dependent viscosity μ, the volume flow rate per unit surface area q for a material with permeability κ, the pressure drop ΔP can be expressed by

(25) $\begin{array}{cc}\Delta P=-\frac{\mu }{\kappa }q& \left(\mathrm{eq}.6\right)\end{array}$

(26) For a conical projection 11, the pressure drop at height h*, which is measured from the tip 11′ of the conical projection 11 and is equivalent with the height at which the coating 15 is terminated, is given by

(27) $\begin{array}{cc}\Delta P=-\frac{\mu }{2\mathrm{\pi \kappa }}\frac{Ie}{\rho m}\frac{1}{1-\cos \phi }\left(\frac{1}{h^{*}.Math.\tan \phi }-\frac{1}{R}\right)& \left(\mathrm{eq}.7\right)\end{array}$
where ΔP needs to be chosen small enough to allow passive propellant 3 flow through the porous medium, but large enough to enable ion emission with average charge-to-mass ratio e/m required for the operation of the ion thruster 1.

(28) In the third embodiment (FIG. 3c), the propellant-impermeable coating 15 further extends from said first side 12.sub.1 over an adjacent portion 17 of the reservoir 2. It shall be understood, that the coating 15 on the portion 17 of the reservoir 2 and the coating 15 on the portion 16 of the projection 11 are independent from each other in that the coating 15 can be extended over none of the two portions 16, 17 (resulting in the second embodiment, FIG. 3b), over one of the portions 16, 17, or over both portions 16, 17. Moreover, any such coating 15 can optionally be used together with a base 12, at least said first side 12.sub.1 of which is made of material impermeable to the propellant 3 as in the first embodiment (FIG. 3a), i.e. coating said first side 12.sub.1.

(29) In the embodiments of FIGS. 3a to 3c, the base 12 is, e.g., a cuboid or a cylinder and the second side 12.sub.2 of the base 12 connected to the reservoir 2 is opposite to the first side 12.sub.1 of the base 12 which supports the projections 11. How-ever, this is not necessary, as the propellant 3 could also flow through the base 12 from, e.g., a lateral side thereof. An example for such a situation is also shown in FIG. 1b, where the base 12 of the crown-shaped emitter 4 is ring-shaped with an inner and an outer circumference, one or both of which being said second side 12.sub.2 from which flow of propellant 3 from the reservoir 2 is provided to the projections 11 projecting from the top of the ring-shaped base 12, which, in this case, constitutes said first side 12.sub.1. Moreover, the emitter 4 in the example of FIG. 1b has a coating 15 according to the abovementioned third embodiment (FIG. 3c): The coating 15 extends both over the portion 16 of the projections 11 and the portion 17 of the reservoir 2.

(30) Moreover, the propellant-impermeable coating 15 may, optionally, also be repellent, i.e. non-wetting, to the propellant 3. In the present embodiments, the coating 15 is made of an epoxy resin. However, other materials which are impermeable and repellent to the propellant 3 known to the skilled person may be used for the coating 15.

(31) Relating to FIG. 3d, the accumulation of propellant 3 is inhibited by preventing propellant 3 seeping through the base 12; this effect can be supported based on the following: The pressure Δp in a meniscus M formed by a liquid propellant 3 of surface tension γ can be described by the Young/Laplace equation:

(32) $\begin{array}{cc}\Delta p=\gamma \left(\frac{1}{R_1}+\frac{1}{R_2}\right)=2\gamma R_m& \left(\mathrm{eq}.1\right)\end{array}$
where R.sub.1 and R.sub.2 are the principal radii of curvature of the menisci M, R.sub.m is the mean curvature, and γ is a function of temperature, which, e.g. for liquid indium, can be described in the form of
γ.sub.in=a+bt+ct.sup.2  (eq. 2)
where t is the temperature (in centigrade) and the coefficients (for liquid indium) are:
a=568; b=−0.04; c=−0.0000708.

(33) The relationship between a contact angle θ and the Gibbs interfacial energies 6 between solid and gas (SV), solid and liquid (SL), and liquid and vapor (LV) is given by Young's equation
δ.sub.SV=δ.sub.SL−δ.sub.LV cos θ  (eq. 3)

(34) These relationships determine a minimum distance that two adjacent projections 11 shall be separated with, to avoid connection of the two menisci M formed between the base 12 and the projection 11. When the minimum distance is not kept, the force containing the meniscus M around a projection 11 would vanish as the radii increase for a meniscus M that combines with a neighboring meniscus M into one liquid body. Hence, the negative pressure inside the meniscus would decrease and no forces would act that could prevent the liquid accumulation to further increase over time.

(35) As the physical properties of the liquid change with temperature and other environmental conditions, the extent of the minimum distance would need to account for these effects.

(36) The possibility of avoiding the occurrence of growing liquid accumulations in the vicinity of projections 11 and especially between two neighboring projections 11 is to inhibit propellant 3 seeping through the base 12. Avoiding such accumulations can further be supported by providing said first side 12.sub.1 of the base 12 with a material that has a larger contact angle θR to the liquid propellant 3 compared to the material of the projections 11 (and optionally the remaining base 12), i.e. the first side 12.sub.1 is repellent to the propellant 3. Hence, when the coating 15 is also repellent to the propellant 3, the projections 11 may optionally be closer to each other, as depicted in FIG. 3c.

(37) It shall be understood that when the base 12 itself is propellant-impermeable and has a larger uniform area (not shown) and the projections 11 project from merely a sector of this area, not necessarily the whole area but only said sector around each of the projections 11, i.e. particularly between neighboring projections 11, may be coated with said repellent material.

(38) On the basis of FIGS. 1b and 4 an optional internal guiding structure 18 for the propellant 3 shall be explained.

(39) The guiding structure 18, which is comprised by the reservoir 2, enhances the flow of propellant 3 towards said second side 12.sub.2 of the base 12. Therefore, the propellant guiding structure 18 has good wetting characteristics with respect to the propellant 3. In case of indium as propellant 3, the guiding structure 18 is, for example, coated with a layer 19 of tantalum. Tantalum may be applied by a gas phase process like CVD in order to form the layer 19 that is grown into the tank material creating an inseparable nanoscale surface alloy. Such tantalum layer 19 has crystalline features significantly improving the wetting characteristics of indium on the walls of the reservoir 2.

(40) To enhance the passive flow of propellant 3 from the reservoir 2 towards the emitter 4, the guiding structure 18 comprises wettable guiding baffles 20, also referred to as fins, which are introduced into the reservoir 2. These fins 20 lead the propellant 3 either directly to said second side 12.sub.2 of the base 12 of the emitter 4, or via an optional central, wettable feed tube 21 (FIG. 1b) of the guiding structure 18, which itself is connected to said second side 12.sub.2 of the base 12.

(41) The guiding structure 18 also prevents unintended propellant movement inside the reservoir 2 when the propellant 3 is kept in liquid state.

(42) The invention is not restricted to these specific embodiments described in detail herein but encompasses all variants, combinations, and modifications thereof that fall within the frame of the appended claims.