Liquid metal plasma facing components for electric thrusters

Abstract

An electric thruster uses electrical energy to generate and accelerate a propellant to produce thrust through electrostatic, electromagnetic, or electrothermal forces for the purpose of spacecraft propulsion. Exposed thruster surface in acceleration channels, electrodes, grids, cathodes, anodes, nozzle surfaces, or magnet pole pieces are exposed to the energized propellant during operation, including interactions from sputtering, chemical effects and thermal effects. A portion of the surfaces exposed to the energized propellant is covered by a sacrificial liquid.

Description

LIQUID METAL PLASMA FACING COMPONENTS FOR ELECTRIC THRUSTERSINVENTOR: MICHAEL T. KOTSCHENREUTHERCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to provisional application serial no. 63559990 filed on March 1, 2024, which is incorporated herein in its entirety by reference.FIELD OF INVENTION

[0002] This invention generally relates to the area of electric thrusters for spacecraft and satellites.BACKGROUND OF THE INVENTION

[0003] It is well-known that Electric Thrusters (ETs) have fundamental advantages over chemical thrusters. Their propellant exhaust velocity can immensely exceed that which is possible by chemical reactions. ETs excel at the crucial metric of specific impulse. To accomplish this, the propellant molecules or atoms are typically accelerated to energies of the order of several eV to hundreds of eV (electron volts). In many cases, the high atomic velocity is facilitated by having the propellant in a plasma state during acceleration. For simplicity, we will refer to the fuel as atoms (but this term is to be considered here to include molecules and other collections of atoms as well). An ineluctable consequence of the high energy of the atoms is that they cause erosion if they come into contact with solid surfaces. This is because the binding energy of atoms in solids is only of the order of eV, and since the atomic energy of propellant atoms is well in excess of this, a collision with atoms in the solid results in some of the latter being “knocked out” of the surface. This is the well- known process of sputtering. Such erosion seriously limits the lifetime of many electric propulsion systems. Examples of this include, but are not limited to, Hall Effect Thrusters (HETs), Gridded Ion Thrusters (GIDs), Magneto Plasma Dynamic thrusters (MPDTs), Pulsed Plasma Thrusters (PPT) and Arcjet Thrusters. This invention relates to a means to substantially increase the lifetime of ETs by reducing the consequences of erosion.

[0004] Erosion causes undesirable limitations on ETs. It can limit the lifetime of a thruster. Or, if other measures known in the art are used to curtail erosion, this often results in a reduction of the performance of the ET. Hence, a means to eliminate the effect of erosion in ETs could improve the lifetime of a thruster, improve the performance, or possibly both.

[0005] Accordingly, improvements are sought to reduce the impact of erosion in thrusters.SUMMARY OF THE INVENTION

[0006] The problem of severe erosion due to plasma exposure arises in another technological area: magnetically confined plasmas for controlled fusion. Examples of devices where this is a problem include tokamaks, spherical tokamaks, and stellarators. Within the fusion technology arena, an approach that is being developed to counteract erosion is to use liquid Plasma Facing Components (PFCs) with surfaces that can be replenished in situ. A thin liquid layer coats a solid substrate, the liquid faces the plasma, and the liquid protects the solid from erosion. The liquid can be replenished to allow operation for a long time. Replenishment might occur continuously while the device is in operation, or it might be accomplished in between periods of operation without disassembling the device.

[0007] In the context of ETs, we will use the term PEC to refer to areas that are exposed to the electrically energized propellant. This is often a plasma, but even in ET variants where it is not, we will still use the term PFC to refer to areas exposed to electrically energized propellant.

[0008] Liquid Metal (LM) Plasma Facing Components (PFCs) for fusion applications must be able to withstand far more severe conditions than those in ETs, in terms of the flux of high-energy atoms striking a surface or the heat flux on the surface from those particles. In a fusion reactor, the amount of area that needs to be covered by LM PFCs also exceeds that in ETs by orders of magnitude. And the intensity or violence of plasma events to dislodge the liquid from the substrate are, in many cases, higher in fusion experiments than in ETs. So, although LM PFCs are not presently fully mature for application to fusion devices, LM PFC concepts developed in fusion can be capable of meeting many missions in the ET area, at a lower level of technology capability than is required for fusion missions.

[0009] By having the capability of being replenished in situ, liquid surfaces that are subject to erosion (by impact of high-energy atoms) do not change their functional capabilities or shape over time. As long as a “make-up” liquid is added to the surface to replace the eroded liquid, device operation could continue endlessly, no matter how long the erosion lasts or how strong it is. The amount of liquid mass needed for this can be estimated to be relatively small and acceptable for many ET missions. This mass is, for example, often in the range of orders of magnitude less than the propellant mass.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the Figures, wherein like reference numerals refer to similar elements throughout the Figures.

[0011] FIG. 1 illustrates one embodiment where the energized propellant 100 interacts particularly strongly with the wall 101 near the propellant exit region. A liquid 110 covers a substrate 102 in this region.

[0012] FIG. 2 illustrates an embodiment where liquid at 110 is replenished from a reservoir 120 through flow in a channel along walls 150.DETAILED DESCRIPTION

[0013] Within fusion technology, one way that a liquid is held in place as a PFC is by wetting a surface that is designed to utilize wetting and capillary forces. In the field of PFCs for magnetically confined plasmas for fusion energy, such surfaces are referred to as a Capillary Pore System (CPS). These CPSs have small pores or other structures that are wet by the liquid. Wetting and capillary forces are also operative in the low-gravity environments of spacecrafts, and can hold a liquid in place in an ET. In other words, the liquid can be held in place passively by these forces.

[0014] Other forces can be used to hold the liquid in place. For example, electromagnetic forces can be utilized. As one example of many, this can be accomplished as described in the US patent application, Publication No. 2024 / 0206045, entitled "Magnetic means for coating surfaces with liquids" byinventor Michael T. Kotschenreuther, which is incorporated herein in its entirety by reference.

[0015] Under the action of erosion (e.g., sputtering), the liquid coating of surfaces will tend to be depleted: the eroded atoms leave the liquid and do not return. The liquid can be replenished from liquid in a separate reservoir. Various forces can be utilized to cause liquid in the reservoir to flow to the surfaces where liquid is being depleted.

[0016] Capillary forces could replenish the liquid coating surfaces in the ET. This action of capillary forces can be somewhat analogous to the action of liquids in heat pipes. Heat pipes are used in spacecraft to move heat from one region to another. In a heat pipe, liquid is evaporated from a surface that is covered by liquid (in a hot region). Liquid molecules or atoms leave this surface by evaporation, tending to deplete the liquid. The aspect of a heat pipe that is similar to some possible embodiments of the present invention is that liquid can be transported to the surface where it is being depleted, and the motivating impetus for this can be capillary action. This maintains the liquid coating, despite the action of the energized propellant that would tend to cause depletion of the liquid coating.

[0017] So, in the present invention, the liquid that is eroded (e.g., by sputtering) or evaporated from the surface facing the propellant region can be replenished by using capillary action to transport liquid from a separate reservoir to that surface. This mechanism has the advantage of being passive, that is, not requiring external means like pumps to be employed.

[0018] Alternatively, or in addition, electromagnetic forces might also be used to transport liquid to replenish that liquid on the surface from which it is being eroded. Such forces include jxB forces (that is, forces caused by the presence of a current j and a magnetic field B in the liquid, in the direction of the vector cross product of j and B). The relevant current might be applied externally, as by an electromotive force applied to the liquid by means external to the liquid. Many thruster types already have a magnetic field when in operation, and this could provide the magnetic field. Hence, a jxB force arises from the induced current. The currents would be induced in a direction and of a magnitude so that jxB forces move the liquid from a reservoir to the liquid coated surface in the thruster.

[0019] Alternatively, or in addition, the current might be generated by the electromotive force of the Seebeck effect acting between the LM and its substrate. The Seebeck effect causes an electromotive force between dissimilar materials, to drive current when there is a temperature gradient. This concept has been proposed for a different application, namely, the removal of heat from plasma PFCs in magnetically confined plasmas. (See “Lithium-metal infused trenches (LiMIT) for heat removal in fusion devices”, D.N. Ruzic et al 2011 Nuclear Fusion 51 102002.) Temperature differences can arise in an ET, for example, from the heating action of the energized propellant impacting the surface coated by liquid, leading to a Seebeck effect induced current in the LM. The resulting jxB forces can be used in the present invention, to induce flow from a reservoir to a surface in a thruster to replenish liquid on said surface that is eroded by being exposed to energized propellant. This mechanism also has the advantage of being passive, that is, not requiring external power to be applied to the liquid to induce a current.

[0020] Let us consider some ways that liquid PFCs can be used to combat erosion in some examples of ETs. These are but examples of many possible embodiments of this invention.

[0021] Hall Effect Thrusters (HETs)

[0022] Hall Effect Thrusters are one of the most prevalent types of thruster. They are used in many earth-orbiting satellites for orbit maintenance and attitude control. They are also considered for lunar, mars, and deep space missions, and for “space tugs.”

[0023] Many of these missions require very long operating lifetimes. And other satellite missions are evolving to often require longer lifetimes. Erosion is a significant problem for these missions, mentioned as examples, as well as other missions.

[0024] Recently, design concepts have evolved to reduce erosion in the main channel of the HET, by shifting the maximum of the magnetic field outside the channel. This is called “magnetic shielding.” But this then leads to other problems. Although lifetime is improved, this strategy exposes other elements of the thruster to erosion (e.g., the magnet poles and the cathode). And portions of the satellite or spacecraft can be exposed to erosion as well because the exhaust plume of energized propellantspreads out more with magnetic shielding. Erosion of these elements can then become the lifetime limiting process.

[0025] The use of liquid PFCs could address erosion issues in HETs, either with or without magnetic shielding.

[0026] By the use of liquid PFC to combat erosion on the magnet pole pieces, cathodes or other areas, the effects of erosion on those areas might be, in effect, virtually eliminated.

[0027] In addition, important thruster performance metrics are often reduced in various ways by magnetic field configurations (such as magnetic shielding) that target low erosion. Some of these include:

[0028] a) The thruster plume dispersion is increased, so thrust per unit power is reduced

[0029] b) The electron temperature tends to be increased, so that multiple ionization states of the propellant tend to be generated, reducing thrust per unit power

[0030] c) Propellant mass utilization can be reduced by affecting ionization.

[0031] These disadvantages of magnetic shielding, and others, are recognized within the field.

[0032] The use of liquid PFCs could allow a solution to the erosion problem without incurring the reduction in some performance metrics, including those above. Or the use of liquid PFCs in conjunction with some degree of magnetic shielding could allow operation with longer life with less performance reduction than would be possible using magnetic shielding alone.

[0033] Considering the latter possibility in more detail, the use of conductors in the channel has been shown to reduce the performance of magnetically unshielded thrusters, but it has little effect on the performance of magnetically shielded thrusters. By using a combination of magnetic shielding and using LM PFCs in the main channel near the exit, or just beyond the exit, performance reductions might be eliminated or reduced.

[0034] The exhaust plume from a HET can also cause erosion to other areas of the spacecraft. A liquid covered “shield” between the plume and those areas could reduce or eliminate this. And other parts of the spacecraft might be covered with liquids to protect them from erosion too.

[0035] Gridded Ion Thrusters (GITs)

[0036] A major drawback of GITs is the grid as it is in the path of the high energy ions, so it is subject to considerable erosion. These grids could be coated with very thin layers of a liquid, including possibly a liquid metal (LM). LM PFCs for fusion applications often have LM coatings in the range of several hundred microns. Even rather narrow grid structures could be protected from erosion by a liquid cover of such thickness.

[0037] The LM PFCs use wetting as a very effective mechanism to maintain such thin LM layers. They also frequently use molybdenum as substrate material, and GIT grids are often made of molybdenum as well. Several low melting point LMs have excellent compatibility with molybdenum, with extremely low erosion and strong wetting to keep them in place. One example of such an LM is tin (Sn), but there are numerous others.

[0038] A layer of LM can be maintained over time in various ways. In some embodiments capillary forces may suffice to draw liquid along the grid. Other methods may also be used to maintain the coating of LM on the grid as well. For example, as has been experimentally demonstrated in LM PFCs for fusion applications, that in the presence of a heat flux into the LM (which the accelerated ions would provide in a GIT), the Seebeck effect together with a weak magnetic field can provide propulsion of the LM along a substrate. A small magnetic field was used in the NEXT GIT, and this type of magnetic field could also be used for the novel purpose just mentioned. The Seebeck effect, for an LM on a substrate of molybdenum, tungsten, or other solid metal, causes small currents in the LM which, together with the external magnetic field, give jxB forces that can move the LM along channels to coat the metal. The grids could incorporate this structure to have a slow continuous flow of LM to keep them “wet”, or periodically recoat the grid with LM using this method.

[0039] Magneto Plasma Dynamic thrusters (MPDTs)

[0040] Magneto Plasma Dynamic thrusters have the potential to be very efficient at higher power levels. However, they have serious erosion issues with the electrodes. These electrodes are in intimate contact with the plasma, and they must be, since theytransfer current to the plasma to provide the motive force on the plasma which is also the propellant.

[0041] Consequently, especially at high power levels, the erosive effect of the plasma on the electrodes is quite strong. This results in a short lifetime of the electrodes, which strongly limits the utility of the thruster.

[0042] These electrodes could be coated with LM PFCs, so that eroded material could be replenished either in operation or between periods of operation.

[0043] Pulsed Plasma Thrusters and Arcjet Thrusters

[0044] Pulsed Plasma Thrusters and Arcjet Thrusters also suffer from erosion problems due to contact of surfaces with the hot plasma. Liquids could be used to coat such surfaces. One example of such surfaces is the electrodes.

[0045] Other electric thruster types also have problems with erosion, similar to the specific examples above. Liquid coatings on surfaces could be used to address this problem for many variants of ETs.

[0046] Further aspects of the use of liquid PFCs in ETs are described below.

[0047] An issue arises with LMs exposed to high-energy atoms. It has been found that the sputtering yield of many LMs increases very strongly for LM temperatures above about 300-500 C. Such temperatures, or higher, often arise in the surfaces in ETs. High sputtering for the liquids is disadvantageous because it would require more liquid to be supplied to replenish the sputtered material. This in turn would require that additional liquid be onboard the spacecraft or satellite for this purpose. Increasing the mass of the spacecraft is a disadvantage so increased sputtering of liquids at high temperature can be an undesirable phenomenon.

[0048] But there are means to reduce or eliminate the increased sputtering at high temperature. This is discussed at length in the patent application entitled “Liquid metal compositions for use as plasma facing components”, U.S. Patent Publication No. 2024 / 0203610, by inventor Michael T. Kotschenreuther, which is incorporated herein in its entirety by reference. That application discloses means to greatly reduce this increased sputtering at higher temperature, by employing alloys with multiple elements. These alloys segregate elements to the surface that reduce or eliminate the enhanced sputtering with higher temperature. Elements to segregate to the surface for this purpose include metals with a higher melting point. A means to segregate theseelements is to add to the alloy a small amount of non- metallic elements such as, but not limited to, O, N, F, P, S, C, and perhaps B and H. These same methods can be advantageous for liquids used in ETs. These methods could be adapted to accommodate the specific circumstances of ETs. For example, iodine (which is a halogen like F and Cl), is also considered as a propellant in some ETs (such as HETs). This iodine might play the role like F and Cl, of inducing surface segregation of a suitable metal to avoid strong sputtering at high temperatures. Such iodine could be added to the LM passively by its contact with the propellant plasma of iodine, or it could be added to the LM before it is used as a PFC.

[0049] Such methods can be used for ET liquids. Reducing the mass of the replenishment liquid would be advantageous. Because lower Z elements are lighter, it could be an advantage to have lower Z elements preferentially sputtered. But sometimes, the sputtering coefficient of low Z elements is high. Thus, the segregated surface might be low Z or high Z, depending upon the material that leads to the lowest mass loss rate from sputtering and evaporation.

[0050] There is another possible benefit of using liquids. Liquids could be used to supply the propellant for a thruster, by having the propellant dissolved in the liquid. When said propellant is normally solid, this could greatly ease the process of supplying the propellant to the thruster where it is energized. As but two examples of many, consider bismuth or zinc as propellants. These elements have been considered as a propellant for HETs, and could be used for other ETs. However, since they are solid by nature except at elevated temperatures, there is additional complexity in supplying this fuel to the thruster. But such metals could be dissolved in another liquid. One example of many could be an alloy of zinc or bismuth with gallium, indium, or tin. These alloys have far lower melting points than zinc and bismuth. This liquid could be supplied to the ET and the zinc or bismuth could be evaporated or sputtered to supply the propellant. The vapor pressure of gallium, indium, and tin is far less than zinc or bismuth, so these would not be evaporated. After supplying the fuel, the remaining liquid could be recirculated to be “refreshed” with new zinc or bismuth and then used to supply fuel again. This could be done continuously. Elements other than zinc or bismuth could also be used, for example, other metals with a low ionization potential and high vapor pressure. This can make it practical toconsider many advantageous elements for propellants that would otherwise be extremely difficult to use for this purpose.

[0051] As but one possible example of this, consider an MPD thruster. The electrodes, which are usually subject to strong erosion, could be coated by a liquid containing, as some of its constituents, propellant elements. The propellant would be evaporated or sputtered from the surface to supply the active propellant in the plasma region. The remaining liquid would protect the surface, and could be recirculated to incorporate new propellant after some residence time on the electrode.

[0052] As yet another embodiment of this invention, it is possible that the replenishing liquid can be solidified on the surface to be protected, so that the surface is primarily covered by a solid in operation, but is replenished from a reservoir while the surface is heated above the melting point, which allows liquid to easily flow.

Claims

CLAIMSWhat is claimed is:

1. An improved electric thruster that utilizes electrical energy to generate and accelerate a propellant to produce thrust, the improvement comprising a sacrificial liquid covering at least a portion of a thruster surface exposed to interactions with an energized propellant, including sputtering, chemical effects, and thermal effects.

2. The electric thruster of claim 1, wherein the exposed surface of the thruster includes at least one of an acceleration channel, electrode, grid, cathode, anode, nozzle surface, and magnet pole piece.

3. The electric thruster of claim 1, wherein the liquid is composed primarily or entirely by metal.

4. The electric thruster of claim 1, wherein the liquid is held in place by wetting of the surface it covers.

5. The electric thruster of claim 1, wherein the liquid is held in place by electromagnetic forces.

6. The electric thruster of claim 1, wherein the liquid is replenished from a reservoir.

7. The electric thruster of claim 6, wherein liquid is transported from the reservoir to the exposed surface at least partially by capillary forces.

8. The electric thruster of claim 6, wherein liquid is transported from the reservoir to the exposed surface at least partially by the action of electromagnetic forces.

9. The electric thruster of claim 8, where the electromagnetic forces arise at least partially due to electromotive action of the Seebeck effect between the liquid and the surface it covers.

10. The electric thruster of claim 1, wherein the liquid contains elements that are evaporated or sputtered to become propellant.

11. The electric thruster of claim 1, wherein the liquid comprises some components that segregate to the surface facing the energized propellant, so that sputtering or evaporation are dominated by the segregated components.

12. The electric thruster of claim 1, wherein the liquid on the surface absorbs the propellant, or some components of the propellant, that impinge upon it from the energized propellant.

13. The electric thruster of claim 12, wherein the absorbed propellant or components of the propellant segregate to the surface and reduce the amount of other elements in the liquid that are sputtered or evaporated from the surface.

14. The electric thruster of claim 1, wherein the propulsion mechanism includes, but is not limited to, electrostatic, electromagnetic, electrothermal, or any combination thereof, for spacecraft propulsion, including at least one of orbital maneuvers, station-keeping, attitude control, and deep-space propulsion.

15. An electric thruster comprising: a propulsion system that utilizes electrical energy to generate and accelerate a propellant to produce thrust, wherein the propulsion mechanism includes at least one of electrostatic, electromagnetic, and electrothermal forces; one or more exposed surfaces of the thruster exposed to interactions with an energized propellant, including sputtering, chemical effects and thermal effects during operation; and wherein at least a portion of the exposed surfaces is reconditioned during spaceflight by flowing a molten material onto the exposed surface and allowing the molten material to solidify on the exposed surface.

16. The electric thruster of claim 1 used for spacecraft propulsion, including at least one of orbital maneuvers, station-keeping, attitude control, and deep-space propulsion.

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