CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Phase application of International Application No. PCT/AT2018/060159 filed Jul. 24, 2018 which claims priority to Austrian Patent Application No. A 60138/2017 filed Dec. 12, 2017, the disclosures of which are incorporated herein by reference.

TECHNICAL FIELD

The disclosed subject matter relates to an ion thruster, in particular to an ion thruster for propulsion of spacecrafts.

BACKGROUND

Increased sophistication of missions for pico- and nano-satellites or general spacecrafts requires efficient and light-weight propulsion systems. For proper attitude, magnetorquers, tethers or reaction wheels are insufficient and do not allow, e.g., flight formation or any other mission which requires change of speed (Δv) capabilities. Small spacecrafts with tight power and mass budgets are reluctant to embed chemical or cold-gas based propulsion systems due to their limited specific impulse.

Electric propulsion systems offer a promising alternative. Avoiding moving parts drastically reduces system complexity and thus guarantees high reliability and durability. For example, ion thrusters and in particular field-emission electric propulsion (FEEP) systems are highly attractive for missions with increased specific impulse demands.

Ion thrusters create thrust by electrically accelerating ions as propellant; such ions can be generated, e.g., from neutral gas (usually xenon) ionized by extracting electrons out of the atoms, from liquid metal, or from an ionic liquid. Field-emission electric propulsion (FEEP) systems are based on field ionization of a liquid metal (usually either caesium, indium, gallium or mercury). Colloid ion thrusters, also known as electrospray thrusters, use ionic liquid (usually room temperature molten salts) as propellant.

The emission sites of ion thrusters are projections which have the shape of cones, pyramids, triangular prisms, or the like. To achieve the strong electric field necessary for ion extraction, the projections are sharp-tipped or sharp-edged to utilize the field-concentrating effect of the tip or edge.

Applying a strong 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's projection. In FEEP ion thrusters, neutral atoms of liquid metal at the apex of the Taylor cone evaporate from the surface. In the strong electric field between the emitter and the extractor, due to field emission negative electrons tunnel back to the surface, changing the formerly neutral atoms to positively charged ions. The thusly created ions are extracted from the Taylor cone and accelerated by the electric field. This principle of creating positive ions and accelerating them by the very same electric field is used to generate thrust. In colloid ion thrusters, already charged ions of an ionic liquid are extracted from the Taylor cone and accelerated by the electric field. The thrust can be controlled by the strength of the electric field. The sharper the emission site, the smaller is the base of the Taylor cone, leading to a higher efficiency of the thruster at any given ion current.

For transporting propellant to the sharp tip or edge of each projection passive forces, like capillary effects, and/or external forces, like pressure differences or centrifugal forces, are employed.

Three different types of emitters for transporting and emitting propellant are known. Firstly, emitters with solid projections, e.g. needles, are used, wherein the emitter and its projections have surfaces which are wetted by the propellant. Due to adhesion on the wetting surface of the emitter, the emitter and each projection is covered with a film of propellant. This technology is particularly attractive in terms of performance as the propellant flow impedance is high, but is also very prone to contamination or any effects that could com-promise or disrupt the propellant film. Solid emitter projections of this type are known, e.g., from US 2011/192968 A1 or US 2009/114838 A1 for colloid ion thruster applications.

Secondly, nozzle-type emitters with projections penetrated by capillary channels are used for propellant transport. Such capillary emitters have the advantage that the projections are resistant to contamination and the manufacturing is simple and reliable. This type of projections is known, e.g., from AT 500412 A1, U.S. Pat. No. 4,328,667 B for FEEP ion thrusters or from K. Huhn et al, “Colloid Emitters in Photostructurable Polymer Technology: Fabrication and Characterization Progress Report”, IEPC-2015-120, July 2015 for a salt-based colloid ion thruster. However, the exit opening of the capillary needs a minimum diameter mainly governed by manufacturing capabilities, thus leading to a larger Taylor cone and, hence, to lower efficiency in terms of thrust per propellant mass, i.e. a smaller specific impulse. To at least partly counteract this disadvantage, it is known from the cited prior art to cover the tips of the channeled projections with a material that is not wettable by the propellant to reduce the size of the Taylor cone.

Thirdly, porous emitters are known, e.g. from US2016/0297549 A1 or D. Krejci et al., “Design and Characterization of a Scalable Ion Electrospray Propulsion System”, IEPC-2015-149, July 2015 for ionic liquid ion thrusters. The material of the porous emitters and the projections thereof is wetting in respect to the propellant used. Such porous emitters combine the advantages of said first and second types of emitters as the porous projections transport high volume of propellant both inside and on their outer surfaces and allow for sharp tips or edges. Hence, porous projections offer both high specific impulse and robustness against contamination and the ion thrust can be precisely controlled. Using such porous emitters in long-term operation may, however, lead to undesirable loss of propellant or other functional and performance degradation or impairment which sometimes even causes a system breakdown.

BRIEF SUMMARY

It is thus an object of the disclosed subject matter to provide an ion thruster which is not only efficient and reliable but also durable.

This object is achieved with 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 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 supporting said projections and has pores or channels for providing flow of propellant from the reservoir to said projections.

The disclosed subject matter is based on the finding that the functional degradation or impairment as well as the loss of propellant in porous emitter type thrusters is a consequence of uncontrolled accumulation of propellant on the base between and around the porous projections due to propellant seeping through the base. This also leads to system breakdown in long-term operation. By making said first side of the base entirely impermeable to propellant, said seeping through the base and the accumulation of propellant can effectively be prevented and functional degradation or system breakdown can be avoided in the long-run as well as during manufacturing and ground-handling. Still, the advantage of the porous projections in terms of specific impulse and robustness against contamination is maintained.

In an advantageous embodiment, the entire base is made of a material impermeable to the propellant. Such a base can be manufactured easily and is reliable in use. While being impermeable for the propellant, the base is provided with porous or open channels connecting the projections with the reservoir for providing the necessary flow of propellant.

Optionally, the pores or channels of the base are covered with a material that is wettable by the propellant, which intensifies the capillary effect for ensuring passive propellant flow.

Alternatively or in addition thereto, it is favorable when said first side is coated with a coating impermeable to the propellant. Thereby, the base can be manufactured from a wide variety of materials—even from the same, particularly porous, material as the projections, which effectuates a very smooth flow of propellant. Nevertheless, the coating is entirely impermeable to the propellant, i.e., when made of porous material, the pores are blocked by the coating. If desired, the base and the projections can be a single, unitary piece of porous material manufactured in one step or, on the other hand, be separately manufactured and then connected, e.g. by additive manufacturing, welding or the like.

In an embodiment thereof, the coating extends over an adjacent portion of each projection. Hence, the projections can be arranged closer to one another on the base without accumulation of propellant between the projections. While keeping the same maximum thrust of the ion thruster, the size of the emitter can thereby be further reduced.

To prevent propellant from leaking at the connection of the emitter to the reservoir, it is beneficial when the coating extends over an adjacent portion of the reservoir.

The coating can be made of a wide variety of materials which also depend on the propellant. In an embodiment, said coating is also repellent to the propellant. Such a coating which is repellent to the propellant, i.e. non-wetting, prevents possible dripping of propellant from the projections and/or creeping of propellant along the surface. Thereby, the reliability of the ion thruster is further increased. Optionally, the coating is made of an epoxy resin, which has proven to be simple in use and reliable.

In a favorable embodiment the base and the projections are made of porous tungsten. Tungsten is very durable and can be produced having fine pores and sharp tips or edges. Moreover, when using liquid indium as propellant, tungsten also provides excellent wetting characteristics for the propellant, thereby increasing the reliable passive force of the capillary effect for transporting propellant within the ion thruster.

While the projections may be sharp-edged triangular prisms or sharp-tipped pyramids, in an advantageous embodiment the projections are needle-shaped, i.e. narrow, pointed cones. This shape effectuates a small Taylor cone and the circular cross section of the cones facilitates a homogenous flow of propellant.

In an optional embodiment the emitter has a multitude of projections arranged in a circle on said first side. Thereby, a single circular window in the extractor can be provided to generate a uniform electric field for all projections simultaneously. This is easier in manufacturing and alignment with the projections than a separate window in the extractor for each projection, which is common practice for ion thrusters.

For facilitating the flow of propellant, the reservoir optionally comprises an internal propellant guiding structure which leads to said second side of the base.

