Conductive Liquid Propellant Pulsed Plasma Thruster

Abstract

In one aspect of the present invention, a plasma thruster device is provided that includes an insulating substrate, the substrate including one or more supply channels for supplying a conductive liquid to a bridge structure, and further including electrical terminals. The bridge structure is configured to form an electrically conductive bridge when the conductive liquid is applied thereto. The bridge structure is configured to form contact areas in electrical contact with the electrical terminals, thereby connecting the contact areas. The bridge structure is configured to form a plasma of the conductive liquid when the conductive liquid is ionized by a current peak flow circuit connecting the contact areas via the electrical terminals.

Description

[Technical field]

[0001] The present invention relates to plasma thruster devices. [Background technology]

[0002] Classical pulsed plasma thrusters are known propulsion technology, e.g. used in satellites. Although the thrust-to-power ratio of pulsed plasma thrusters (PPTs) is limited, their simplicity, reliability, and often the use of solid (e.g. PTFE) propellants make them attractive for small satellite thrusters. These pulsed plasma thrusters can operate at high frequencies, so that a nearly continuous operation of the thruster can be obtained. Such known systems typically have two main electrical circuits. The first main electrical circuit is an ignition circuit, which may have, for example, a capacitive circuit for storing electrical power and a switching circuit for releasing this power and generating an electric arc. This electric arc causes ablation and ionization of a small portion of the propellant into a low-energy plasma. The second main electrical circuit generates an electric discharge through the formed low-energy plasma, which generates a Lorentz force due to the interaction of the magnetic field with the discharge current through the plasma. This Lorentz force accelerates the plasma out of the thruster. One advantage of PPTs is their simplicity in design and operation. This means that PPTs are extremely robust and can be made extremely small in nature, which is an advantage for modern small spacecraft. One drawback is that the thruster efficiency of classical PPTs is very low, resulting in a relatively low thrust-to-power ratio. Furthermore, the anode and cathode plates of the accelerator stage and the anodes and cathodes of the igniters (e.g. spark plugs) of the igniter discharge stage are subject to erosion.

[0003] The background of the advanced pulsed plasma thruster concept is described in "Design of a High-Energy, Two-Stage Pulsed Plasma Thruster" by TE Markusic, YCF Thio, and J. T Cassibry, presented at the 38th AIAA Joint Propulsion Conference, Indianapolis, Indiana, July 7-10, 2002. In this proposed structure, liquified lithium is pumped between electrodes located at one end of an acceleration channel, and the lithium droplets grow large enough to completely bridge the distance between the electrodes, thus closing an electrical circuit and thus discharging a high-power capacitor, which ionizes the lithium droplets. The resulting plasma is accelerated and ejected from the acceleration channel. Lithium has a low density (0.53 g / cm3) and can be liquified by heating it above its melting point of 454 K. Due to the size of the droplets, the generated plasma is slow and has low thrust power. A second electric acceleration stage is used to impart most of the energy for the acceleration stage to the plasma by an electric circuit, increasing the plasma velocity to generate thrust. This multi-stage process typically lasts for a few microseconds to perform a complete cycle that can be repeated. Also, in this known thruster, the electrodes for ionizing the propellant corrode after continuous use, so that the life span is limited, and the structure, materials, and geometry of this known plasma thruster are very limited in practical use. Therefore, the development of a small and reliable thruster device is desirable, but the system needs to be improved before miniaturization is possible. Summary of the Invention

[0004] In one aspect of the invention, the features recited in claim 1 are provided. Specifically, the plasma thruster device comprises an insulating substrate provided with electrical terminals and including one or more supply channels for supplying a conductive liquid to a bridge structure, the bridge structure is configured to form an electrically conductive bridge when a conductive liquid is applied; The bridge structure is configured to form contact areas in electrical contact with the electrical terminals, the bridge structure thereby connecting the contact areas, and the bridge structure is arranged to form a plasma of the conductive liquid when the conductive liquid is ionized by a current peak flow circuit connecting the contact areas via the electrical terminals. Using a liquid as a source material for ionization allows the conduction path to be regenerated after a current pulse. The electrical connection to the part that turns into plasma can be made through a line through which the liquid is supplied. This avoids electrode erosion, since the electrical contacts in direct contact with the plasma are liquids, which are replenished after each discharge. This feature provides regenerative electrodes and bridges between these electrodes, to which high voltages of several kV can be applied. This voltage application causes a large current with sharp peaks of several kA to flow through the bridge structure, which quickly heats up, melts and turns into plasma. The plasma expands at several km / s without the need for a dedicated accelerator stage. This single stage conductive liquid propellant pulsed plasma thruster has a good thrust-to-power ratio and can be used regeneratively to prevent wear or failure due to ionization of the propellant. This can have a high volumetric density, allowing it to be miniaturized for use in, for example, nanosats, and in some embodiments can generate high velocity (several km / s) plasma without the need for Lorentz force accelerators. [Brief description of the drawings]

[0005] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0006] [Figure 1] (A+B) An embodiment of a conductive liquid propellant pulsed plasma thruster device is shown. [Diagram 2] (A+B) Plan views of one embodiment of the present invention are shown. [Diagram 3] (A+B) A schematic graph of a current peak flow circuit. [Figure 4](A,B,C) Schematic process scheme for regeneratively operating a thruster device. [Diagram 5] FIG. 1 shows an exploded system view of a complete propulsion system of which a conductive liquid propellant pulsed plasma thruster is a subsystem. [Figure 6] 1 shows a diagram of thrust versus power for a propulsion system. [Figure 7] (A+B) An embodiment of an electrically conductive bridge is shown. [Figure 8] (A+B) Alternative structures of the electrically conductive bridge are shown. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by a person of ordinary skill in the art to which this disclosure pertains when read in the context of the description and drawings. It will also be understood that terms such as those defined in commonly used dictionaries should be interpreted as having a meaning consistent with the meaning in the context of the relevant art, and not interpreted in an idealized or overly formal sense unless expressly defined as such herein. In some cases, detailed descriptions of well-known devices and methods may be omitted so as not to obscure the description of the present system and method. The terminology used to describe certain embodiments is not intended to limit the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural, unless the context clearly indicates otherwise. The term "and / or" includes any or all combinations of one or more of the associated listed items. It will be further understood that the terms "comprise" and / or "comprising" specify the presence of the referenced features, but do not exclude the presence or addition of one or more other features. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In the case of conflict, the present specification, including definitions, will control.

[0008] The "substrate" may be a ceramic substrate or any other suitable non-conductive substrate, such as a silicon or silicon-like substrate (e.g. Pyrex). The substrate may be part of the satellite facing the bridge. The substrate is non-conductive, does not react with the conductive liquid, is hard, rigid and corrosion resistant. The "current peak flow circuit" may be a conventional circuit suitable for activating the plasma thruster device, i.e., ionization or plasmafication of the bridge circuit. Examples are shown in Figures 1 and 3.

[0009] The present invention, in some embodiments, relates to the field of nano satellites, and in particular CubeSats. Satellites typically have plasma thrusters to maintain or change course while orbiting the Earth. Space propulsion systems work on the principle of accelerating a working mass (propellant) to a high speed, thereby generating thrust and varying the speed of the spacecraft. The maneuverability of a satellite is usually expressed as the satellite's speed increase (or ΔV) provided by the propulsion system. Each type of maneuver requires a certain ΔV. If a satellite has to perform a series of specific maneuvers, its propulsion system must be able to generate a certain total ΔV, which is the sum of the ΔVs of the individual maneuvers. The total ΔV that a propulsion system can provide depends on the amount of propellant on board and the efficiency of generating thrust using this propellant. The "efficiency of a propellant" is usually expressed as "specific impulse" (Isp). This is the total impulse that a propulsion system can provide per unit of propellant weight (gravimetric specific impulse) or per unit of propellant volume (volumetric specific impulse). Due to the small size of CubeSats, the available propellant storage volume is limited, and therefore the total impulse (or total ΔV performance) of the thruster is also limited. The present invention relates to direct plasmaization of the propellant in a pulsed plasma thruster, eliminating the need for separate ignition and acceleration devices, thereby allowing the use of high density conductive liquids as propellants. Direct plasmaization, conductive liquid propellant pulsed plasma thrusters can provide improved thruster efficiency for nanosats while not occupying significant satellite volume.

[0010] FIG. 1 shows a schematic top view (A) and side view (B) of an embodiment in which a pulsed plasma thruster device 10 has a current peak flow circuit 30 (C). When the bridge 13 of the pulsed plasma thruster device 10 is shorted through the bridge circuit 12, it is ionized by the current peak flow circuit 30 to form a plasma. The current peak flow circuit 30 emits current into the bridge 13 to ionize it, which propels the plasma jet away from the substrate 11 by electrothermal acceleration. The current peak flow circuit is preferably a single stage circuit. A single stage circuit does not distinguish between physical multi-stage processes, such as in conventional pulsed plasma thruster types that distinguish between a plasma generation stage and an acceleration stage. In contrast, in a single stage plasma generation process, a current peak flow is applied to an electrical terminal, which instantly ionizes the bridge structure. Because the current pulse is very short and concentrated in the bridge, the propellant efficiency is very high, and therefore no second stage is required for thrust generation. This reduces complexity and eliminates problems associated with erosion of the electrodes of the second stage. The intensity of the pulse ensures that a large proportion of the bridge material is substantially transformed into plasma, increasing the energy efficiency. The current pulse is very short, typically on the order of less than 50 nanoseconds, more preferably on the order of less than 10 nanoseconds, and only a small portion of the energy is lost as heat. For example, the current peak flow circuit 30 comprises a capacitor that is charged to a high voltage, a switch, and a transmission line to the thruster device 10. When the capacitor is discharged through the transmission line to the thruster device 10, the plasma is propelled away from the substrate at a speed of up to 3 km / s. The bridge material 12, i.e., the conductive liquid, in which the high-speed plasma is formed in the bridge 13, has a relatively low electrical resistance, and the overall dynamic characteristics of the current peak flow circuit 30 are optimized to provide most of the capacitor's energy to the bridge 13 of the thruster device. By way of example and not of limitation, a resistance of about 2 Ω is considered to be the maximum value for the bridge resistance in some applications. The bridge structure can be small, approximately 200×300×5 micrometers in size, although other dimensions are suitable depending on the application and the propellant used.Using the values ​​of density and volume of the conductive liquid in the bridge, it is possible to calculate the mass of propellant that will be converted to plasma during each pulse cycle. To form a plasma, the material must first be heated to its boiling point, vaporized, and converted to plasma. Using appropriate values ​​such as the specific heat and enthalpy of vaporization, it is possible to calculate the amount of energy required to vaporize the bridge. Additional energy is required to further heat this vapor and convert it to a high temperature plasma. The resistance of the bridge 13 depends greatly on its shape, thickness, and length to width ratio, but should be reasonably low, for example, on the order of 0.1 to 5 ohms.

[0011] FIG. 1b shows a bridge 13 provided on an electrically insulating circuit board 11. The substrate 11 is provided with shallow basins formed by basin boundaries 120 and provided with electrical terminals 122. For example, the basins provide a conductive liquid layer thickness of less than 100 microns. One or more supply channels 123 are provided to supply the conductive liquid 12 to the basins 120. Preferably, the connections to the bridge structures widen and / or thicken sharply in the direction away from the central bridge structure so that the current density and resistance drop sufficiently quickly that these paths do not heat up and turn into plasma. To achieve this, the basins 120 forming the bridges 13 shape the conductive liquid 12 into a suitable shape, in this example a butterfly shape, and electrically connect it to electrical terminals 122 provided in or on the substrate 11. The basins 120 are thus configured to form a low-resistance electrically conductive bridge structure 13 provided on the insulating substrate 11 when the conductive liquid 12 is supplied thereto.

[0012] The bridge structure 13 provides an electrical connection (bridge) between the anode and the cathode and is arranged to form a plasma when the bridge structure 13 is ionized by a current peak flow circuit. The current peak flow circuit is provided by, for example, the current peak flow circuit 30 of FIG. 1 or an alternative circuit as shown, for example, but not limited to, in FIG. 3. In a preferred embodiment, the electrical terminal 122 is provided by a metal interconnect pad that is located under the conductive liquid 12 at the contact area 132 of the depression 120. Other suitable connections to the current peak flow circuit are feasible. Although the depression 120 is shown with a constant depth, the contact area / sides may have different depths relative to the bridge zone 13. Preferably, the bridge structure and the contact area extend along the substrate and can form a plasma jet substantially away from the substrate. For example, the depression provides a conductive liquid layer thickness of less than 100 microns for the bridge. The depression can be provided with wetting structures, such as localized roughening or materials that improve the wetting behavior of the liquid, to form an optimal bridge structure, preferably with a layer thickness of less than 10 microns. Schematically, a connection feed is shown between a reservoir 130 containing a conductive liquid and a feed 123 connected to the cavity 120. The anode and cathode sides of the cavity 120, which are electrically isolated from one another, have propellant reservoirs. The reservoirs, the feed channels, and / or the cavity may include heaters for liquefying a conductive liquid, such as a liquid metal. The feed reservoir is arranged to contain the conductive liquid and is coupled to one or more of the feed channels. It may include a liquid inlet and outlet mechanism for injecting and ejecting the conductive liquid from the feed reservoir.

[0013] In Fig. 2, the tapered zone II extends from the contact areas I to the bridge zone III to form a butterfly bridge structure. The bridge zone III defines the direction of the current flow along the shortest connecting path i between the contact areas I. The bridge zone III preferably has an elongation perpendicular to the shortest connecting path i. That is, at least a portion of the bridge zone III preferably has a width w defined between the opposing parallel sides, which is greater than a length l defined by the length of the parallel sides. In another preferred embodiment, the bridge zone is connected to the tapered zone II through a rounded edge in an intermediate zone IIIa between the bridge zone III and the tapered zone II to optimize the current flow and, in particular, optimize the plasma formation of the bridge structure 13 in the bridge zone III.

[0014] FIG. 3 shows an exemplary electrical setup of the plasma thruster device 10 in a current peak flow circuit 30. L and R are essentially parasitic in nature, i.e. as small as possible, and energy is unloaded in the bridge structure 13 after closing the switch S. The resistance of the bridge is important for the overall function of the thruster device, since it is part of the dynamic discharge of the capacitor to the bridge after closing the switch. The electrical circuit of the thruster device system consists of the capacitor C, the switch S, and the transmission line, all of which may be provided by a microcircuit. The circuit has a parasitic inductance L and a resistance / impedance R. The current peak flow circuit is coupled to the electrical terminal 122 of the bridge structure 13. The current peak flow circuit comprises a circuit for providing a current peak flow to the electrical terminal to ionize the bridge circuit 13.

[0015] The current in such a system can be written as:

[0016]

number

[0017] U O is the voltage across the capacitor, ω = circular frequency √(1 / LC), L = induction of the circuit, τ = time constant of the circuit (2L / R).

[0018] An example of such a discharge can be seen in FIG. 3B, which shows a 2 kV discharge with C=250 nF, R=200 mΩ, and L=20 nH.

[0019] FIG. 4 shows a schematic process scheme for regeneratively operating a thruster device. Starting from FIG. 4a, a plasma thruster device 10 is shown in top and side views before discharge. The device is provided by an electrically insulating (ceramic or other electrically non-conductive material) substrate and includes a shallow butterfly-shaped reservoir. The reservoir is filled with a conductor of conducting liquid, forming a conducting "bridge" in the middle. The conducting liquid can be fed to the reservoir via a feed channel, e.g. a small capillary at the bottom. Alternatively, an (electromagnetic) pumping device can be used that can heat the conducting liquid at the same time. The conducting liquid can be an ionic liquid, a molten salt, a liquid metal, or any other substance that can be used in liquid form and has sufficient electrical conductivity. The liquid can be a pure substance or a mixture, or a fluid with suspended solid particles. Ideally, the liquid has a low or negative vapor pressure so that it does not evaporate on its own when exposed to a vacuum space. Also, a melting point of about room temperature is preferred. This is because spacecraft are typically maintained at near room temperature, and once a liquid is at this temperature, less energy is required to turn it into a liquid and keep it liquid. Finally, high density liquids are desirable for space applications, since the limiting parameter in small satellites is usually volume, not mass. High density propellants allow for high specific impulse to volume.

[0020] In general, liquid metals are suitable for the intended space application, since metals have sufficient conductivity and high density. Examples of pure metals that can be used as propellants include gallium, indium, tin, cadmium, lead, bismuth, lithium, sodium, potassium, and mercury. Alloys of these and other metals are also of interest. All of these examples have different suitability for applications due to their inherent properties, such as density, conductivity, reactivity, toxicity, vapor pressure, melting point, molecular weight, specific heat, surface tension, surface wetting properties, chemical compatibility with other materials, and other possible properties. Gallium and its low melting alloys, such as gallium-indium eutectic and gallium-indium-tin ("GalInStan"), are suitable. The supply of conductive liquid to the bridge after each discharge can be accomplished in several ways. These include: 1) "normal" mechanical pumping systems such as rotary or positive displacement pumps; 2) pressure-fed pumping by pressurizing the system's propellant tanks with some kind of gas; 3) electromagnetic pumping; 4) magnetic forces applied by moving electromagnets or permanent magnets (if the liquid has sufficient magnetic susceptibility or sufficient ferromagnetic properties (e.g., due to suspended iron particles in the liquid); or 5) capillary action (the inherent affinity of the liquid for the surface of the system, or electrowetting on a dielectric (EWOD)).

[0021] In one embodiment, the bridge material can be liquid gallium, which is relatively non-toxic, has a low melting point (30°C), a high density (5900 kgm-3), and good electrical properties. Additionally, gallium has negligible vapor pressure, thus preventing evaporation when exposed to a vacuum space.

[0022] 4B shows the thruster device 10 during discharge when a current peak flow is released from the current peak flow circuit (not shown). The rapid dissipation of electrical energy causes explosive ionization of the bridge 13. An expanding plasma is released, which generates a small force (thrust) in the opposite direction.

[0023] FIG. 4C shows the bridge in schematic regeneration mode after the discharge of FIG. 4B. Initially, there is a gap 15 between the left and right conductive liquid reservoirs, which interrupts the electric circuit 30 (see FIG. 1). To regenerate the bridge structure 12, one or more supply channels are arranged to repeatedly inject conductive liquid into the recesses before providing a current peak flow. Conductive liquid then flows from the reservoirs to the center, thus closing the gap between the reservoirs 16. During this process, the reservoirs are resupplied with conductive liquid from the supply channels. At the end of this process, the bridge is fully restored and ready for another discharge back to FIG. 4A.

[0024] 5 shows a schematic system diagram of an architecture that may be used in a plasma propulsion device according to the principles detailed above. The propulsion system may further include subsystems such as:

[0025] Thrust Generation System (TGS) This subsystem includes a plasma thruster device as disclosed above, arranged to generate a small thrust using the principle of regenerative bridge structures. The device may include one or more regenerative bridge structures (e.g., in an array) and an electrical circuit including one or more switches and one or more capacitors (a switched capacitor array or SCA). Additionally, a heater may be required to keep the liquid metal propellant in a liquid phase.

[0026] Propellant Feed System (PFS) This subsystem stores conductive liquid propellant, keeps it above its melting point (30° C. for gallium and 10° C. for Galinstan), and supplies it to the TGS, as shown, for example, in FIG 1. The PFS may include a Propellant Storage Tank (PST), a Conductive Liquid Propellant (PROP), a Propellant Injection and Ejection Assembly (FDA), a Thermal Insulation System (TIS), a Capillary Feed Assembly (CFA), and optionally a Tank Heater (TH).

[0027] Power Control System (PCS) This subsystem contains power electronics for distributing power to the various subsystems and subassemblies, as well as for generating high voltages to charge the capacitors of the current peak flow circuits. The PCS may consist of a High Voltage Power Supply (HVPS), a Low Voltage Power Control System (LVPC), and a Digital Control Unit (DCU).

[0028] A conductive liquid propellant pulsed plasma thruster as disclosed herein uses a conductive liquid propellant (such as liquid gallium) rather than an insulating solid propellant.

[0029] Conductive liquid propellant pulsed plasma thruster devices do not require an ignition device because the propellant is already conductive, and therefore generate one discharge per pulse (rather than an "ignition" discharge and a "main" discharge).

[0030] An electrically conductive liquid propellant pulsed plasma thruster device uses a switch to close an electrical circuit and trigger a discharge.

[0031] The discharge in a conductive liquid propellant pulsed plasma thruster device is an order of magnitude shorter than in a conventional pulsed plasma thruster device (i.e., ∼0.5 μs instead of ∼10 μs), allowing for a larger discharge current and improved energy coupling with the propellant.

[0032] The conductive liquid propellant pulsed plasma thruster device does not have a physical electrode to generate a discharge. The propellant cavity acts as an electrode and regenerates after discharge. Therefore, the conductive liquid propellant pulsed plasma thruster device is not affected by electrode erosion.

[0033] Specific impulse to weight is directly related to the exhaust velocity of the propulsion system.

[0034]

number

[0035] In this equation, Isp_grav is the specific impulse by weight [s], Ueff is the effective exhaust speed [ms-1], and g0 is the gravitational acceleration at sea level [ms-2]. This equation shows that to obtain a large specific impulse by weight (high mass efficiency), the propulsion system must be able to accelerate the propellant at a high speed. In electric or thermoelectric plasma propulsion systems, the relationship between power consumption, specific impulse, and thrust level is given by the following equation:

[0036]

number

[0037] In this formula, P is the power consumption [W], Isp is the weight specific impulse [s], g0 is the gravitational acceleration at sea level [ms-2], and ηt is the thrust efficiency [-], which is the ratio of the kinetic jet power of the exhaust plume to the electrical input power to the propulsion system. The thrust efficiency is the product of several sub-efficiencies that take into account the losses of the various energy conversion steps in the propulsion system. Based on experimental data, we can assume a value of at least ηt = 0.25, which is a conservative estimate of the thrust efficiency. Since it is important for nanosats that the propulsion system occupies as little volume as possible, we can optimize nanosat propulsion systems for the maximum volumetric specific impulse, which is simply the product of the weight specific impulse and the propellant density.

[0038]

number

[0039] In this formula, I vol is the volumetric specific impulse [kgsm -3 ], and ρ p is the density of the propellant [kgm -3

[0011] . Therefore, to obtain a large specific impulse by volume, it is preferable for the propulsion system to operate at a large specific impulse by weight and / or to use a high density propellant. The disclosed plasma thruster uses as a propellant a conductive liquid, e.g. a liquid metal such as gallium or galinstan. This has a density 2.7 times that of the solid propellant used in conventional plasma thrusters that use solid PTFE as the propellant (i.e. 2200 kgm -3 5900kgm -3 ). The weight specific impulse of the propulsion system can be calculated by Equation 1 and can be equal to 408s at a plasma velocity of 4000 m / s. This is a conservative estimate and can be significantly higher. The volumetric specific impulse of the propulsion system can be calculated by Equation 2 and is the product of weight specific impulse and propellant density. If the weight specific impulse is 408s and the propellant density is 5907 kgm-3 (density of gallium at 1 atm and 298.15 K), the volumetric specific impulse can be about 2.4×10^6 kgsm-3 or more. This means that a conductive liquid propellant pulsed plasma thruster can operate at 2.7 times lower weight specific impulse than a conventional plasma thruster while having the same volumetric specific impulse and a significantly higher thrust-to-power ratio. Since the thrust-to-power ratio is inversely proportional to the weight specific impulse, a thrust-to-power ratio of 2.7 times is obtained as a result. The conductive liquid propellant pulsed plasma thruster concept has the potential to achieve significantly higher thrust-to-power ratios at the same volumetric specific impulse or significantly higher volumetric specific impulse at the same thrust-to-power ratio compared to conventional plasma thrusters. Figure 6 shows a diagram of thrust (in mN) versus power of the propulsion system. The amount of power available to the propulsion system depends heavily on the size of the satellite (i.e., the area of ​​its solar panels). For nanosats, the power available for propulsion can be between 10W and 15W. Assuming a power budget of 10W, the propulsion system can generate a thrust of about 0.75mN. The total ΔV that can be delivered by the propulsion system depends on the amount of propellant on board and the specific impulse of the propulsion system. This relationship is given by Tsiolkovsky's rocket equation:

[0040]

number

[0041] In this formula, m0 is the initial satellite mass including propellant [kg], and mp is the propellant mass [kg]. The total propellant mass (mp) depends on the volume allocated to the propulsion system and the volume loading factor of the propulsion system (i.e., the fraction of the propulsion system volume occupied by propellant). Consider a hypothetical nanosat with the following mass and volume distribution:

[0042] Satellite mass without propellant: 5kg. Total satellite volume (including propulsion system (PS)): 6L. Volume allocated to PS: 1L. With a propellant loading rate of 75% in the propulsion system, the total propellant mass is 4.4kg and the initial satellite mass (including propellant) is 9.4kg. Substituting these values ​​into Equation 4, the total ΔV is 2300ms-1.

[0043] Alternative Embodiments Figure 7a shows an embodiment of an electrically insulating substrate 110. This substrate includes one or more supply channels 123.1 and 123.2 for supplying a conductive liquid to a bridge structure 120 configured to form an electrically conductive bridge when a conductive liquid is applied. In this example, the supply channels are formed by opposed orifices that connect to a supply reservoir (not shown). In Figure 7b it is further shown that additional supply channels can be provided in the form of small orifices 130 in the bridge substrate 110 through which the liquid is supplied. This can have the advantage of fixing the bridge geometry in place. These orifices can have a capillary action or be fed by an active supply mechanism such as an electromechanical pump (not shown).

[0044] Figures 8a and 8b show an alternative configuration of an electrically insulating substrate 110. The substrate includes one or more supply channels 123.1 and 123.2 for supplying a conductive liquid to a bridge structure 120 configured to form an electrically conductive bridge when a conductive liquid is applied.

[0045] In Fig. 8a, the bridge 120 is a shallow meniscus formed between an annular orifice 123.1 and a central orifice. Both of these orifices can be fed by a feeding mechanism of the type described above. The substrate 110 in Fig. 8a can have an extension 115 extending, for example tubular, from the substrate 110 to direct the plasma generated by the bridge structure 120 in the axial direction of the central orifice 123.1 away from the substrate.

[0046] Figure 8b shows an alternative bridge structure. A liquid bridge is formed by the cohesive forces of opposed orifices 123.1, 123.2 fed by a liquid supply mechanism. The bridge 120 may be free-standing, i.e. it does not need to be in contact with the substrate 110.

[0047] 1. The operating principle of the present invention allows the use of high density propellants, resulting in a high volumetric ISP. While a conventional pulsed plasma thruster may have a similar weight ISP, the present device can improve the volumetric ISP by allowing high density propellants. The volumetric ISP is the product of the weight ISP and the propellant density. This provides the advantage of forming devices in small volumes, for example in nanosats where volume is a limiting factor.

[0048] 2. The single stage direct plasma generation, without the pre-ionization stage and second acceleration step as in conventional thrusters, provides thruster pulses on significantly shorter time scales than conventional pulsed plasma thrusters. Therefore, a high thrust-to-power ratio can be achieved due to the high thruster efficiency on these short time scales. This offers the advantage of more efficient energy conversion, but limits the weight ISP due to the absence of a second acceleration stage. Thus, the thruster of the present invention is more energy efficient in providing specific impulse while using high mass propellants.

[0049] 3. The liquid bridge formed by the bridge structure simultaneously suppresses the deterioration of the electrodes. The electrodes are made of liquid metal that can be regenerated by continuously supplying conductive liquid to the bridge structure.

[0050] Although exemplary embodiments have been shown for the systems and methods, alternative ways may be envisioned by one of ordinary skill in the art having the benefit of this disclosure to achieve similar functions and results. For example, some components may be combined or divided into one or more alternative components.

[0051] For example, the above discussion is intended to be merely illustrative of the present system, and should not be construed as limiting the appended claims to any particular embodiment or group of embodiments. Thus, while the present system has been described in detail with reference to certain exemplary embodiments, it should also be recognized that numerous modifications and alternative embodiments may be devised by those skilled in the art without departing from the scope of the present system and method as set forth in the following claims. Accordingly, the specification and drawings are to be regarded as illustrative, and not intended to limit the scope of the appended claims.

[0052] In interpreting the appended claims, it should be understood that the word "comprising" does not exclude the presence of elements or acts other than those recited in a given claim. The words "a" or "an" preceding an element do not exclude the presence of a plurality of such elements. Reference signs in the claims do not limit their scope. Several "means" can be represented by one or more identical or different items, or by implemented structures or functions. Unless otherwise stated, any of the disclosed devices or parts thereof can be combined together or separated into different parts. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. an insulating substrate (11) including one or more supply channels (123) for supplying a conductive liquid (12) to the bridge structure (13) and provided with electrical terminals (122); the bridge structure (13) being configured to form an electrically conductive bridge when the conductive liquid (12) is applied; the bridge structure (13) is configured to form a plurality of contact areas (132) in electrical contact with the electrical terminals (122) when the conductive liquid (12) is applied, the bridge structure (13) thereby electrically connecting the plurality of contact areas (132) when the conductive liquid (12) is applied; the bridge structure (13) is arranged to form a plasma of the conductive liquid (12) when the conductive liquid (12) is ionized by a current emitted by a current peak flow circuit (30) formed by connecting the plurality of contact areas (132) via the electrical terminals (122). A plasma thruster device (10).

2. The plasma thruster device (10) of claim 1, wherein the bridge structure (13) and the plurality of contact areas (132) extend along the insulating substrate.

3. The plasma thruster device (10) of claim 1 or 2, wherein the current peak flow circuitry comprises a circuitry for providing a current peak flow to the electrical terminals for ionizing the electrical conductive bridge.

4. 4. The plasma thruster device (10) of claim 3, wherein the one or more supply channels (123) are coupled to a bottom of the recess (120) for repeatedly injecting conductive liquid (12) into the recess (120) prior to providing the current peak flow to regenerate the electrically conductive bridge.

5. The plasma thruster device (10) of any one of claims 1 to 4, wherein the current peak flow circuit (30) is a single stage circuit.

6. the one or more supply channels (123) are provided by capillaries; The plasma thruster device (10) of any one of claims 1 to 5, wherein the conductive liquid (12) is supplied by capillary action through the supply channel.

7. The plasma thruster device (10) of any one of claims 1 to 5, wherein the conductive liquid (12) is supplied through the supply channel by an electromagnetic pump.

8. The plasma thruster device (10) of any one of claims 1 to 7, wherein the conductive liquid (12) is a liquid metal having a liquid state in a temperature range of -50°C to +100°C.

9. The plasma thruster device (10) of claim 8, wherein the liquid metal (12) comprises one of the group consisting of gallium, mercury, cesium, rubidium, and galinstanium.

10. a supply vessel (130) arranged to contain the conductive liquid (12); the supply vessel is coupled to the one or more supply channels (123); The plasma thruster device (10) of any one of claims 1 to 9, wherein the supply vessel includes a liquid inlet and outlet mechanism for injecting and ejecting the conductive liquid from the supply vessel.

11. The plasma thruster device (10) of claim 5, further comprising a heater (124) for heating the cavity (120) and the supply channel (123) to liquefy the conductive liquid (12).

12. 12. The plasma thruster device (10) of claim 1, wherein the bridge structure (13) is formed by a tapered recess extending from the contact areas (I) to a bridge zone (III) that defines a direction of current flow along a connecting path between the contact areas (I).

13. 13. The plasma thruster device (10) of claim 12, wherein the bridge zone (III) is connected to the tapered recess through a rounded edge.

14. The plasma thruster device (10) of any one of claims 1 to 13, wherein the electrical terminals (122) are provided by means of metal pads.

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