Metallic plasma thruster and control circuit for satellites

Abstract

The Pulsed Metal Plasma Thruster (MPT) cube comprises multiple thrusters. Each thruster has a first cathode electrode, a trigger electrode separated from the first electrode by sufficient insulator to sustain an initiating plasma, and a porous anode electrode positioned a distance away from all sides of the cathode electrode. The cathode electrode can be either the inner or outer electrode. A power supply supplies a high-voltage pulse to the trigger electrode sufficient to initiate a plasma on the surface of the insulator relative to the cathode electrode. This plasma travels between the anode electrode and the cathode electrode of a selected thruster, thereby generating a thrust pulse.

Description

[Technical Field]

[0001] The present disclosure relates to plasma thrusters for use in satellites. In particular, the present disclosure relates to metal plasma thrusters (MPTs) that generate pulsed thrust through a series of plasma generation cycles. [Background technology]

[0002] The next decade is expected to see a dramatic increase in the number of small satellites launched into low Earth orbit (LEO). These satellites range in mass from pico- and femto-satellites (less than 1 kg) up to nano- (1 kg–10 kg) and micro- (10 kg–100 kg). Generating useful in-space propulsion is challenging for low-mass pico- and femto-satellites because the mass of the fuel required for even small maneuvers exceeds the mass of the satellite. Larger satellites, such as micro-satellites (100 kg–500 kg), and even larger satellites, can utilize a wide range of well-tested space propulsion systems, including chemical rockets (hydrazine fuel, Busek), electrothermal arcjets (Aerojet), or electric propulsion thrusters, such as Hall thrusters and ion engines (Busek).

[0003] Thousands of nano- and microsatellites are expected to be deployed into low Earth orbit (LEO) to perform multiple functions, from imaging the Earth for agricultural and disaster management purposes to providing internet services. These satellites require onboard propulsion systems for stationkeeping, attitude control, or orbital maneuvering. While electric propulsion systems are more fuel-efficient than chemical propulsion systems, new forms of electric propulsion are required to meet the low mass and small size constraints imposed by these satellites. Scaling down existing electric propulsion engines, such as xenon ion engines and Hall thrusters, to power levels of approximately 1 W to 10 W is not practical. The unavoidable additional mass of propellant storage tanks, flow control devices, and piping in xenon ion engines, along with the increased magnetic field associated with miniaturization in Hall thrusters, unacceptably reduces the overall efficiency of Hall thrusters at these power levels.

[0004] One type of prior art plasma thruster is known as the PTFE pulsed plasma thruster (PPT). This PTFE pulsed plasma thruster uses a PTFE propellant in a configuration in which the PTFE is positioned between two electrodes, a plasma is generated between the electrodes, and the PTFE is consumed in a series of high-velocity plasma jets that generate the desired thrust. A drawback of PTFE is that the plasma is generated across the electrically insulating PTFE, necessitating the generation of a high initiation voltage. Therefore, a typical energy storage device is a high-voltage capacitor that is charged to approximately 2 kV per discharge cycle. Additionally, prior art PPTs require a spark plug trigger to initiate the discharge. This spark plug trigger is charged to an even higher voltage, typically 5 kV to 10 kV, because the plasma is generated against the insulating PTFE propellant. PPTs thus use high-voltage components, which require a larger insulating gap in the thruster assembly than is required in alternative low-voltage thrusters that form the plasma on a pre-metallized film surface. The plasma from the PPT that generates thrust is composed of carbon and fluorine ions. As such, the exhaust velocity of this carbon / fluorine plasma is in the range of 5 km / s to 6 km / s, a relatively low exhaust velocity in contrast to other alternative thruster types, which can be in the range of 8 km / s to 20 km / s. The amount of propellant required to accomplish a space mission is exponentially dependent on the exhaust velocity. Therefore, the relatively low exhaust velocity of prior art PPTs makes them a less desirable option.

[0005] Another type of thruster is the vacuum arc thruster (VAT), which uses two electrodes in combination with an insulator coated with a very thin deposited metal layer. Prior art VATs rely on energy stored in an inductor to generate a discharge plasma across the insulator, resulting in metal vaporization and corresponding thrust. In prior art VATs, the inductor is first charged to a first current threshold through a switch, triggering the switch's opening. When the switch opens, inductive energy is released, creating an induced voltage peak LdI / dt. This induced voltage peak LdI / dt generates a plasma arc by initially forming microplasmas across a microgap formed by a break in a thin conductive surface applied to the surface of an insulating separator positioned between the anode and cathode electrodes. Multiple initial microplasma sites assist in the initiation of the main plasma discharge. These microplasmas spread into the surrounding space, allowing current to flow directly from the cathode to the anode along a plasma discharge path with lower resistance (hundreds of milliohms) than the initial thin-film surface discharge path. The current that was flowing through the solid-state switch before it was actively opened (for about 100µs to 500µs) is completely switched to the vacuum arc load. A typical current of about 100A (for about 100µs to 500µs) is conducted at a voltage of about 25V to 30V. As a result, most of the magnetic energy stored in the inductor is converted into a plasma pulse. The drawback of this VAT is that during the charging cycle, the storage inductor and I 2R is the energy dissipated by the switch losses. During this phase of increasing inductor energy, current flows through the inductor and switch but not the arc discharge, because the voltage required to trigger the arc is generated only after the switch is opened. Both the inductor and switch are dissipative elements, and therefore a portion of the energy in each cycle is dissipated as heat in these elements. Another drawback of VAT is the redeposition of metal ejected from the cathode onto the insulating layer. Some prior art VAT geometries rely on arranging the anode and cathode together as parallel electrode plates that extend beyond the insulator in the non-thrust direction to prevent plasma formation in those regions. However, devices with this configuration have a shorter-than-desirable cycle life before film redeposition occurs that exceeds the amount that the inductor can vaporize at the start of the arc. Ultimately, this limits the number of thrust cycles the device can achieve. Furthermore, wear on the cathode electrode creates an asymmetry that reduces the number of usable discharges. Another prior art VAT geometry has a series of ring electrodes acting as anode / cathode pairs. This configuration requires the use of an energy storage inductor as a combined energy storage and particle redirection structure to redirect ejected ions perpendicular to the desired thrust direction, guided by the axial magnetic field of the storage inductor and an external cathode, as described in commonly owned U.S. Patent No. 7,518,085.

[0006] A different type of prior art plasma thruster is the field emission electric propulsion (FEEP) device. This device ejects ions as a continuous stream from a needle electrode. The ions in the FEEP are supplied by a non-toxic liquid salt stored in a passive tank. The liquid is wicked along the electrode tip by capillary action. The salt can consist of both positive and negative ions with masses in the range of several hundred daltons. By periodically changing the polarity of the extraction potential, positive and negative ions are extracted, forming a charge-neutral ion beam that is ejected from the thruster. If only positive ions are extracted from the salt-containing ions, a separate electron gun is required to neutralize the beam. Without a separate electron gun, the ejection of ions would cause the spacecraft to assume a negative potential, which would in turn pull the ions back toward the spacecraft, resulting in no thrust. The conventional focused ion beam (FIB) source on which these FEEP thrusters are based uses a potential of approximately 10 kV across a gap of approximately 1 mm. In the case of FEEP thrusters, this potential is even lower (approximately 1 kV), and therefore these gaps must be even smaller (approximately 100 μm). The narrow gaps and high electric fields between the electrodes in this device create challenges in manufacturing millions of emitters that operate reliably in sync, resulting in reliability and manufacturability issues. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] U.S. Patent No. 7,518,085 Summary of the Invention [Problem to be solved by the invention]

[0008] It is desirable to provide an improved plasma thruster that operates with ions at higher ejection velocities than those in a PPT, has improved reliability and lifetime over prior art VATs, and uses components that are simpler to manufacture and more operationally reliable than FEEP thrusters. [Means for solving the problem]

[0009] In one embodiment, the present disclosure relates to a metal plasma thruster comprising a porous anode, an inner trigger electrode, an inner insulator surrounding the inner trigger electrode and having a conductive coating, the conductive coating having a microgap, the conductive coating being in contact with the inner trigger electrode, a metal cathode surrounding the inner insulator, the metal cathode having an area and a surface exposed to the anode, and an outer insulator surrounding the metal cathode, the outer insulator having a conductive coating, the metal cathode has an end portion and a surface exposed to the porous anode; the inner insulator has an extent from the end portion that is smaller than that of the metal cathode; the conductive coating of the outer insulator has an extent from the cathode surface that is smaller than that of the metal cathode; the metal cathode surface is square, rectangular, or circular; and the outer trigger electrode has one or more conductive regions for forming a starting plasma with the metal cathode.

[0010] In another embodiment, the present disclosure relates to a metal plasma thruster comprising a porous anode, a metal cathode, an outer insulator surrounding the metal cathode, the outer insulator having a conductive coating, the conductive coating having a microgap, and an outer trigger electrode in contact with the conductive coating of the outer insulator, the metal cathode having a termination end and a face exposed to the porous anode, the face of the metal cathode being square, rectangular, or circular, and the outer trigger electrode having one or more conductive regions for forming a starting plasma with the metal cathode.

[0011] In yet another implementation, the present disclosure relates to a metal plasma thruster comprising: an inner trigger electrode; an inner insulator surrounding the inner trigger electrode; a cathode electrode having a surface, the cathode electrode surrounding the inner insulator; an outer insulator surrounding the cathode electrode; an outer trigger electrode surrounding the outer insulator and having a surface substantially continuous with the central cathode electrode surface and the outer insulator, the surface of the outer insulator providing a plasma-forming surface for the surface of the cathode electrode, at least one of the inner insulator or the outer insulator having a first surface that is coplanar with the central cathode electrode surface and a second surface located proximate the first surface and shielded from redeposition of material ejected from the central cathode surface; and a porous anode positioned opposite the cathode surface.

[0012] In yet another implementation, the present disclosure relates to a metal plasma thruster comprising: an inner trigger electrode having a surface; an inner insulator surrounding the inner trigger electrode; a cathode electrode surrounding the inner insulator; an outer insulator surrounding the cathode electrode; an outer trigger electrode having a surface, at least one of the inner insulator or the outer insulator having a first surface parallel to the cathode electrode surface, the inner insulator or the outer insulator having at least one second surface that shields against redeposition of material ejected from the cathode electrode surface; and a porous anode electrode positioned opposite the cathode electrode surface.

[0013] In yet another embodiment, the present disclosure relates to a power supply for a metal plasma thruster (MPT) having an anode output, a reference output, and a trigger output, the power supply comprising: a first capacitor coupled to a charging source and connected to the reference output and having a voltage terminal; an inductor positioned between the anode output and the capacitor voltage terminal; a switch element positioned between the anode output and the reference output; a second capacitor disposed between the anode output and the trigger output; and a controller that maintains the switch open during a charging period until the first capacitor reaches a threshold voltage or alternatively maintains the first capacitor at the threshold voltage until a thrust event is required, the controller then closes the switch for a period sufficient to allow the inductor to generate a current necessary to form a trigger arc when the switch is later opened, the trigger arc being formed between a trigger electrode connected to the trigger output and a cathode connected to the reference output, the trigger arc then causing plasma formation between the cathode connected to the reference output and an anode connected to the anode output.

[0014] In one example of the present disclosure, a metal plasma thruster (MPT) has an inner trigger electrode surrounded by an inner insulated trigger plasma initiator, the inner insulated initiator comprising an insulator having a conductive surface with a microgap and surrounded by a cathode electrode. The cathode electrode is surrounded by an outer insulated initiator comprising an insulator having a conductive surface with a microgap, the outer insulated initiator being surrounded by an outer trigger electrode. In one example configuration, a porous anode electrode is positioned a spaced distance from the face of the cathode.

[0015] In another example configuration, the thruster has a cathode electrode with a surface that generates plasma on the processing surface of the inner insulator and inner trigger electrode or on the processing surface of the outer insulator and outer trigger electrode. The inner or outer trigger electrode generates plasma on the inner or outer insulator, respectively. Once the plasma is generated near the cathode surface, a very long thrust cycle is initiated, where the plasma is generated between the cathode surface and a porous anode, and metal ions of the plasma are hydrodynamically accelerated through the porous anode, generating the thrust cycle. For use on a satellite, multiple thrusters are positioned on a cube to provide three-axis thrust application.

[0016] The inventors have realized that the disclosed metal plasma is much more energy efficient than prior art PTFE PPTs, which require a high-voltage trigger and release a slow propellant, or VATs, which require a storage inductor with a charging cycle in which approximately 25% to 50% of the stored energy is lost. The present MPT charges the inductor over a very short time (less than 100 μs) compared to the arc discharge time (approximately 3 ms to 6 ms). This means that less than 5% of the charge stored in the MPT storage capacitor is discharged by the time the switch is opened, allowing more than 95% of the charge to flow into the arc discharge plasma and contribute to thrust. In contrast, VATs require the switch to be opened after the pre-discharge current has reached its maximum value, resulting in less than 50% of the stored charge flowing into the arc.

[0017] In another example of the present disclosure, a power supply has a ground reference, a trigger output, and an anode output for connection to the cathode, trigger, and anode of a metal plasma thruster, respectively. The power supply has a controller that implements a variable reservoir capacitor charging time and a variable inductive switch time. The power supply and controller operate to generate thrust events from the metal plasma thruster, the power supply having a first reservoir capacitor with one terminal connected to a charging source and the other terminal connected to a ground reference. An inductor is positioned between the first capacitor charging source terminal and the anode output. A second capacitor is coupled from the anode output to a trigger output of the power supply. A trigger electrode of the plasma thruster is located sufficiently close to the cathode electrode to initiate a plasma arc. The anode output and the reference output are periodically connected together by a switch element operated by the controller. The controller operates to maintain the switch element in an open state until the first capacitor is charged to a threshold voltage and / or a thrust cycle event is required, and then closes the switch for a sufficient period of time to allow the inductor to charge a current sufficient to cause an LdI / dt voltage spike to arc between the MPT trigger electrode and the cathode electrode when the switch is opened. The switch remains open for the duration of the thrust cycle, during which the plasma transitions from between the trigger electrode and the cathode electrode to between the cathode electrode and the anode electrode, thereby generating thrust through the thrust cycle. This thrust continues until the current is insufficient to sustain the plasma, at which point the charging of the first capacitor, switch closure, switch opening, and thrust cycle are repeated. Each cycle results in the generation of a pulsed thrust event. [Brief explanation of the drawings]

[0018] [Figure 1A] FIG. 1 is a front view of a metal plasma thruster with an axial trigger electrode. [Figure 1A-1]FIG. 1 is a cross-sectional side view of a metal plasma thruster during a trigger cycle. [Figure 1A-2] FIG. 1 is a cross-sectional side view of a metal plasma thruster during a thrust cycle. [Figure 1B] FIG. 1 is a front view of a metal plasma thruster with a circumferential trigger electrode. [Figure 1B-1] FIG. 1 is a cross-sectional side view of a metal plasma thruster during a trigger cycle. [Figure 1B-2] FIG. 1 is a cross-sectional side view of a metal plasma thruster during a thrust cycle. [Figure 2] FIG. 1 illustrates a power supply and controller for a plasma thruster. [Figure 3] FIG. 10 is a cross-sectional view of an insulator for an axial trigger thruster. [Figure 3A] FIG. 4 is a diagram showing the configuration of an insulator for illustrating the details of FIG. 3. [Figure 3B] FIG. 4 is a diagram showing the configuration of an insulator for illustrating the details of FIG. 3. [Figure 3C] FIG. 4 is a diagram showing the configuration of an insulator for illustrating the details of FIG. 3. [Figure 3C-1] FIG. 4 is a diagram showing the configuration of an insulator for illustrating the details of FIG. 3. [Figure 3D] FIG. 4 is a diagram showing the configuration of an insulator for illustrating the details of FIG. 3. [Figure 3E] FIG. 4 is a diagram showing the configuration of an insulator for illustrating the details of FIG. 3. [Figure 3F] FIG. 4 is a diagram showing the configuration of an insulator for illustrating the details of FIG. 3. [Figure 3G] FIG. 4 is a diagram showing the configuration of an insulator for illustrating the details of FIG. 3. [Figure 3H] FIG. 4 is a diagram showing the configuration of an insulator for illustrating the details of FIG. 3. [Figure 4] FIG. 10 is a cross-sectional view of an insulator for a circumferential trigger thruster. [Figure 4A] FIG. 5 is a detailed view of FIG. [Figure 4B] FIG. 5 is a detailed view of FIG. [Figure 4C] FIG. 5 is a detailed view of FIG. [Figure 4D] FIG. 5 is a detailed view of FIG. [Figure 4D-1] FIG. 5 is a detailed view of FIG. [Figure 4E] FIG. 5 is a detailed view of FIG. [Figure 4F] FIG. 5 is a detailed view of FIG. [Figure 4G] FIG. 5 is a detailed view of FIG. [Figure 4H] FIG. 5 is a detailed view of FIG. [Figure 4I] FIG. 5 is a detailed view of FIG. [Figure 5] FIG. 2 is a schematic waveform diagram relating to the operation of a plasma thruster. [Figure 6] 1 is a graph of anode current and anode voltage during a thrust cycle. [Figure 7] 1 is a graph of the operation of a plasma thruster from a trigger cycle to a thrust cycle. [Figure 8] 1 is a graph of a trigger cycle waveform. [Figure 9] FIG. 10 is a waveform diagram of a failed trigger. [Figure 10A] FIG. 1 is a cross-sectional view of a dual axial / circumferential trigger thruster geometry over the life of the thruster. [Figure 10B] FIG. 1 is a cross-sectional view of a dual axial / circumferential trigger thruster geometry over the life of the thruster. [Figure 10C] FIG. 1 is a cross-sectional view of a dual axial / circumferential trigger thruster geometry over the life of the thruster. [Figure 10D] FIG. 1C is a detailed view of the plasma start and trust events of FIGS. 10A and 10B. [Figure 10E] FIG. 1C is a detailed view of the plasma start and trust events of FIGS. 10A and 10B. [Figure 11] FIG. 1 illustrates an example of a square plasma thruster having four plasma trigger electrodes arranged on each side of the square. [Figure 11A] FIG. 12 is a cross-sectional view of FIG. [Figure 11B]FIG. 12 is a detailed view of the inner trigger electrode and inner insulator option for FIG. 11. [Figure 11C] FIG. 11C is a cross-sectional view of the optional inner trigger electrode and inner insulator of FIG. 11B. [Figure 12] This is a perspective view of a three-axis thruster cube. [Figure 13] FIG. 13 is a schematic diagram of a thrust power supply connected to a trigger driver for driving the three-axis thruster cube of FIG. 12. DETAILED DESCRIPTION OF THE INVENTION

[0019] FIG. 1A shows a front view of a first configuration of a metal plasma thruster (MPT). A central trigger electrode 110 is separated from a cathode surface 108 by an annular insulator 101, on which the starting plasma is formed. The insulator 101 can be formed from glass, ceramic, alumina (aluminum oxide), or any insulator with a high melting point. In one example of the present disclosure, a cured epoxy resin and cured epoxy laminate (e.g., glass-reinforced epoxy laminate FR4) can be used to form an insulator with a surface that provides conductive microgaps between the trigger electrode and the cathode for forming a plasma starting spark across multiple microgaps. FIG. 1B shows an alternative circumferential trigger configuration, in which a central cathode surface 113 is separated by an outer circumferential trigger electrode 109, and the insulator 111 is positioned within the radial extent of the trigger electrode 109. FIG. 1A, the axial trigger configuration, is a front view best understood in combination with FIG. 3, a side cross-sectional view, and FIG. 1B, the circumferential trigger configuration, is best understood in combination with FIG. 4, a side cross-sectional view.

[0020] The cathode 108 of FIG. 1A or the cathode surface 113 of FIG. 1B may be formed from any material, but preferably from a metal that provides a combination of a relatively high erosion rate, which implies a relatively high mass flow rate during operation, and a high pumping speed. Suitable materials include any of the elements lithium (Li), carbon (C), magnesium (Mg), aluminum (Al), titanium (Ti), chromium (Cr), niobium (Nb), molybdenum (Mo), tantalum (Ta), tungsten (W), platinum (Pt), or uranium (U). The following table provides a list of atomic mass units (AMU), maximum metal ion velocities (m / s), and specific impulses I SP (change in momentum per unit amount of propellant consumed) for each cathode element.

[0021] [Table 1]

[0022] The propellants listed in Table 1 are not exclusive; any conductive solid element or alloy may be used as the electrode material in an MPT, from low mass lithium to high mass depleted uranium and beyond. A comprehensive list of cathode metals suitable for the present disclosure, sorted by melting point, includes magnesium, aluminum, radium, barium, strontium, cerium, europium, ytterbium, calcium, lanthanum, praseodymium, silver, neodymium, actinium, gold, samarium, copper, promethium, uranium, manganese, beryllium, gadolinium, terbium, dysprosium, nickel, holmium, cobalt, erbium, yttrium, iron, scandium, thulium, palladium, protactinium, lutetium, titanium, thorium, platinum, zirconium, chromium, vanadium, rhodium, hafnium, technetium, ruthenium, iridium, niobium, molybdenum, tantalum, osmium, rhenium, tungsten, and carbon. These metals, sometimes referred to as "tier 1" metals, may exist in the cathode in elemental form or as alloys with other metals.

[0023] Alternative metals with lower melting points that can be used as cathodes in the MPT include francium, cesium, gallium, rubidium, potassium, sodium, indium, lithium, tin, polonium, bismuth, thallium, cadmium, lead, and zinc. These materials are sometimes referred to as "tier 2" metals and can exist in elemental form or as alloys with other metals. Within the scope of this disclosure, the "tier 1" and "tier 2" metals listed above are understood to be suitable for use as cathode materials for MPTs, either alone or alloyed with other metals, for use in forming the MPT cathodes of this disclosure. Although these metals are classified by melting point, this is not intended to suggest a ranking in terms of preference for use in the cathodes of the MPT.

[0024] This MPT has a wide range of I SP whereas prior art PPT's PTFE (Teflon®) are limited to a narrower range of values ​​of about 525 seconds to 600 seconds.

[0025] The anode 114 for the axial trigger thruster of FIG. 1A or the circumferential trigger thruster of FIG. 1B is a conductive electrode positioned at a distance from the cathode (108 or 113) surface, the anode 114 having a porosity greater than 80% to allow accelerated metal ions to pass through. The anode 114 can have any shape, including a screen, an annular ring, or a torus with a diameter equal to or greater than the diameter of the cathode 108 of FIG. 1A or the trigger electrode 109 of FIG. 1B. The anode may be formed from any conductive material, a refractory metal, or a stainless steel alloy, such as an alloy containing at least steel and chromium, and has the maximum practical porosity so that metal ions are accelerated away from the cathode but propagate through the porous anode.

[0026] 1A-1 shows a cross section of the MPT of FIG. 1A, where a trigger electrode 110 generates a pulsed voltage that is applied between the trigger electrode 110 and the cathode 108 and exceeds the breakdown voltage of the insulator 101. In the MPT of FIG. 1A-1, a high voltage pulse is applied between the trigger lead 102 and the grounded cathode lead 104 using the power supply 201 of FIG. 2.

[0027] FIG. 2 shows a power supply 201 for use with any of the previously described plasma thruster 207 configurations, including an axial trigger thruster 207A or an circumferential trigger thruster 207B. A third thruster 207C is of dual-trigger geometry, with both inner and outer trigger electrodes, positioned at different axial locations as illustrated in FIGS. 10A, 10B, and 10C. Power supply 202 charges energy storage capacitor C1 206. Controller 203 determines the charge rate, thrust release frequency, switch 208 closure timing, and switch 208 closure duration according to the required thrust, material consumption, trigger requirements, and other input parameters. Switch 208 is normally off and may be an insulated gate bipolar transistor (IGBT), field-effect transistor (FET), bipolar transistor, or any other controllable switching element capable of withstanding high voltages (greater than 1 kV) and 100 A current. The switch 208 has a low switch-on resistance (less than 30 mΩ) and a fast switching time (greater than 500 ns). During the time that the switch 208 is off, the trigger 102 and the grounded cathode 104 are equipotential to each other. When the voltage across C1 206 reaches a threshold voltage determined by the controller 203 and the controller 203 requests a thrust event, a trigger voltage Vg is delivered to the switch 208 as a short pulse of duration T1 (FIG. 5), which closes the switch 208 and allows current to flow through L1 for the duration of T1. This current is sufficient to generate a starting plasma when the switch 208 opens at the end of T1, generating a positive voltage spike across the switch 208 that is coupled to the trigger lead 102 via the trigger capacitor C2 210. The resulting trigger plasma formed between the trigger electrode 110 / 109 of FIGS. 1A and 1B and the cathode electrode 108 / 113 of FIGS. 1A and 1B results from current flowing through the circuit formed by CL, L1, C2, trigger lead 102, and cathode lead 104.The period T1 during which the switch 208 is enabled by the gate Vg is selected to extend only long enough for plasma initiation. Then, the plasma propulsion period T2 (FIG. 5) begins, during which a primary plasma is generated from the cathode 108 / 113 (corresponding to the cathode lead 104) to the anode 114 (corresponding to the anode lead 106) via the circuit C1 206, L1 204, the anode lead 106, and the cathode lead 104. To minimize triggering energy losses, the inductor L1 204 and the period T1 are selected to be the smallest possible value sufficient to generate a sufficiently high voltage (LdI / dt) to trigger a micro-discharge plasma across the surface of the metallized insulator (region 301 in FIG. 3 or region 401 in FIG. 4 having the surface profile 402 and the central axis 403), thereby promoting arc breakdown between the cathode 104 and the anode 106.

[0028] A first capacitor C1 206 is charged by DC current drawn directly from a power source 202, such as a spacecraft solar power source (typically up to 30V-100V or other suitable voltage), or at a charging rate regulated by a controller 203. Initially, both the switch (IGBT or MOSFET 208) and the trigger plasma path across insulator 101 (FIG. 1) from trigger electrode 110 to cathode 108 or through insulator 111 from trigger electrode 109 to cathode 113 are open; the vacuum gap between the cathode (108 or 113, respectively) and anode 114 also ensures that no current flows back through that gap to power ground. When C1 206 is fully charged, a trigger signal Vg is sent by controller 203 to the control input of the switch, closing switch 208. As current flows from C1 206 through inductor L1 204, switch 208, and back to capacitor ground, it increases current in L1 204, eventually allowing a sufficiently high LdI / dt voltage to be generated to strike a trigger electrode arc when switch T1 208 is opened. In one example, after 30 μs to 100 μs, control input Vg returns to 0 V, opening switch 208, and the voltage (LdI / dt) generated by L1 204 is delivered to the trigger electrode through C2 210, striking a plasma across the insulator gap between the trigger electrode and the cathode electrode.

[0029] The inductor 204 has a sufficiently high inductance to generate an adequate breakdown voltage (e.g., in the range of 200V to 1000V) across the insulator gap between the cathode 108 / 113 and the trigger electrode 110 / 109. Therefore, the inductor 204 can be small and low mass, in contrast to prior art VATs, where the inductor must have a high enough inductance to store the required arc energy applied between the anode and cathode in that configuration. In this MPT, the inductor is not the primary energy source for the arc; it only serves to generate the LdI / dt voltage spike required to trigger the arc. Charge and energy for the arc are primarily provided by the storage capacitor C1.

[0030] FIG. 5 shows the operating waveforms of a metal plasma thruster. An energy storage capacitor (C1 206 in FIG. 2) is charged by an external power source to generate a voltage Vc 504. When the capacitor voltage reaches a threshold voltage 502, in response to a command from controller 203 in FIG. 2, switch 208 is triggered by Vg 508 during the T1 period, causing switch 208 to close. The trigger T1 period removes a small amount of charge from the capacitor, which is used to generate a trigger plasma, such as 120 shown in FIG. 1A-1, or similarly across the insulator in region 301 in FIG. 3 or region 401 in FIG. 4. The trigger T1 period is followed by plasma thrust period T2 in FIG. 5, in which the plasma spreads between the cathode surface and anode 114 in FIG. 3 or 4, as shown as 122 in FIG. 1A-2. During T2, the stored charge in the capacitor is reduced by the generation of plasma, and eventually the capacitor voltage or arc current drops below a voltage level that cannot sustain the plasma. This cycle repeats at the next trigger Vg event, with the repetition period T3 being limited by the current supplied to the capacitor by the external charging circuit.

[0031] FIG. 6 shows the current and voltage during a primary cathode-to-anode arc period lasting an example 3 ms thrust period, corresponding to period T2 in FIG. 5. Waveform 602 shows an example anode electrode current, and waveform 604 shows an example anode electrode voltage relative to a grounded cathode, both for an example fixed arc resistance of 30 mΩ and an arc sheath potential drop of 19 V. One component of the applied voltage that generates the plasma is the sheath potential and the ohmic voltage drop across the arc. The sheath potential is a well-known phenomenon in arc discharges, where ions from the primary discharge plasma in the anode-cathode gap travel back across the cathode sheath, thereby gaining approximately 10 eV to 15 eV of energy per singly charged ion. These ions further heat the cathode locally at multiple locations, supporting the field-enhanced thermionic emission of electrons needed to carry the arc current from the cathode to the anode. These current-carrying electrons pass through another thin sheath at the anode, gaining energy (about 10 eV) and depositing that energy as heat at the anode. The sum of the cathode and anode sheath potentials is 19 V in this example. At a peak current of 230 A, the resistive drop is 230 A * 30 mΩ ≒ 7 V. Adding this 19 V sheath drop results in a total voltage across the arc (between the cathode and anode) of 26 V at peak current. This terminal voltage between the cathode and anode is called the burning voltage of the arc.

[0032] 7 shows the waveforms during the interval from time 701 to time 703, which corresponds to the switch transition from T1 (Vg on) to T2 (Vg off). Plot 704 shows that capacitor C1 is charging and switch 208 is closed until time 701. During this first interval, the current through C1 206, L1 204, and switch 208 is I until the switch is opened at time 701, initiating the trigger plasma. L1702. During the interval from 701 to 703, the current path transitions from C1 to L1 to C210 to the trigger electrode 102, after which the current flows from the anode 106 to the cathode 104. At time 703, the plasma spreads from the cathode 108 / 113 to the anode 114, and the current continues to increase from time 703 onwards as the plasma grows between the cathode and anode (electrodes), resulting in a substantial decrease in resistance within the plasma and eventually insufficient energy stored in the capacitor 206 to sustain the plasma. Plot I L1 702 shows a linear rise to a maximum of approximately 70 A at time 701 during the first 90 μs of T1, an example of when the switch is closed, followed by a slight drop and then a sinusoidal rise. The anode current plot 702 is for the example of FIG. 2, with component values ​​for this example of Cl=10.5 mF, Ll=70 pF, and a total loop resistance (total resistance of capacitor, coil, and closed switch) of 40 mΩ.

[0033] FIG. 8 shows the waveform of a time segment including the trigger interval, beginning approximately 45 μs after switch 208 is opened at the end of T1 interval 807. The switch opening causes its internal resistance to rise sharply, causing the current through switch 208 to drop sharply. The interruption of current flow through switch 208 causes the voltage applied to trigger electrode Vtrig 806 to rise sharply, generating plasma at the trigger electrode. At that moment, trigger electrode current 804 begins to flow, generating more trigger plasma. As the trigger plasma fills the gap between the cathode and anode, the plasma current path transitions from between the cathode and trigger electrode to between the cathode and anode, causing the trigger capacitor to drive current in the opposite direction, causing trigger current 804 to reverse polarity. Inductor current I L1802 changes slope during the trigger plasma interval, 45 μs to 55 μs, as the current begins to drop due to a sudden increase in resistance in switch 208. This change in current contributes to the LdI / dt voltage, which increases, shunting current through switch 208 to trigger electrode 102 to form the initial trigger arc to the cathode. When the switch is opened (approximately 43 μs in this FIG. 8 example), the current I in coil L1 L The sudden drop in 802 causes the LdI / dt voltage V TRIG806 occurs. This initial fast (approximately 200 ns) voltage spike of 120 V passes through blocking capacitor C2 210, which is selected to provide low impedance (less than 1 kΩ) for the fast (less than 1 μs) rise time pulse, generating a flashover plasma (120 in FIG. 1A-1) across the metallized (approximately 10 kΩ to 1 kΩ) insulator surface (region 301 in FIG. 2 or region 401 in FIG. 4) between the trigger electrode (110 in FIG. 3 or 109 in FIG. 4, respectively) and the cathode (108 in FIG. 3 or 113 in FIG. 4, respectively). An example trigger current from the trigger electrode to the cathode electrode is as high as 45 A (suggesting a flashover resistance of approximately 2.7 Ω), generating the plasma 120 of FIG. 1A-1 sufficient to bridge the main cathode-anode gap and initiate breakdown 122 across the main cathode-anode gap of FIG. 1A-2. Arc breakdown occurs across the plasma bridge, which has a resistance of less than 50 mΩ / s, allowing a higher current to flow from C1 through L1, resulting in arc discharge. The much higher resistance of the trigger path ensures negligible current flow through the trigger electrode path, as shown by trigger current plot 804, which returns to zero at approximately 55 μs as the anode arc current rises. Furthermore, blocking capacitor C2 210 exhibits an increasingly higher impedance as the current rises, rising from less than 1 mΩ during the 100 ns switch voltage rise time to a maximum of over 6000 mΩ during the slower portion of the main arc. Therefore, the trigger path is only active for about 10 μs after switch 208 opens. The trigger electrode current 804 goes negative at 50 μs in FIG. 8, at which point the main plasma arc from cathode to anode ignites because blocking capacitor C2 210 now drives current in the reverse direction through the arc. The voltage across switch 208 drops to about 33 V after about 53 μs, which is the arc burning voltage (sheath drop plus resistance drop in the arc resistance).In this example, the voltage reaches a peak of about 400V and then drops to the arc burning voltage of about 33V.

[0034] FIG. 9 illustrates the consequences of an excessively short trigger interval T1 or insufficient voltage being generated to initiate a plasma at the trigger electrode. The voltage generated by discontinuously interrupting the inductor current is LdI / dt, and therefore care must be taken not to exceed the breakdown voltage of the switch 208. This protection is achieved by an overvoltage device 209, which may be any device with a fast enough response time to limit the voltage exceeding a threshold and provide protection to the switch during transient voltage events. One such device is a metal oxide varistor (MOV) 209. In the example waveforms of FIG. 9, the switch voltage 904 reaches 1200 V, and weak or delayed plasma formation results in the MOV 209 shunting a portion of the current, as shown in waveform 902, thereby protecting the switch 208 from overvoltage breakdown. The clamping voltage of this protection device 209 is selected to prevent overvoltage damage to the switch 208. For reference, the inductor current 906 is also shown.

[0035] These various charging times, plasma arcing times, cycle times, and circuit configurations are provided for illustrative purposes only, and many other variations are possible. The plasma arc has been described as being activated at discrete thrust events using discrete energy levels stored in an inductor. Alternatively, a DC voltage source may be placed between the anode and cathode electrodes so that when the initiator electrode initiates the plasma arc, it maintains the plasma arc in a steady state until the DC power source is removed. In discrete pulse mode, the energy storage capacity of the energy storage capacitor is expressed as follows:

[0036]

number

[0037] where C is capacitance and V is the energy storage capacitor. Furthermore, the capacitance, plasma electrode shape, spacing, and ohmic resistance determine the time interval over which the arc is maintained. Also, the given durations and waveforms are exemplary in nature with respect to the components used and are not intended to limit the values ​​of these components or the duration and time of the waveforms produced by them. In one example of the present disclosure, the initiation (microgap spark) lasts for about 1 μs, and the plasma thrust (plasma current flowing between the cathode and anode) lasts for about 1 ms or about 5 ms.

[0038] An example of the insulator surface shape in axial trigger region 301 of FIG. 3 is shown in detail in FIG. 3A, and an example of the circumferential insulator surface in region 401 of FIG. 4 is shown in detail in FIG. 4A. The shape of these surfaces, which maintain a thin coating of plasma-forming metal, is crucial to the long-term operation of the plasma thruster. By forming a stepped surface of insulator 101, as shown in the example insulator in region 301 of FIG. 3A, to create a flat insulator surface 304 and proximal surfaces 305 and 306 located below cathode face 302, the surface resistance of the recessed segment of insulator 306 remains nearly constant over multiple plasma cycles, thereby extending the life of the thruster. One specific function of this insulator 101 shape is to shield one or more surfaces of the insulator from redeposition of metal ejected from the face of cathode 108 / 113. The example proximal surfaces 305 or 306 in FIG. 3A can perform this function. The surface of the insulator 101 may rely on an initial metallization or deposition, such as graphite particles approximately 1 μm thick. The microgaps between the particles create multiple diffusion regions where microarcs form within the small gaps in the metallized surface, which are crucial for plasma arc initiation. The insulator's surface resistance is preferably in the range of tens of ohms to thousands of ohms, although other resistance values ​​may be used to form microplasmas in the gaps between the metallized surfaces. Another insulator surface profile is shown in Insulator Detail 3B, which shows a sloped section with a shielded surface 314 adjacent to the deposition surface 312. Insulator Detail 3C shows a sawtooth section with a shielded surface 320 and an exposed surface 318. Alternative Insulator Detail 3C-1 shows a sawtooth section with a peak 321 that creates a gradient in material removal / deposition between two surfaces on either side of the peak (one fully shielded, the other partially shielded). FIG. 3D shows a graded insulator 326 .This insulator 326 may be used with a trigger electrode 110 having a surface 330 that is flush with the recessed region of the insulator 101, or may extend to a height where the trigger electrode 110 is flush with the cathode 108, as shown by profile 328. Furthermore, the trigger electrode profile may be flat as shown, or may taper to a tip (not shown), bevel to the flat surface 328 (not shown), or may be rounded (not shown). Figures 3E, 3F, and 3G show additional embodiments of the insulator 101 having surfaces 318, 324, 328 / 330, respectively. These surfaces are shielded from redeposition of the plasma emitted from the cathode by their shape for the return of plasma metal ions. If the incidence of redeposition is low, the flat surface 331 of Figure 3H may be used. The insulator 101 may have any shape that provides equilibrium for the evaporation and redeposition of cathode material. In one example, at least one surface is shielded from redeposition, or in another example, a balance is achieved between erosion due to the plasma trigger event and redeposition due to the return of metal ions after the trigger event. The insulator 101 may have other non-planar surface characteristics, such as wavy surface features, that create a balance between metal deposition and metal ejection due to each plasma thrust event, or a roughened surface that maintains microplasma formation within the microgaps in the metallized portions. Thus, a design constraint for this insulator may be that the total thickness of new material deposited in each pulse on the shielding surfaces, such as 305 and 306 in FIG. 3A (or the shielding surfaces of other insulator examples in FIGS. 3B-3H ), must be equal to the thickness eroded by each trigger flashover thrust event. In one example of the present disclosure, the depth of surface 306 relative to surface 304 and the width of the grooves forming surfaces 305 and 306 are selected to have an aspect ratio of 2 or greater, and a deposition / erosion balance is enforced on at least one of surfaces 305 or 306. The aspect ratio of the feature is understood to be the depth of the feature parallel to the cathode central axis divided by the width of the feature parallel to the face of the cathode.When this balance is achieved, the insulator 101 functions as a reliable trigger with a reproducible surface resistance over millions of pulses, e.g., in the 5 kg orbit raising example described later in this application, the total number of pulses over a 14 day mission at 2 Hz is 2.5 x 10. 6The total mass of propellant burned during this period is approximately 30 g. A single molybdenum (Mo) cathode in an MPT with a diameter of 30 mm erodes this mass to a depth of 4 mm. The insulator 301 is shown in detail in Figure 3A. With successive plasma discharge events, metal ions from the cathode 108 tend to accumulate at the insulating interface 304. This accumulation can reduce the surface resistance to the trigger current. Groove 306 shields the proximal region of the insulator from metal ion redeposition. The groove depth is selected so that the amount of material removed by a plasma ignition event is equivalent to the amount of new material redeposited by the main plasma arc event. In this way, material redeposition replaces the lost material, allowing the igniter to remain operational even after millions of ignition events. While the circumferential trigger configuration of FIG. 4A exhibits similar grooves formed by surfaces 406 and 406 shielded from redeposition of ions emitted from cathode 113, surface 404 is expected to experience more deposition than surfaces 406 or 407. The insulator configuration of the axial trigger electrode of FIG. 3 can be utilized in the circumferential trigger configuration of FIG. 4 by mirroring the insulator features, thereby maintaining the surface profile located near the central trigger electrode even when the trigger electrode is circumferentially shaped. For example, the insulator profile of FIG. 3A (having a stepped surface 306 adjacent to trigger electrode 110) may be transformed into the insulator profile shown in FIG. 4A (maintaining the stepped surface 406 adjacent trigger electrode 109) by mirroring the insulator profile. Similarly, features 414, 417, and 416 of the insulator 101 of the circumferential trigger of FIG. 4B may be transformed from features 312, 314, and 316, respectively, of the corresponding insulator 101 of the axial trigger of FIG. 3B by mirroring the surface features. Thus, the insulator profiles shown in Figures 3C-3H may be similarly modified for use with circumferential triggering, as shown in Figures 4C-4I, respectively, to maintain shielding of at least one proximal surface from redeposition of plasma ions emitted from the cathode. The material of insulator 101 / 111 need not be a high temperature ceramic.This is because the insulator surface is only briefly exposed to a small amount of trigger plasma during part T1, which is a small portion of the plasma duration T1 + T2. Suitable insulators 101 / 111 are those that support a surface metal deposition of approximately 1 micron and have a melting point sufficient to withstand the energy transferred from the trigger arc over repeated cycles.

[0039] In one example of the present disclosure, the MPTs have diameters ranging from 5 mm to 40 mm. In another example of the present disclosure, referring to FIG. 2, the MPTs are arranged on one or more surfaces of a cube measuring approximately 10 cm x 10 cm x 10 cm. One or more surfaces of the cube contain multiple MPTs, each with an anode lead 106 with a capacitor C2 210 for its respective MPT trigger and a commonly connected cathode lead 104. In this configuration, a single power supply 201 is used on one face of the cube to control a particular orthogonal thrust direction, and each MPT (or group of MPTs) located on a particular surface is connected via a switch element (reed relay or other low-resistance switch) to connect one group of MPTs at a time and isolate the other groups of MPTs. In this manner, one or more MPTs can be selected until the useful life of that thrust pulse expires, after which the MPT with its associated capacitor 210 is isolated and a new MPT and associated capacitor 210 can be selected for continued operation.

[0040] 10A, 10B, and 10C show a metal plasma thruster with a dual trigger geometry, where the dual triggers have different operating depth ranges to extend the usable life of the cathode. A central inner trigger electrode 1008 is surrounded by an inner insulator 1010, which can have any insulator shape for a coaxial insulator, as described in FIGS. 3A-3H. The inner insulator 1010 is not in contact with the inner trigger electrode 1008. and outer insulator 1004 respectively.The surfaces 1040 and 1044 of the metal cathode 1006 are treated with a conductive surface that forms a microgap, such as a surface graphite treatment, to initiate the plasma by providing a microgap for arc formation. The outer surface of the metal cathode 1006 is covered by an outer insulator 1004, which has an area of ​​conductive surface, such as a graphite surface 1044 on its inner surface, and a short area 1020 on its outer surface for contacting the outer trigger electrode 1002. Arrows 1024 and 1022 indicate the initial plasma initiation event in FIGS. 1A-1 and 1B-1, respectively, while a thicker arrow 1023 indicates the subsequent plasma thrust event shown in FIGS. 1A-2 and 1B-2. FIG. 10A shows the initial state of the metal plasma thruster with 100% cathode propellant present from the terminal end 1015 to the cathode face area 1017. For this first stage cathode propellant shown in FIG. 10A , the outer trigger electrode 1002 may be actuated alone, or the outer trigger electrode 1002 may be actuated in combination with the inner trigger electrode 1008, or the inner trigger electrode 1008 and the outer trigger electrode 1002 may be actuated in any order required to uniformly deplete the cathode 1006 during alternating or sequential thrust events.

[0041] In one example of the present disclosure shown in Figures 10A, 10B, and 10C, the inner insulator 1010 has a partially or fully conductive surface treatment 1040 to form a microgap from the cathode opposite the cathode face to a distance of, for example, 30% to 60% of the original cathode extent L3, shown as L1 1011. Here, the original cathode extent from the origin of the cathode 1015 to the cathode face is shown as L3 1013. As the cathode 1006 erodes, this cathode extent decreases to L4 2019, shown in Figure 10B, and eventually to L5 2019, shown in Figure 10C. The inner insulator 1010 has a conductive or partially conductive surface treatment 1040 that forms a microgap over a range L2 1012 from the terminal end 1015 to less than the original cathode extent L3 1013, and the outer insulator 1004 has a conductive or partially conductive surface treatment 1044 over a range L1 1011 from the original cathode surface 1017 to less than the terminal end 1015. As the extent of the cathode 1006 decreases from L3 1013 in Figure 10A to L4 1019 in Figure 10B, the starting arc path on the surface of the outer insulator 1004 increases, but the starting arc path on the inner insulator 1010 remains unchanged. Eventually, the arc path length 1044 will extend to a length that will not reliably initiate the plasma, and the surface treatment 1040 of the inner insulator 1010 will dominate plasma initiation during the cathode life from range L4 1019 in FIG. 10B to the propellant end of life (EOL) 1021 in FIG. 10C.

[0042] Figure 10D shows a detailed region 1030 of the insulator 1004 and electrode 1002 of Figure 10A. The microgap initiation surface comprises graphite particles 1032 deposited on the outer insulator 1004, which are overcoated with a cathodic redeposit 1034 of a metal, e.g., Mo, from a previous plasma thrust event. Figure 10D shows the initial phase, in which a spark in the microgap of the surface coating 1032 forms a microplasma 1038, which provides an electrical path for a full plasma discharge in the thrust phase shown in Figure 10E. The initiation event of Figure 10D can last on the order of microseconds, while the thrust event of Figure 10E lasts on the order of milliseconds.

[0043] The axial trigger mechanism of FIG. 3 and the circumferential trigger mechanism of FIG. 4 can be extended to a rectangular or square geometry for a square plasma thruster, as shown in FIG. 11, with four trigger electrodes 1102A, 1102B, 1102C, and 1102D, an outer insulator 1104 having a microgap conductive surface for initiating the plasma, and a cathode 1106. FIG. 11A shows a cross-sectional view of FIG. 11A, including the porous anode 114 shown in the first embodiment without the inner trigger electrode 1120 or inner insulator 1124. In another example of the present disclosure, the additional elements of the inner trigger electrode 1120 and inner insulator 1124 shown in FIG. 11B for detail 1120 of FIG. 11 may be used. FIG. 11D shows a corresponding cross-sectional view of the inner trigger electrode 1120 and inner insulator 1124, which has a surface coated with a conductive material that forms a microgap as described above. In another variation of the present disclosure, only the inner trigger electrode 1120 and inner insulator 1124 may be used for triggering, and the plasma thruster does not include the outer insulator 1104 and outer trigger electrodes 1102A, 1102B, 1102C, and 1102D. The geometry of FIG. 11A is preferable for using the entire surface when used with the thruster cube 1200 of FIG. 12. In one example of the disclosure of FIG. 11, the square or rectangular cathode 1106 of FIG. 11 can be triggered using a single trigger electrode or any combination of trigger electrodes 1102A, 1102B, 1102C, and 1102D. In another variation of the present disclosure, shown in detail 1120, the inner trigger electrode 1122 and inner insulator 1124 are used without the surrounding outer trigger electrodes 1102A, 1102B, 1102C, and 1102D.

[0044] 10A, 10B, 10C, 10D, and 10E are understood to be applicable to, but not limited to, circular or square cathode geometries. Furthermore, in one example of the present disclosure, the outer trigger electrode 1002 may be formed as two or more separate conductors, thereby allowing for individual triggering of plasma across the face of the cathode by individually actuating the trigger electrode segments, as described with respect to FIG.

[0045] FIG. 12 shows an example three-axis thrust control cube. Here, five of the six exterior surfaces have thrusters mounted to provide orthogonal thrust directions. In the example of FIG. 12, each surface of thrust cube 1200 has four thrusters mounted in an orthogonal configuration: lower thruster 1206 provides a +Z thrust, upper thruster 1202 provides a -Z thrust, left thruster 1204 provides a +X thrust, and right thruster 1210 provides a -X thrust. The forward thruster 1210 provides a +Y thrust. In a typical configuration for satellite thrusters, each surface on opposite sides of the satellite should have a thrust cube, with the cube having a surface configured to provide a -Y thrust in the opposite direction. A typical satellite configuration may use two to eight or more thrusters like those in FIG. 12. Each surface may have any number of thrusters, which may be actuated individually or in groups to achieve orthogonal, granular thrust events. By mounting the three-axis thrust cubes 1200 at uniform offsets and uniform orientations around the satellite's center of mass, control of the rotation rate and static position relative to the center of mass can be achieved by first firing opposite thrusters in opposite directions to initiate rotation, and then firing opposite thrusters to stop rotation. As is well known in the satellite thruster prior art, discrete thrust events, such as those generated by plasma thrusters positioned on the thrust control cubes 1200, can be used to precisely position and orient a satellite and move it from one orbital position to another.

[0046] FIG. 13 is a schematic diagram of an example triaxial thruster for use with the thrust cube of FIG. 10. Power supply 1301 operates as described in FIG. 2, with DC power supply 1302 charging energy storage capacitor 1308 C1 as previously described. Capacitor C1 may be two 21 mF capacitors in parallel. As described with respect to FIG. 2, trigger switch 1310 closes for a period of time sufficient to maximize the instantaneous current in inductor L1 1306, after which switch 1310 is released, allowing the current stored in inductor L1 1306 to be routed to one or more selected thrusters in thruster cube assembly 1318. The trigger switch 1310 is formed using four IGBT devices, and storage inductor L1 1306 may be a 70 μH inductor, providing a current that rises approximately linearly to 150 A over 320 μs. Blocking capacitor 1330, which may be of a value such as 500 nF, couples a trigger voltage to a selected trigger electrode via a trigger switch, such as an insulated gate bipolar transistor (IGBT) 1326. Selection of individual thrusters is accomplished using thruster select inputs 1332 coupled to isolation drivers 1328, which may be opto-isolators or other isolation switch elements that allow low-voltage control and isolation of the activated thruster 1318 from high transient voltages. Each driver, such as 1328, is coupled to a trigger switch 1326, which may be an IGBT 1326, that is in either an on or off state. When the IGBT switch 1326 is in the on state, an instantaneous current is coupled through blocking capacitor 1330 and applied to the collector (drain) electrode of the trigger switch, such as 1334. In the on state, the trigger switch 1334 is connected to an emitter (source) 1336 that couples a corresponding trigger current to a trigger electrode of the thruster 1324 .This initiates a trigger plasma from the cathode to the trigger electrode, which initiates a primary arc discharge from the cathode electrode 1324 to the anode electrode 1320, generating thrust as previously described. Each driver in the trigger driver 1316 is configured such that an individual thruster 1338-1322 is individually selected for each thrust event. As previously described, the duration of the plasma provided by the circuit of FIG. 13 is determined primarily by the geometry of the energy storage device 1306 and the thruster electrodes, and can be in the range of several orders of magnitude of 6 μs or less.

[0047] In an alternative embodiment of the present disclosure for providing continuous plasma thrust as described above, a switchable DC power supply may be applied between the anode electrode 1312 and the cathode electrode 1314 of Figure 13. A selected thruster is initiated with a voltage pulse to the initiator electrode (replacing the capacitor 1330) sufficient to initiate a plasma in the selected thruster, which will run continuously until the switchable DC power supply is removed.

[0048] The individual thruster geometries used in the thruster cube 1318 are shown as circumferential triggering geometries of thrusters 1338-1322 in FIG. 13, including any of the configurations shown in FIG. 1B, 4, 4A, or 4B. An example of a circumferential triggering thruster 1338 is shown with an outer trigger electrode 1342, an inner cathode electrode 1342, and insulator 1346. The thrusters 1338-1322 could alternatively have any of the axial geometries described above with respect to FIG. 1A, 1B, 3, 3A, 3B, 3C, 3D, 3E, 3F, or 3D, 4, 4A, 4B, 4C, 4D, 4E, 4F, 4G, 4H, 4I, 10A, 10B, 10C, or 11A. In the variation of Figure 11 with or without an inner trigger and inner insulator or with or without an outer trigger and outer insulator, either of the insulator profiles shown in the series of figures in Figure 3 and the series of figures in Figure 4 can be used for the shape of the inner or outer insulator to ensure equilibrium between the cathode material that deposits and the cathode material that vaporizes during a plasma thrust event.

[0049] Additionally, the thrusters 1338-1322 of the thruster cube 1318 may be located on a single surface or multiple surfaces of the cube in FIG. 10, and any number of thrusters may be located on a particular surface. In certain configurations of the satellite, an orbital direction vector may exist, and thrust that is not symmetrical about the line of action (or line of motion) may induce angular momentum on the satellite, which may cause undesirable rotation. To address this, it may be preferable to locate symmetric pairs of thrusters on certain surfaces, with one thruster on each side of the line of action, operating in pairs to prevent rotational momentum from being included in the delivered force. These are example configurations presented for understanding this disclosure and are not intended to limit the scope of this disclosure.

[0050] In one example of the present disclosure, each thruster on the surface of the thruster cube is configured to connect to the anode conductor 1312 and cathode conductor 1314 of the power supply 1301, and each thruster is individually selectable using the thruster input 1332. This thruster input 1332 selects a particular thruster to receive a trigger pulse and generate thrust, while unselected thrusters that do not receive a trigger pulse remain passive. This allows for fine-grained control of the pulsed plasma from one or more orthogonal surfaces. In one example of the present disclosure, the pulsed power supply 1301 and trigger driver 1316 are packaged within an internal housing, as shown in FIG. 10. This housing has a thruster 1318 on its outer surface, as shown in FIG. 10, which allows for low-loss coupling from the inductor 1306 to the selected thruster on the thruster cube 1318.

[0051] Many such configurations are possible, and this particular example is presented solely for illustrative purposes.

[0052] Compared to VAT devices, the MPT differs from prior art VATs in three distinct ways. (1) MPTs have plasma arc currents in the range of about 200 A to 300 A or more, while VATs have currents of about 100 A. This reflects the characteristics of the underlying energy storage elements, namely, that inductors (high resistive losses) deliver lower currents than capacitors (low resistive losses). (2) MPT arcs are used as storage elements, resulting in a VAT It has a longer duration (approximately 3ms to 12ms) compared to the arc duration (approximately 100μs to 500μs). (3) The inductance of an MPT to initiate the plasma is approximately 20 μH to 80 μH, whereas that of a VAT, which stores plasma energy within the inductor, is approximately 1 mH. The inductor of an MPT stores less than 2% of the energy delivered to the arc in each cycle.

[0053] The higher ISP values ​​obtained by MPT result in higher fuel efficiency than PPT for space missions. As is well known from the rocket equations, the amount of propellant that must be expelled from a spacecraft for a given mission in orbit at a given expulsion rate depends strongly (exponentially) on this expulsion rate. The propellant mass M p is the initial mass of the spacecraft M o It is correlated with.

[0054]

number

[0055] where ΔV is the velocity change required for the orbital maneuver, and u e is the exhaust velocity of the propellant. Equation 1 shows that for a given operation, the higher the exhaust velocity, the lower the mass of propellant required.

[0056] As an example, consider raising a spacecraft from a 500 km orbit above Earth to a higher 700 km orbit. These orbits place the spacecraft significantly higher than the 411 km orbit of the International Space Station (ISS), thus avoiding disruption to the ISS's orbit and posing a potential threat to crewed missions. Equations of orbital mechanics can be used to calculate the velocity change (defined as ΔV) required to accomplish such an orbital maneuver. This velocity change is 110 m / s. Table 2 lists three different types of propulsion systems: (1) a metal plasma thruster, which is the subject of this disclosure and uses molybdenum as a candidate propellant; (2) a pulsed plasma thruster (PPT) sold by Busek; and (3) a FEEP thruster sold by ACCION.

[0057] [Table 2]

[0058] If we assume that we are using each of these thrusters to launch a 5 kg spacecraft (such as a nanosatellite) from a 500 km orbit to a 700 km orbit (this maneuver requires a ΔV of 110 m / s), the propellant mass M required by each of these engines is p is calculated from Equation 1 and presented in the last row of the table above. Both the Mo MPT and ACCION FEEP require approximately 30 g for combustion, while the Teflon® (PTFE) PPT requires approximately 100 g. The advantage of the higher ISP of the MPT and FEEP is that less propellant mass is required.

[0059] The thrust efficiency of all these electric propulsion systems is primarily determined by the energy cost of generating ions from a solid (Teflon® PPT or MPT) or liquid (FEEP) state. FEEP is the most efficient of these three systems because electroevaporation is a direct, non-thermal extraction via quantum tunneling across a potential barrier. Both Teflon® PPT and MPT utilize a thermal process to ionize atoms. In such a thermal process, the ionization cost is much higher: approximately 100 eV per atom, compared with approximately 10 eV per atom for FEEP. As a result, the thrust-to-input power ratio of FEEP is higher than that of Teflon® PPT or MPT. However, the mass (for a given power) of MPT is much lower than that of FEEP. Therefore, the thrust-to-mass ratios of MPT and FEEP are comparable.

[0060] The examples provided in this disclosure are intended to aid in understanding the disclosure, which can be implemented in many different ways. It is understood that the example values ​​for the inductor, first capacitor, second capacitor, voltage and current, trigger electrode-cathode electrode distance, insulator gap, and insulator surface profile and shape are exemplary and not limiting of the disclosure. The disclosure is defined by the appended claims. Quantities referenced within approximate order ranges are understood to be 10 times or more greater, or 10 times or less less than the referenced quantity. In one variation, the trigger current is periodically measured and used to adaptively change the threshold voltage for initiating a thrust cycle to a higher or lower voltage level. In this way, detecting increased metal deposition on the insulator surface (increased V / I peak at arcing) results in a change to the threshold voltage, increasing the erosion rate, and detecting increased metal erosion (decreased V / I peak at arcing) results in a change to the threshold voltage in the opposite direction. The controller's threshold adjustment direction to higher or lower voltages can be based on measurements or can be adaptive based on electrode wear, number of pulse discharges, or other algorithms. [Explanation of symbols]

[0061] 101 Annular insulator 102 Trigger lead, trigger electrode, trigger 104 Cathode lead, cathode 106 Anode lead, anode 108 cathode surface, cathode, cathode electrode 109 Outer circumferential trigger electrode 110 Central trigger electrode 111 Insulator 113 Central cathode surface, cathode 114 Anode, porous anode 120 Plasma 122 Dielectric breakdown 201 Power supply 202 Power supply 203 Controller 204 Inductor L1, Coil L1 206 Energy storage capacitor C1, circuit C1 207 Plasma Thruster 207A Axial Trigger Thruster 207B Circumferential Trigger Thruster 207C Third Thruster 208 Switch T1 209 Overvoltage devices, metal oxide varistors (MOVs), protection devices 210 Blocking capacitor C2, trigger capacitor C2 301 area 302 cathode surface 304 Flat insulator surface, insulating interface 305 proximal surface 306 Proximal surface, insulator, groove, stepped surface 312 Sedimentary Surface 314 Shielding Surface 318 Exposed Surface 320 Shielding Surface 321 Peak 326 Angled Insulator 328 Profile, Flat Surface 330 surface 331 flat surface 401 area 402 Surface Profile 403 Center axis 404 Surface 406 Stepped Surface 414 Feature Part 416 Feature Part 417 Features 502 Threshold Voltage 504 Voltage Vc 508 Vg 602 Waveform 604 Waveform 701 hours 702 Anode Current Plot 703 hours 704 plots 804 Trigger electrode current 806 Trigger electrode Vtrig 807 T1 section 902 waveform 904 Switch Voltage 906 Inductor Current 1002 Outer trigger electrode 1002 Electrode 1004 Outer insulation 1006 Metal cathode 1008 Central inner trigger electrode 1010 Inner insulator 1011 Range L1 1012 Range L2 1013 Cathode range L3 1015 Termination, cathode 1017 Cathode surface area 1019 Range L4 1020 range 1022 Arrow 1023 Thicker Arrow 1024 arrows 1030 Detail area 1032 Graphite particles, surface coating 1034 Cathode redeposit 1038 Microplasma 1040 Surface treatment part 1044 Surface treatment, graphite surface, arc path length 1102A Outer Trigger Electrode 1102B Outer trigger electrode 1102C Outer trigger electrode 1102D Outer trigger electrode 1104 Outer insulator 1106 Cathode 1120 Inner trigger electrode 1122 Inner trigger electrode 1124 Inner insulator 1200 Thruster Cube, Thrust Cube, 3-Axis Thrust Cube, Thrust Control Cube 1202 Upper Thruster 1204 left thruster 1206 Lower Thruster 1210 Forward thruster 1210 right thruster 1301 Power supplies, pulse power supplies 1302 DC power supply 1306 Energy storage device, storage inductor L1 1308 Energy Storage Capacitor 1310 Trigger Switch 1312 Anode electrode, anode conductor 1314 Cathode electrode, cathode conductor 1316 Trigger Driver 1318 Thruster cube assembly, thruster, thruster cube 1320 Anode electrode 1324 Thruster 1326 Insulated Gate Bipolar Transistor (IGBT), IGBT Switch, Trigger Switch 1328 Separate Driver 1330 Blocking Capacitor 1332 Thruster selection input 1334 Trigger Switch 1336 Emitter Source 1338 Circumferential Trigger Thruster 1342 Inner cathode electrode, outer trigger electrode 1346 Insulator 2019 L4, L5

Claims

1. 1. A metal plasma thruster, comprising: a porous anode; an inner trigger electrode; an inner insulator surrounding the inner trigger electrode and having a first conductive coating, the first conductive coating having a microgap, the first conductive coating being in contact with an area of ​​the inner trigger electrode; a metal cathode surrounding the inner insulator and having a first extent; an outer insulator surrounding the metal cathode, the outer insulator having a second conductive coating over a portion of an outer surface of the metal cathode and across a face of the outer insulator, the second conductive coating having an additional microgap; an outer trigger electrode in contact with the second conductive coating; Equipped with the metal cathode has a region of the cathode surface exposed from a terminal end to the porous anode; the inner insulator has a second extent from the terminal end that is smaller than the first extent; the second conductive coating has a third extent along the inner surface of the outer insulator starting at the cathode face that is smaller than the first extent.

2. The metal cathode may be selected from magnesium (Mg), aluminum (Al), radium (Ra), barium (Ba), strontium (Sr), cerium (Ce), europium (Eu), ytterbium (Yb), calcium (Ca), lanthanum (La), praseodymium (Pr), silver (Ag), neodymium (Nd), actinium (Ac), gold (Au), samarium (Sm), copper (Cu), promethium (Pm), uranium (U), manganese (Mn), beryllium (Be), gadolinium (Gd), terbium (Tb), dysprosium (Dy), nickel (Ni), holmium (Ho), cobalt (Co), erbium (Er), yttrium (Y), iron (Fe), scandium (Sc), thulium (Tm), palladium (Pd), protactinium (Pa), lutetium (L), or the like.

10. The metal plasma thruster of claim 1, wherein the metal plasma thruster is made of an alloy or element comprising at least one of: titanium (Ti), thorium (Th), platinum (Pt), zirconium (Zr), chromium (Cr), vanadium (V), rhodium (Rh), hafnium (Hf), technetium (Tc), ruthenium (Ru), iridium (Ir), niobium (Nb), molybdenum (Mo), tantalum (Ta), osmium (Os), rhenium (Re), tungsten (W), carbon (C), francium (Fr), cesium (Cs), gallium (Ga), rubidium (Rb), potassium (K), sodium (Na), indium (In), lithium (Li), tin (Sn), polonium (Po), bismuth (Bi), thallium (Tl), cadmium (Cd), lead (Pb), and zinc (Zn).

3. 10. The metal plasma thruster of claim 1, wherein at least one of the first conductive coating or the second conductive coating comprises graphite.

4. 10. The metal plasma thruster of claim 1, wherein the cathode surface is square or rectangular.

5. 10. The metal plasma thruster of claim 1, wherein the outer trigger electrode is square or rectangular.

6. 6. The metal plasma thruster of claim 5, wherein said outer trigger electrode comprises a plurality of conductive segments.

7. 7. The metal plasma thruster of claim 6, wherein at least one of the conductive segments of the plurality of conductive segments includes a straight edge that is parallel to an edge of the metal cathode.

8. 10. The metal plasma thruster of claim 1, wherein the cathode surface is circular.

9. 2. The metal plasma thruster of claim 1, wherein a discharge formed in the microgap or the additional microgap initiates a plasma formed between the metal cathode and the porous anode.

10. 10. The metal plasma thruster of claim 1, wherein at least one of the inner insulator or the outer insulator is formed from at least one of ceramic, alumina, glass, or epoxy laminate.

11. 4. The metal plasma thruster of claim 3, wherein the graphite is disposed on the inner insulator or a surface of the outer insulator facing the porous anode.

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