JP7804151B2 - Metallic plasma thruster and control circuit for satellites - Google Patents

Description

This disclosure relates to plasma thrusters for use on satellites. In particular, this disclosure relates to metal plasma thrusters (MPTs) that generate pulsed thrust through a series of plasma generation cycles.

Over the next decade, the number of small satellites launched into low Earth orbit (LEO) is expected to increase dramatically. These satellites range in mass from pico- and femto-satellites, weighing less than 1 kg, to nano-satellites (1 kg-10 kg) and micro-satellites (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).

Thousands of nano- and micro-satellites are expected to be deployed into low Earth orbit (LEO) to perform multiple roles, 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 Xe ion engines and Hall thrusters, to power levels of approximately 1 W to 10 W is impractical. The unavoidable additional mass of propellant storage tanks, flow control devices, and piping in Xe 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.

One type of prior art plasma thruster is known as the PTFE pulsed plasma thruster (PPT). This PTFE pulsed plasma thruster uses 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 that require a larger insulating gap in the thruster assembly than is required in alternative low-voltage thrusters that form plasma on a pre-metallized film surface. The thrust-generating plasma from a PPT 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.

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 flowing through the solid-state switch before it is actively opened (between approximately 100 μs and 500 μs) is completely switched to the vacuum arc load. A typical current of approximately 100 A (between approximately 100 μs and 500 μs) is conducted at a voltage of approximately 25 V to 30 V. As a result, most of the magnetic energy stored in the inductor is converted into a plasma pulse. A drawback of this VAT is the energy dissipated during the charging cycle through the storage inductor and I ² R switch losses. During this phase of increasing energy stored in the inductor, current flows through the inductor and switch but not into the arc discharge because the voltage required to trigger the arc is generated only after the switch is opened. Both the inductor and the switch are dissipative elements, and therefore a portion of the energy during each cycle is dissipated as heat in these elements. Another drawback of the VAT is the redeposition of metal ejected from the cathode onto the insulating layer. One prior art VAT geometry relies 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, exceeding the amount that the inductor can vaporize at arc initiation. 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 that function as an anode/cathode pair. This configuration, as described in commonly owned U.S. Patent No. 7,518,085, requires the use of an energy storage inductor as a combined energy storage and particle redirection structure to redirect emitted ions perpendicular to the desired thrust direction, which are guided by the axial magnetic field of the storage inductor and an external cathode.

A different type of prior art plasma thruster is the field emission electric propulsion (FEEP) device, which 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 be composed of both positive and negative ions with masses in the range of several hundred daltons. By periodically changing the polarity of an 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 a FEEP thruster, 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.

U.S. Patent No. 7,518,085

It would be desirable to provide an improved plasma thruster that operates with ions at higher ejection velocities than those found in PPTs, has improved reliability and lifetime over prior art VATs, and uses components that are simpler to manufacture and more operationally reliable than FEEP thrusters.

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 conductive coating The porous anode includes an outer insulator having a microgap therebetween, and an outer trigger electrode in contact with the conductive coating of the outer insulator. The metal cathode has an end and a surface exposed to the porous anode. The inner insulator has an area from the end smaller than that of the metal cathode. The conductive coating of the outer insulator has an area from the cathode surface smaller than that of the metal cathode. The metal cathode surface is square, rectangular, or circular. The outer trigger electrode has one or more conductive regions for forming a starting plasma with the metal cathode.

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 an end portion and a surface exposed to the porous anode, the surface 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.

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 cathode electrode surface; and a porous anode positioned opposite the cathode surface.

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, wherein at least one of the inner insulator or the outer insulator has a first surface parallel to the cathode electrode surface and the inner insulator or the outer insulator has 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.

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 includes: 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 is formed between a trigger electrode connected to the trigger output and a cathode connected to the reference output. The trigger arc then causes plasma formation between the cathode connected to the reference output and an anode connected to the anode output.

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, which comprises an insulator having a conductive surface with a microgap and is surrounded by a cathode electrode. The cathode electrode is surrounded by an outer insulated initiator, which comprises an insulator having a conductive surface with a microgap, and the outer insulated initiator is surrounded by an outer trigger electrode. In one example configuration, a porous anode electrode is positioned a spaced distance from the surface of the cathode.

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. When plasma is generated near the cathode surface, a very long thrust cycle is initiated, where 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 achieve three-axis thrust application.

The inventors have realized that the disclosed metal plasma is much more energy efficient when compared to 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.

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 storage capacitor charging time and a variable inductive switch time. The power supply and controller operate to generate a thrust event from the metal plasma thruster, and the power supply has a first storage 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. The 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 generation 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.

FIG. 1 is a front view of a metal plasma thruster with an axial trigger electrode. FIG. 1 is a cross-sectional side view of a metal plasma thruster during a trigger cycle. FIG. 1 is a cross-sectional side view of a metal plasma thruster during a thrust cycle. FIG. 1 is a front view of a metal plasma thruster with a circumferential trigger electrode. FIG. 1 is a cross-sectional side view of a metal plasma thruster during a trigger cycle. FIG. 1 is a cross-sectional side view of a metal plasma thruster during a thrust cycle. FIG. 1 illustrates a power supply and controller for a plasma thruster. FIG. 10 is a cross-sectional view of an insulator for an axial trigger thruster. FIG. 4 is a diagram showing the configuration of an insulator for illustrating the details of FIG. 3 . FIG. 4 is a diagram showing the configuration of an insulator for illustrating the details of FIG. 3 . FIG. 4 is a diagram showing the configuration of an insulator for illustrating the details of FIG. 3 . FIG. 4 is a diagram showing the configuration of an insulator for illustrating the details of FIG. 3 . FIG. 4 is a diagram showing the configuration of an insulator for illustrating the details of FIG. 3 . FIG. 4 is a diagram showing the configuration of an insulator for illustrating the details of FIG. 3 . FIG. 4 is a diagram showing the configuration of an insulator for illustrating the details of FIG. 3 . FIG. 4 is a diagram showing the configuration of an insulator for illustrating the details of FIG. 3 . FIG. 4 is a diagram showing the configuration of an insulator for illustrating the details of FIG. 3 . FIG. 10 is a cross-sectional view of an insulator for a circumferential trigger thruster. FIG. 5 is a detailed view of FIG. 4. FIG. 5 is a detailed view of FIG. 4. FIG. 5 is a detailed view of FIG. 4. FIG. 5 is a detailed view of FIG. 4. FIG. 5 is a detailed view of FIG. 4. FIG. 5 is a detailed view of FIG. 4. FIG. 5 is a detailed view of FIG. 4. FIG. 5 is a detailed view of FIG. 4. FIG. 5 is a detailed view of FIG. 4. FIG. 5 is a detailed view of FIG. 4. FIG. 2 is a schematic waveform diagram relating to the operation of a plasma thruster. 1 is a graph of anode current and anode voltage during a thrust cycle. 1 is a graph of the operation of a plasma thruster from a trigger cycle to a thrust cycle. 1 is a graph of a trigger cycle waveform. FIG. 10 is a waveform diagram of a failed trigger. 1A-1C are cross-sectional views of a dual axial/circumferential trigger thruster geometry over the life of the thruster. 1A-1C are cross-sectional views of a dual axial/circumferential trigger thruster geometry over the life of the thruster. 1A-1C are cross-sectional views of a dual axial/circumferential trigger thruster geometry over the life of the thruster. FIG. 1C is a detailed view of the plasma start and trust events of FIGS. 10A and 10B. FIG. 1C is a detailed view of the plasma start and trust events of FIGS. 10A and 10B. FIG. 1 illustrates an example of a square plasma thruster having four plasma trigger electrodes arranged on each side of the square. FIG. 12 is a cross-sectional view of FIG. FIG. 12 is a detailed view of the inner trigger electrode and inner insulator option for FIG. 11. FIG. 11C is a cross-sectional view of the optional inner trigger electrode and inner insulator of FIG. 11B. FIG. 1 is a perspective view of a three-axis thruster cube. 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.

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 may 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, the side cross-sectional view, and FIG. 1B, the circumferential trigger configuration, is best understood in combination with FIG. 4, the side cross-sectional view.

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 velocity. 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 identifies each cathode element in atomic mass units (AMU), maximum metal ion velocity (m/s), and specific impulse _I (change in momentum per unit amount of propellant consumed).

The propellants listed in Table 1 are not exclusive, and any conductive solid element or alloy, from low mass lithium to high mass depleted uranium and beyond, may be used as an electrode material in MPT. 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.

Alternative metals with lower melting points that may 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 may exist in elemental form or as alloys with other metals. For purposes 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 categorized by melting point, this is not intended to suggest a ranking in terms of preference for use in the cathodes of the MPT.

This MPT achieves a wide _{ISP range} of 800 seconds to 2400 seconds, while the prior art PPT PTFE (Teflon) is limited to a narrower range of values of about 525 seconds to 600 seconds.

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, with 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 to allow metal ions to propagate through the porous anode while being accelerated away from the cathode.

Figure 1A-1 shows a cross section of the MPT of Figure 1A. Here, 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 Figure 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 Figure 2.

FIG. 2 illustrates 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 configured in a dual-trigger geometry with both inner and outer trigger electrodes, each positioned at a different axial location as illustrated in FIGS. 10A, 10B, and 10C. The power supply 202 charges an energy storage capacitor C1 206. A 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. The 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 and conducts current 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 trigger electrode 110/109 of Figures 1A and 1B and cathode electrode 108/113 of Figures 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 switch 208 is enabled by gate Vg is selected to extend only long enough for plasma initiation. Thereafter, plasma propulsion period T2 (Figure 5) begins, during which a main plasma is generated from cathode 108/113 (corresponding to cathode lead 104) to anode 114 (corresponding to anode lead 106) via circuit C1 206, L1 204, anode lead 106, and cathode lead 104. To minimize triggering energy losses, inductor L1 204 and period T1 are selected to be the smallest possible value sufficient to generate a voltage (LdI/dt) high enough 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 surface profile 402 and central axis 403), thereby promoting arc breakdown between cathode 104 and anode 106.

The first capacitor C1 206 is charged by DC current drawn directly from the power source 202, e.g., the spacecraft's solar power source, or at a charging rate regulated by the controller 203 (typically up to 30V-100V or other suitable voltage). Initially, both the switch (IGBT or MOSFET 208) and the trigger plasma path across the insulator 101 (FIG. 1) from the trigger electrode 110 to the cathode 108 or the trigger plasma path through the insulator 111 from the trigger electrode 109 to the cathode 113 are open; the vacuum gap between the cathode (108 or 113, respectively) and the 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 the controller 203 to the control input of the switch, closing the 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 when switch T1 208 is opened to strike a trigger electrode arc. In one example, after 30 μs to 100 μs, the 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 via C2 210, striking a plasma across the insulator gap between the trigger electrode and the cathode electrode.

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, but serves only to generate the LdI/dt voltage spike required to trigger the arc. Charge and energy for the arc are primarily provided by the reservoir capacitor C1.

Figure 5 shows the operating waveforms of a metal plasma thruster. An energy storage capacitor (C1 206 in Figure 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 Figure 2, switch 208 is triggered by Vg 508 during the T1 period, thereby closing switch 208. 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 Figure 1A-1, or similarly across the insulator in region 301 in Figure 3 or region 401 in Figure 4. The trigger T1 period is followed by plasma thrust period T2 in Figure 5, in which the plasma spreads between the cathode surface and anode 114 in Figure 3 or Figure 4, as shown as 122 in Figure 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.

FIG. 6 shows the current and voltage during a main 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 main 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. Furthermore, the ions locally heat the cathode at multiple locations, supporting the field-enhanced thermionic emission of electrons required 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 (approximately 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.

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 increases as I _L1 702 until the switch is opened at time 701 and a trigger plasma is initiated. During the interval from 701 to 703, the current path transitions from C1, L1, C210, and 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 current continues to increase from time 703 onward 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 decrease and then a sinusoidal rise. The anode current plot 702 is for the example of FIG. 2 , with component values for this example: Cl=10.5 mF, Ll=70 pF, and a total loop resistance (total resistance of the capacitor, coil, and closed switch) of 40 mΩ.

FIG. 8 shows the waveform of a time segment including the trigger interval, beginning approximately 45 μs when switch 208 is opened at the end of T1 interval 807. The switch's 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, sparking 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 reverse direction, causing trigger current 804 to reverse polarity. During the trigger plasma interval, 45 μs to 55 μs, inductor current I _L1 802 changes slope as the current begins to drop due to the sudden rise in resistance in switch 208. This change in current contributes to an LdI/dt voltage that rises, causing current flowing through switch 208 to be shunted 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), a sudden drop in current I _L 802 in coil L1 generates an LdI/dt voltage V _TRIG 806 at the coil output node. This initial fast (approximately 200 ns) voltage spike of 120 V passes through blocking capacitor C2 210. This blocking capacitor C2 210 is selected to provide low impedance (less than 1 kΩ) for a fast (less than 1 μs) rise time pulse to generate a flashover plasma (120, FIG. 1A-1) across the metallized (approximately 10 kΩ to 1 kΩ) insulator surface (region 301, FIG. 2 or region 401, FIG. 4) between the trigger electrode (110, FIG. 3 or 109, FIG. 4, respectively) and the cathode (108, FIG. 3 or 113, FIG. 4, respectively). An example trigger current from the trigger electrode to the cathode electrode is on the order of 45 A (implying a flashover resistance of approximately 2.7 Ω), generating plasma 120, FIG. 1A-1, sufficient to bridge the main cathode-anode gap and initiate breakdown 122 across the main cathode-anode gap, 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 that negligible current flows 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 at the 100 ns rise time of the switch voltage to a maximum of over 6000 mΩ during the slower portion of the main arc. Thus, the trigger path is only active for approximately 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 33V after about 53 μs, which is the arc burning voltage (sheath drop plus the resistance drop in the arc resistance). In this example, the voltage reaches a peak of about 400V and then drops back down to the arc burning voltage of about 33V.

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

These various charging times, plasma arc discharge 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 given by the following equation:

where C is the 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 approximately 1 μs, and the plasma thrust (plasma current flowing between the cathode and anode) lasts for approximately 1 ms or approximately 5 ms.

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 of 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, with the microgaps between the particles creating multiple diffusion regions where microarcs form within the small gaps in the metallized surface, which are crucial for plasma arc initiation. The insulator preferably has a surface resistance 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 portion with a shielded surface 314 adjacent to the deposition surface 312. Insulator Detail 3C shows a sawtooth portion with a shielded surface 320 and an exposed surface 318. Alternative Insulator Detail 3C-1 shows a sawtooth portion with a peak 321 that creates a gradient in material removal/deposition between two surfaces on either side of the peak (one fully shielded and the other partially shielded). FIG. 3D shows a sloped 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). FIGS. 3E, 3F, and 3G show additional embodiments of the insulator 101 having surfaces 318, 324, 328/330, respectively. These surfaces are shaped for return of plasma metal ions, shielding them from redeposition of plasma emitted from the cathode. If the incidence of redeposition is low, the flat surface 331 of FIG. 3H may be used. The insulator 101 can have any shape that provides a balance between the vaporization 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 provide 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 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 imposed on at least one of surfaces 305 or 306. This feature aspect ratio 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 cathode face. When this balance is achieved, insulator 101 functions as a reliable trigger with a reproducible surface resistance over millions of pulses. For example, 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 ⁶ . The total mass of propellant burned during this period is approximately 30 g. A single molybdenum (Mo) cathode in an MPT with a 30 mm diameter erodes this mass over a depth of 4 mm. Insulator 301 is shown in detail in FIG. 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 redeposition by the main plasma arc event. In this way, material redeposition replaces the lost material, and the igniter remains operational even after millions of ignition events. While the circumferential trigger configuration of FIG. 4A shows similar grooves formed by surfaces 406 and 406 being shielded from redeposition of ions emitted from the 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 trigger electrode 110) may be mirrored to modify the insulator profile shown in FIG. 4A (maintaining the stepped surface 406 adjacent trigger electrode 109). Similarly, features 414, 417, and 416 of the insulator 101 of the circumferential trigger of FIG. 4B may be modified 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 FIGS. 3C-3H may be similarly modified for use with a circumferential trigger, as shown in FIGS. 4C-4I, respectively, to maintain shielding of at least one proximal surface from redeposition of plasma ions emitted from the cathode. The material of the insulators 101/111 need not be a high-temperature ceramic. This is because the insulator surface is only briefly exposed to the triggering 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 triggering arc over repeated cycles.

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, with one or more surfaces of the cube having multiple MPTs, each with an anode lead 106 with a capacitor C2 210 for a 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, with each MPT (or group of MPTs) located on a particular surface 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 MPTs with their corresponding capacitors 210 are separated and new MPTs and corresponding capacitors 210 can be selected for continued operation.

Figures 10A, 10B, and 10C show a dual-trigger metal plasma thruster with dual triggers having 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 of the insulator shapes for a coaxial insulator, as described in Figures 3A-3H. Surfaces 1040 and 1044 of the inner and outer insulators 1010 and 1004, respectively , that are not in contact with the inner trigger electrode 1008 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 a conductive surface area, 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 of 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. Figure 10A shows the initial condition 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.

10A, 10B, and 10C, an example of the present disclosure shows an inner insulator 1010 having a partially or fully conductive surface treatment 1040 to form a microgap from the cathode opposite the cathode face to a range, such as 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 FIG. 10B, and eventually to L5 2019, shown in FIG. 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 prevent reliable plasma ignition, and the surface treatment 1040 of the inner insulator 1010 will dominate plasma ignition during the cathode life from range L4 1019 in FIG. 10B to the propellant end of life (EOL) 1021 in FIG. 10C.

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 redeposition 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.

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 plasma initiation, 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 full surface use 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.

It is understood that Figures 10A, 10B, 10C, 10D, and 10E are 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 the plasma across the face of the cathode by individually actuating the trigger electrode segments, as described with respect to Figure 11.

Figure 12 shows an example three-axis thrust control cube, where five of the six exterior surfaces have thrusters mounted to provide orthogonal thrust directions. In the example of Figure 12, each surface of the thrust cube 1200 has four thrusters mounted in an orthogonal configuration: the lower thruster 1206 provides a +Z thrust, the upper thruster 1202 provides a -Z thrust, the left thruster 1204 provides a +X thrust, and the 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 Figure 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 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 the satellite and move it from one orbital location to another.

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. 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 the trigger voltage to the selected trigger electrode through 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 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, which couples a corresponding trigger current to the 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 of 1324 to the anode electrode 1320, generating thrust as previously described. Each driver of the trigger driver 1316 is configured so 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 within a few orders of magnitude less than 6 μs.

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 FIG. 13. A selected thruster is initiated with a voltage pulse to the initiator electrode (replacing capacitor 1330) sufficient to initiate a plasma in the selected thruster, which will run continuously until the switchable DC power supply is removed.

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 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 FIG. 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 FIG. 3 and the series of figures in FIG. 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.

Furthermore, thrusters 1338-1322 of 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 a particular surface, with one thruster on each side of the line of action and 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.

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 the thrusters 1318 on its outer surface as shown in FIG. 10, allowing for low-loss coupling from the inductors 1306 to the selected thrusters of the thruster cube 1318.

Many such configurations are possible, but this particular example is presented solely for illustrative purposes.

In comparison 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 properties of the underlying energy storage elements, namely, that inductors (high resistive losses) will draw lower currents than capacitors (low resistive losses).
(2) MPT arcs have a longer duration (approximately 3 ms to 12 ms) compared to the arc duration in VAT (approximately 100 μs to 500 μs) due to the use of a capacitor as a storage element.
(3) The inductance in an MPT to initiate the plasma is approximately 20 μH to 80 μH, whereas in a VAT, which stores the plasma energy in an inductor, it is approximately 1 mH. The inductor in an MPT stores less than 2% of the energy delivered to the arc each cycle.

The higher ISP values obtained by MPT result in greater 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 at a given expulsion rate for a given mission in orbit depends strongly (exponentially) on this expulsion rate. The propellant mass M _p is a function of the initial mass M _o of the spacecraft.

where ΔV is the change in velocity required for the orbital maneuver and u _e is the propellant exhaust velocity. Equation 1 shows that for a given maneuver, the higher the exhaust velocity, the lower the propellant mass required.

As an example, consider the raising of a spacecraft from a 500 km orbit above Earth to a higher orbit of 700 km. These orbits would 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) the metal plasma thruster, which is the subject of this disclosure and uses molybdenum as a candidate propellant; (2) the pulsed plasma thruster (PPT) sold by Busek; and (3) the FEEP thruster sold by ACCION.

Assuming a 5 kg spacecraft (such as a nanosatellite) is launched from a 500 km orbit to a 700 km orbit using each of these thrusters (this maneuver requires a ΔV of 110 m/s), the propellant mass _Mp required by each of these engines 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.

The thrust efficiency of all these electric propulsion systems is primarily determined by the energy cost of generating ions from the solid (Teflon® PPT or MPT) or liquid (FEEP) state. FEEP is the most efficient of these three systems because electrochemical evaporation is a direct, non-thermal extraction process 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 to 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.

The examples provided in this disclosure are intended to aid in understanding the disclosure, which may 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 manner, 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 to a higher or lower voltage can be adaptively based on measurements, electrode wear, number of pulse discharges, or other algorithms.

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 device, metal oxide varistor (MOV), protection device 210 Blocking capacitor C2, trigger capacitor C2
301 Region 302 Cathode surface 304 Flat insulator surface, insulating interface 305 Proximal surface 306 Proximal surface, insulator, groove, stepped surface 312 Deposition surface 314 Shielding surface 318 Exposed surface 320 Shielding surface 321 Peak 326 Graded insulator 328 Profile, flat surface 330 Surface 331 Flat surface 401 Region 402 Surface profile 403 Central axis 404 Surface 406 Stepped surface 414 Feature 416 Feature 417 Feature 502 Threshold voltage 504 Voltage Vc
508 Vg
602 Waveform 604 Waveform 701 Time 702 Anode current plot 703 Time 704 Plot 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 insulator 1006 Metal cathode 1008 Central inner trigger electrode 1010 Inner insulator 1011 Region L1
1012 Range L2
1013 Cathode range L3
1015 End portion, cathode 1017 Cathode surface range 1019 Range L4
1020 Area 1022 Arrow 1023 Thicker Arrow 1024 Arrow 1030 Detail Area 1032 Graphite Particles, Surface Coating 1034 Cathode Redeposition 1038 Microplasma 1040 Surface Treatment 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 supply, pulsed power supply 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 isolation driver 1330 blocking capacitor 1332 thruster select input 1334 trigger switch 1336 emitter source 1338 circumferential trigger thruster 1342 inner cathode electrode, outer trigger electrode 1346 insulator 2019 L4, L5

Claims (11)

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;
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. 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). The metal plasma thruster of claim 1, wherein at least one of the first conductive coating or the second conductive coating comprises graphite. The metal plasma thruster of claim 1, wherein the cathode surface is square or rectangular. The metal plasma thruster of claim 1, wherein the outer trigger electrode is square or rectangular. A metal plasma thruster as described in claim 5, wherein the outer trigger electrode comprises a plurality of conductive segments. 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. The metal plasma thruster of claim 1, wherein the cathode surface is circular. 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. 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. 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.