Patent Description:
The present disclosure relates to a pulsed cathodic arc propulsion system. In particular, the present invention relates to a pulsed cathodic arc propulsion system suitable for use in a spacecraft, wherein the arc is triggered by an insulated wire or pin surrounded by the material constituting the cathode.

A spacecraft is a machine or vehicle that is designed to operate in space. Such spacecraft include, but are not limited to, rockets, space shuttles, satellites, and space stations. Spacecraft are used for a variety of purposes, including communications, navigation, scientific research and discovery, meteorology, and the like. Space is a near vacuum environment, which presents difficulties in operating, maintaining, and fuelling spacecraft. These difficulties are magnified by the large distances involved and the consequential costs and timings to perform those operations.

Engines used for orbital station-keeping and long-duration spaceflight engines have been implemented using electric and plasma based propulsion systems. These propulsion systems have a high specific impulse, are controllable, and are technically mature and thus reliable. Further, electric and plasma based propulsion systems have a favourable deliverable mass fraction, which is the proportion of the initial mass of a spacecraft that can be delivered to the intended destination of that spacecraft.

A rocket propelled vehicle generates acceleration by discharging propellant at high velocity, resulting in an exchange of momentum due to an unbalanced force. The thrust generated by a propulsion system is the product of the exhaust velocity and the instantaneous rate of change of the mass of the spacecraft due to propellant expulsion. The thrust can be considered as an external force applied to the spacecraft.

Space missions that require the delivery of large payloads must use a prohibitively large propellant mass or the propulsion system must be multi-stage or the propulsion system exhaust velocity must be of the same order or greater than the required change in velocity. Since interplanetary missions typically require velocity changes of tens of kilometres per second, it follows that the exhaust velocity of any thruster employed on such a mission must be comparable to this required change in velocity and sufficiently robust to operate for extended periods. In such a context, electrical propulsion technologies are deemed to be preferable to chemical fuelled rockets, due to the high propellant exhaust velocities and fewer mechanical parts.

One problem that arises with electrical propulsion systems, relative to chemical propulsion systems, is the need to have a power supply. While chemical thrusters carry the energy needed to effect momentum changes inside the chemical bonds of the fuel, electrical propulsion systems need to have an energy source supplied to the system. Since any power supply used for propulsion adds mass and volume, it is necessary to make the electrical propulsion system as energy efficient as possible.

Determining the ratio of jet power to power consumed allows for a simple determination of energy efficiency in a working rocket, with flight rated systems achieving energy efficiency values of approximately <NUM> to <NUM>%. This ratio is also termed "thrust efficiency," as it is the ratio of exhaust kinetic energy to available energy (in chemical thrusters) or input power (for electrical propulsion systems).

All of the gas fuelled electrical propulsion technologies suffer from the same design challenges, namely the added mass from propellant tanks and piping and the mechanical failure modes inherent in valves and pressure regulators.

There are disadvantages to all of the existing thruster systems that use a gaseous fuel. The pressurised tank, piping, valves and pressure regulator systems add mass to the spacecraft that could be given over to payload, as well as introducing manufacturing challenges and failure modes. These drawbacks could be overcome or substantially ameliorated if a reliable and efficient solid fuelled electromagnetic thruster was provided.

Pulsed Plasma Thrusters (PPTs) utilise a solid fuel and are reliable, but have problems with propellant feedstock evaporation between plasma pulses. PPTs ablate and ionise a proportion of the propellant feedstock by creating an arc discharge sheet across the face of a propellant cylinder using a discharging capacitor. The plasma that is created responds to the electric field of the arc and its own self-induced magnetic field, causing the plasma to accelerate down a discharge chamber or nozzle, resulting in moderate thrust. The propellant feedstock is generally a rod of solid material, such as polytetrafluoroethylene (PTFE), marketed as Teflon by E. du Pont de Nemours and Company ("DuPont"). The propellants used by PPTs typically have low melting points compared to most metals. Heat generated during arcing causes evaporation of the propellant between arc pulses, which expends fuel with only marginal thrust having been produced. Consequently, PPTs have much lower system efficiencies when compared to other technologies, as well as lower total thrust.

Thus, a need exists to provide an improved propulsion system suitable for use in a spacecraft.

<NPL> describes earlier work of the inventors describing a proof of concept centre triggered cathodic arc system for use as a spacecraft propulsion system. This study used a titanium cathode with central trigger pin comprised of a <NUM> diameter tungsten wire encased in an alumina sheath of <NUM> external diameter. This study investigated the effect on the total impulse generated by the system for three anode geometries (cylindrical, bell, and plate), two pulse current profiles (square wave and sawtooth) and two pulse durations (<NUM> and <NUM>), but did not investigate triggering characteristics.

<CIT>) describes a pulsed plasma thruster comprising an anode, cathode, voltage source and a solid propellant bar and an initiator. The solid propellant bar extends longitudinally and is held for progressive advancement in a downstream longitudinal direction to a gap between the cathode and anode, and the initiator is used for initiating arc discharge between the anode and cathode by inducing thermionic emission of electrons, which electrons are drawn toward the anode and tend to induce ionization of material on an exposed surface of the bar so as to initiate said arc discharge in a flashover.

<NPL> is a paper describing a hydrodynamic model of a vacuum arc thruster and its plume, and the effect of a magnetic field on the plume expansion and plasma generation of a magnetically enhanced co-axial vacuum arc thruster (MVAT) and a vacuum arc thruster with ring electrodes (RVAT). In this study, it was found that the magnetic field significantly decreases the plasma plume radial expansion under typical conditions.

<NPL> is a paper on the study on cathode spot motion and macroparticles reduction in axisymmetric magnetic field-enhanced vacuum arc deposition and focuses on the use of an axisymmetric magnetic field (AMF) applied to the cathode surface to investigate the influence of the AMF on the cathode spot motion and the macroparticle reduction on TiN films. The results of this study show that the AMF affected the cathode spot motion by redistributing the dense plasma connected with the initiation of the new spot.

The present disclosure relates to an internal wire-triggered pulsed cathodic arc propulsion system according to claim <NUM>.

In a second aspect, the present disclosure provides a spacecraft propulsion system comprising the above-mentioned propulsion system.

In a third aspect, the present disclosure provides a spacecraft comprising the above-mentioned spacecraft propulsion system.

One or more embodiments of the present disclosure will now be described by way of specific example(s) with reference to the accompanying drawings, in which:.

Method steps or features in the accompanying drawings that have the same reference numerals are to be considered to have the same function(s) or operation(s), unless the contrary intention is expressed or implied.

The present disclosure provides a pulsed cathodic arc (PCA) propulsion system suitable for use in spacecraft and related applications. Such use may include, for example, application as a thruster for accelerating a spacecraft. A PCA system uses an arc discharge to generate plasma, wherein the plasma flows in part along electric field lines in order to complete a circuit. The plasma consists of ions, electrons and neutral vapour. The PCA system operates in a vacuum or near vacuum environment, such as space, and uses pulses of very short duration, in the range of <NUM> to <NUM>.

The PCA propulsion system of the present disclosure uses a conductive or semiconductive solid fuel in the form of a solid cathode. A trigger applies a short duration, high current surface flashover across an insulating barrier between a trigger pin or wire and an upper surface of the cathode. The flashover generates cathode spots on the upper surface of the cathode, leading to the creation of plasma. The production of plasma is maintained by a potential difference between the cathode and an anode, and the cathode spots move across the upper surface of the cathode, ejecting ionised material at high velocity in a narrow cone shape directed normal to the cathode surface.

The use of a solid fuel obviates the need for tanks, pipes, valves, or regulators, which are drawbacks of gas fuelled electrical propulsion technologies. Embodiments of the PCA propulsion system use a solid cathode made from a suitable conductive or semiconductive material. Suitable materials will depend on the particular application, but may include, for example, magnesium, titanium, aluminium, calcium, carbon, vanadium, molybdenum, tungsten, tantalum, tin, bismuth, chromium, iron, nickel and alloys or composites thereof. In addition to their conductive or semiconductive nature allowing arc triggering and current flow, such cathode materials typically have a much higher melting point than PTFE, overcoming the efficiency problems exhibited by PPTs, such as evaporation of the propellant feedstock between plasma pulses.

Electrical triggers utilise a short duration, high current surface flash-over across an insulating barrier between a trigger-wire and the face of the cathode. The trigger-wire and insulating spacer can be located anywhere on the face of the cathode.

Electrical triggering methods use a pulse of high voltage, typically in the kilovolt range, over distances of millimetres to create electrical breakdown conditions over the face of an insulating material. These arcs create the intense electric fields necessary for the creation of cathode spots, leading to the creation of plasma. Plasma production is maintained by the potential difference between cathode and anode. The cathode spots move along the cathode surface, ejecting ionised material at high velocity in a cone directed normal to the cathode surface. Advantages of electrical triggering methods include the lack of moving parts and robustness. Further, electrical triggering methods have a repetition rate limited only by the power supply used to drive the trigger.

In an internal wire-triggered electrical triggering system, the trigger may be implemented using a wire made of tungsten, or some other refractory metal, sheathed in an insulating tube, said tube being made of an acceptably temperature and vacuum stable non-conductive material such as alumina, boron nitride, PTFE, or tempered glass, with an end of the trigger wire locally protruding above an upper surface of the cathode, typically but not necessarily at the centre of the cathode, so that the trigger wire and insulating tube are contained within the bulk of the cathode. In contrast, edge-triggering systems use an annular (ring-shaped) trigger surrounding or offset from an annular insulator that separates the cylindrical or tubular cathode from the annular trigger. The trigger element, whether a pin, wire or ring, is rapidly charged and an electric field strong enough to cause breakdown across the surface of the insulator between the trigger and cathode is created. Dense electrical arcs travel down these breakdown paths to the cathode, causing localised plasma generation to occur. The plasma generation sites are known as "cathode spots", and cathode spots in an internal wire-triggered arc are free to traverse the upper surface of the cathode. The plasma initiates a high current discharge on the cathode surface, which is driven by a low impedance power supply capable of delivering currents ranging from tens to thousands of amperes for short periods of time. The trigger pulse is commonly only a few microseconds long and is of low current, as the trigger pulse only exists to close the "switch" needed to fire the arc by creating a low impedance pathway from cathode to anode.

<FIG> is a schematic representation of an internal wire-triggered pulsed cathodic arc propulsion system <NUM> adapted to generate a plasma discharge when operated in a vacuum. The system <NUM> includes an anode <NUM> and a cathode <NUM>. The cathode <NUM> is held in position by a cathode mount <NUM>. The cathode <NUM> is a solid cathode made from magnesium, titanium, aluminium, calcium, carbon, vanadium, molybdenum, tungsten, tantalum, tin, bismuth, chromium, or an alloy or composite thereof. In this example, the solid cathode <NUM> is made from magnesium, and the trigger pin/wire is coaxial with the cylindrical anode, with a short protruding length centrally located in the outer face of the cathode as a "centre-triggered" arc.

The system <NUM> also includes a trigger for triggering an arc on an upper surface of the cathode <NUM>, so as to generate cathode spots and thus lead to the generation of a plasma discharge. Electrical triggering methods use a pulse of high voltage, typically in the kilovolt range, over distances of millimetres to create electrical breakdown conditions over the face of an insulating material. In this example, the trigger is an electrical centre-trigger with an elongated trigger pin <NUM> positioned through the middle of the cathode <NUM>. A trigger insulator <NUM> is positioned between the trigger pin <NUM> and the cathode <NUM>.

In operation, a charging voltage in the range of approximately 50V to 500V is applied between the anode <NUM> and the cathode <NUM>, which establishes an electric field therebetween. A high voltage trigger signal is applied to the trigger pin <NUM>, which creates electrical breakdown conditions over the face of the trigger insulator <NUM> and generates cathode spots on an upper surface of the cathode <NUM>. The trigger signal is in the kilovolt range, from approximately 1kV to 20kV. In one example, the trigger signal is <NUM>.

Material ejected from the cathode spots forms a dense plasma. A "running" or "burning" voltage of approximately 30V to 100V is maintained between the anode <NUM> and the cathode <NUM> while the plasma arc is in operation. In the internal wire-triggered system <NUM> of <FIG>, the cathode spots travel radially outwards from the trigger site, following a dendritic pattern, that is, a semi-random movement or motion with branches that resemble tree branches as those branches radiate from a central position or trunk. This radial motion is driven by repulsion arising from the phenomenon of retrograde JxB motion.

The repulsion of cathode spots is opposite to the expected attraction of two parallel currents. A similar repulsion effect is observed in edge-triggered systems, but since the cathode spots are created on the edge of the cathode the spots travel around the edges of the cathode, sometimes moving towards the centre. Though the spot motion appears macroscopically smooth, it is a result of individual spots extinguishing and new spots igniting at the edge of the crater created by the previous spot. The motion is therefore subject to discontinuities and jumps. <FIG> illustrate cathode spot migration and cathode erosion profiles for an internal wire-triggered cathodic arc propulsion system with a centrally located trigger wire. <FIG> shows the trigger pin <NUM> and eroded material <NUM> on the surface of the cathode. <FIG> illustrate cathode spot migration and cathode erosion profiles for an edge-triggered cathodic arc propulsion system. <FIG> shows eroded material <NUM> eroded from the surface of the cathode.

Since the plasma plume is directed normal to the local cathode surface, thrust is optimised by ensuring that the erosion profile is as flat as possible. An internal wire-triggered system has the advantage that the slope of the eroded zone can be adjusted by tuning the magnitude of the arc current as a function of the radial position of the spots during each pulse. The retrograde motion of cathode spots causes the cathode spots in edge-triggered systems to erode material preferentially from the edge of the cathode, leading to an undesirable convex erosion profile, which leads to a reduction of plasma transport parallel to the longitudinal axis of the system and hence loss of thrust after extended use. While the wear patterns created during the use of an internal wire-triggered cathodic arc result in lowered efficiency compared to a non-eroded cathode surface, the concave erosion results in a smaller drop in performance than a convex erosion profile, which can be mitigated by appropriate pulse shaping.

<FIG> is a schematic representation of an alternative arrangement <NUM> of the internal wire-triggered pulsed cathodic arc propulsion system of <FIG>. The arrangement <NUM> of <FIG> includes an insulator <NUM> in the form of an annular Cathode Spot Inhibitor (CSI). The CSI ring <NUM> acts to stop or inhibit the cathode spots from side-arcing or travelling from the cathode <NUM> onto the cathode mount <NUM> itself, thus increasing system efficiency and longevity. In one arrangement, the CSI <NUM> is made out of a vacuum and thermally stable ceramic, such as alumina, tempered glass, boron nitride, or other suitable material. In one arrangement, the CSI ring <NUM> is secured in place using suitable fasteners, such as bolts, threaded rod, or the like.

<FIG> is a schematic representation of a propulsion system <NUM> embodying the internal wire-triggered pulsed cathodic arc propulsion system <NUM> of <FIG>. The propulsion system <NUM> includes an anode voltage probe <NUM> connected to the anode <NUM>. Connected in parallel to the anode voltage probe <NUM> is an anode current Rogowski coil <NUM> for measuring the current supplied to the anode <NUM>. The propulsion system <NUM> also includes a cathode voltage probe <NUM> connected to the cathode mount <NUM> and a cathode current Rogowski coil <NUM> arranged to measure the current supplied to the cathode <NUM>. The voltage probes <NUM>, <NUM> and Rogowski coils <NUM>, <NUM> are used to measure the plasma parameters.

The system <NUM> further includes a high voltage trigger supply <NUM>, which in the example of <FIG> is rated at approximately 1200V. The high voltage trigger supply <NUM> is coupled to the trigger pin <NUM> via a trigger switch <NUM>. Closing the trigger switch <NUM> has the effect of applying a high voltage trigger signal to the trigger pin <NUM>, to generate electrical breakdown conditions over the face of the trigger insulator <NUM>, as described above. In the example of <FIG>, the trigger signal is approximately <NUM>. The system <NUM> further includes a main capacitor bank <NUM> rated at 21mF in the range from <NUM>-450V. The main capacitor bank <NUM> can dissipate its stored energy through the cathode <NUM> over a period of less than <NUM> millisecond, thus supplying the high power required to activate the cathode spots.

Once the arc has been triggered, the arc forms a low-impedance short circuit between the cathode <NUM> and anode <NUM>, through which the main capacitor bank <NUM> discharges. This means that the trigger circuit acts as a switch for the main capacitor bank <NUM>, allowing current to flow through the cathode <NUM>, then through the plasma to the anode <NUM> and the grounded walls before being earthed. The trigger circuit itself may be controlled using a computer program.

Various configurations of capacitors can supply current to the cathode as a pulse with either a square or a sawtooth profile. The erosion profile of the cathode <NUM> differs depending on the current profile. The main capacitor bank <NUM> ordinarily delivers a sawtooth pulse. In the example of <FIG>, a bank of fast-rising "speed-up" capacitors <NUM> mounted close to the cathode can be connected to the circuit via a switch <NUM> to alter the current profile to deliver a square pulse. In the example of <FIG>, the bank of speed-up capacitors <NUM> supplies an extra <NUM>. 5mF to the total capacitance of the power supply.

<FIG> is an illustration of an alternative arrangement of the propulsion system <NUM> of <FIG>. In the example of <FIG>, the propulsion system <NUM> includes a magnetic nozzle <NUM> and an associated magnetic coil power supply <NUM>. The magnetic nozzle <NUM> can be used to direct a plasma plume <NUM> emitted from the cathode <NUM> by varying a magnetic field applied to an opening of the propulsion system <NUM>.

The propulsion system <NUM> of <FIG> replaces the trigger switch <NUM> with a trigger control system <NUM>. The trigger control system <NUM> may be implemented, for example, by using one or more switches and an associated controller to control actuation of those switches. In one arrangement, the trigger control system <NUM> includes a processor, a memory, and a computer program stored in the memory and adapted to be executed on the processor, whereby when executed the program provides an interface to a user to enable a user to select between an open and closed state of the switch(es), thereby to control application of a high voltage trigger signal from the high voltage trigger supply <NUM> to the trigger pin <NUM>.

The propulsion system <NUM> of <FIG> further includes a pulse control and termination system <NUM>, which controls grounding of the main capacitor bank <NUM> at the conclusion of a pulse to ensure a nominated pulse duration. In one arrangement, the pulse control and termination system <NUM> includes a processor, a memory, and a computer program stored in the memory and adapted to be executed on the processor, whereby when executed the program controls grounding of the main capacitor bank <NUM>.

<FIG> is an illustration of a sawtooth current profile delivered by the main capacitor bank <NUM> and <FIG> is an illustration of a square current profile delivered by the main capacitor bank <NUM> in series with the bank of fast-rising speed-up capacitors <NUM>.

The difference between the current through the cathode <NUM> and the current collected at the anode <NUM> is a measure of the plasma available to produce thrust. The difference between these two currents is termed the Net Ejected Current (NEC), which is a measure of how much plasma is flowing out of the anode <NUM> mouth.

A high NEC indicates that a large degree of plasma material is escaping, and thus generating thrust. Conversely, a low NEC indicates that most of the plasma material is travelling directly to the anode, impacting there and thus generating little net force. Integration of the NEC over the duration of the pulse determines the Net Ejected Charge (NEQ). NEQ correlates strongly with the impulse delivered by a PCA thruster, where impulse is a measure of the total change in momentum experienced by a spacecraft to which the PCA thruster is attached.

After the arc has been initiated, the cathode spots will migrate to the edge of the upper face of the cathode <NUM>, due to the repulsive force between cathode spots. If the arc is not stopped when the cathode spots reach the edge of the cathode face, the cathode spots will move onto the side surfaces of the cathode <NUM>. Since the plasma would then be projected normal to the cathode sides, the plasma would move straight to the wall of the anode <NUM>, rather than exiting the anode mouth. This phenomenon is termed "side-arcing" and can result in major losses in efficiency.

<FIG> are schematic side-view representations of plasma generation using the internal wire-triggered pulsed cathodic arc propulsion system of <FIG>. In particular, <FIG> illustrate the movement of cathode spots and evolution of the arc from triggering to side-arcing. <FIG> illustrates the system <NUM> of <FIG> at a point in time at which an arc <NUM> has been triggered by applying a high voltage of approximately 1200V to an upper end of the trigger pin <NUM>. The high voltage is applied in a pulse to generate a flashover between the trigger pin <NUM> and the cathode <NUM>. A potential difference in the range of <NUM>-400V exists between the cathode <NUM> and the anode <NUM>.

After the arc has been triggered and electron current between the cathode <NUM> and anode <NUM> is sustained, the creation of cathode spots <NUM> occurs close to the location of the trigger <NUM>, as shown in <FIG>. These spots repel each other and are forced outwards as they erode cathode material to form the arc plasma <NUM>, as shown in <FIG>. <FIG> illustrates side-arcing <NUM>. If the arc is not halted by grounding the power supply at the correct time, then the cathode spots will continue to push each other outwards, causing the cathode spots to cross over the edge of the upper surface of the cathode <NUM> and commence eroding material from the side faces of the cathode <NUM> directly to the anode <NUM>. Note that <FIG> is a side-view; all cathode spots are located near the edge of the cathode upper surface, only a few have moved to the side surface to begin side arcing, and those spots that appear to be producing plasma proximal to the trigger location have moved away from the viewer and into the page.

Any PCA thruster embodied in a spacecraft needs to operate for hours to effect any orbital transfer manoeuvre and to be able to commence operations after dormant periods of days to months in the case of orbital station-keeping or course correction manoeuvres. This requires that the arc be able to operate reliably for extended periods of time, with many thousands of pulses triggered reliably.

As the arc fires, it erodes material from the surface of the cathode, gradually wearing away the surface of the cathode and leading to difficulty in triggering the arc if there is too much distance between the trigger pin and the cathode. In one arrangement, the internal wire-triggered pulsed cathodic propulsion system includes a feed mechanism for advancing the eroding cathode to ensure optimal cathode firings. Such a feed mechanism periodically advances the cathode to ensure that the distance between the trigger pin and the cathode is not too great.

In one implementation, the feed mechanism includes a spring biased to advance the cathode relative to the trigger, as the cathode erodes. In another implementation, the feed mechanism includes mechanical actuators to advance the cathode relative to the trigger. Such a mechanical actuator may include, for example, a worm drive, wherein the worm drive includes a worm wheel adapted to engage a worm screw forming part of the cathode. A motor drives the worm wheel, which in turn engages the worm screw to advance the cathode in the desired direction. The worm screw may be integral with the cathode or be attached to an outer surface of the cathode.

One arrangement of the internal wire-triggered pulsed cathodic arc propulsion system includes a trigger feed mechanism to advance the trigger insulator <NUM> and trigger pin <NUM>. Over a period of time of use, it is possible that the wear rate of the trigger insulator <NUM> and trigger pin <NUM> differs from the wear rate of the cathode <NUM>. The trigger insulator <NUM> and trigger pin <NUM> may also be sputtered by ions expelled from the cathode spots. <FIG> is a schematic representation of the internal wire-triggered pulsed cathodic arc propulsion system of <FIG> with the addition of a trigger feed mechanism <NUM>, and a trigger insulator feed mechanism <NUM>. The trigger feed mechanism <NUM> and trigger insulator feed mechanism <NUM> can be used to advance or withdraw the trigger insulator <NUM> and trigger pin <NUM> to ensure optimal operation. As for the feed mechanism described above in relation to the cathode, each of the trigger feed mechanism <NUM> and trigger insulator feed mechanism <NUM> may be implemented using a biased spring, mechanical actuator, such as a worm gear arrangement, or other suitable mechanism.

In order to derive maximum thrust from the plasma discharged from the cathode, it is advantageous to direct the plasma. The plasma expands in a cone normal to the cathode surface from each cathode spot, with the spatial plasma distribution being conical, with an approximately cosinusoidal angular distribution. That is, the plasma is primarily ejected normal to the plane of the cathode surface, with smaller quantities being ejected at angles further from the normal direction. One arrangement applies a magnetic field to focus the plasma, so that a higher proportion of the plasma plume is normally directed and thus enhance the thrust.

One implementation uses permanent magnets to direct the plasma plume. Another implementation uses the discharge current to generate a magnetic field to direct the plasma plume. In this implementation, either the cathode current or anode current, or both the cathode and anode currents, are run through field coils located downstream of the cathode face. Another implementation has a dedicated power supply, such as a separate capacitor bank, to deliver current to the field coils. All implementations provide improved thrust with minimal or no additional power consumption and a small increment in mass.

<FIG> is a schematic representation of an internal wire-triggered pulsed cathodic arc propulsion system <NUM> with a magnetized nozzle <NUM> and a cathode feed mechanism. This magnetic nozzle may be placed before, surrounding, or after the cathode front face, so that the magnetic field lines from the nozzle are converging, parallel or diverging, according to the specific needs of the fuel and charge combination. The propulsion system <NUM> includes a cathode <NUM> attached to a cathode mount <NUM> and an anode <NUM>. A potential difference is applied between the cathode <NUM> and the anode <NUM>.

The propulsion system <NUM> also includes a centrally located electrical trigger system that includes a trigger pin and associated insulator <NUM>. As described with reference to <FIG> and <FIG> and <FIG>, a high voltage applied to the trigger causes an arc discharge on an upper surface of the cathode <NUM>. The resultant plasma plume <NUM> is generally conically shaped and is emitted normally to the upper surface of the cathode <NUM>. The magnetized nozzle <NUM> is implemented using one or more magnets, such as permanent magnets, or magnetic coils. Placing the magnetized nozzle <NUM> at the mouth of the anode <NUM> directs the plasma plume <NUM> to improve the thrust of the propulsion system <NUM>.

The cathode feed mechanism is implemented using a worm gear <NUM> driven by a cathode advancing motor <NUM>. In this arrangement, the worm gear <NUM> is adapted to engage with a worm thread cut into an outer surface of the cathode <NUM>. The worm gear <NUM> may be implemented by cutting a thread into the cathode mount <NUM>, the threads on the cathode <NUM> and the cathode mount <NUM> being the same so as to engage with one another. The motor is coupled to a control system to advance the cathode <NUM> at a predefined rate, based on the erosion, or expected erosion, of the upper surface of the cathode <NUM>. The actual dimensions of the cathode <NUM> will depend on the particular combination of charge voltage, duration, and the material being used. In one arrangement, a cathode made from magnesium and suitable for a long-duration space mission is at least <NUM> long and has a diameter in the range of <NUM> to <NUM>.

A further advantage relating to the internal wire-triggered pulsed cathodic propulsion system of the present disclosure relates to the net negative charge of the plasma exhaust plume. A pulsed arc will therefore not require charge neutralisation systems, such as are needed for Hall Effect thrusters or gridded ion thrusters.

The invention includes a steering device to steer cathode spots across the surface of the cathode. Such a steering device is implemented by switching more capacitors into the discharge at the correct times, which minimises the deposition of eroded material onto the ridges left on the cathode surface by the erosive processes of arc operation. Another implementation not according to the invention of a steering mechanism may be as a side-effect of using a magnetic nozzle, as the imposed magnetic field causes the cathode spots to move in a spiral pattern due to the retrograde JxB effect.

<FIG> illustrates a sample circuit diagram <NUM> showing a capacitor charging power supply <NUM> supplying power to a set of sub-banks of capacitors C<NUM>,. , Cn in parallel with a main capacitor bank Cm, corresponding to the main capacitor bank <NUM> of <FIG>. The main capacitor bank Cm is in series with an inductor Lm and a switch Sm. Each of the sub-banks of capacitors C<NUM>,. , Cn is arranged in a similar configuration, with capacitor C<NUM> in series with an inductor L<NUM> and a switch S<NUM>, and so on through to capacitor Cn being in series with an inductor Ln and a switch Sn. Each parallel arm of the circuit <NUM> includes a diode to ensure unidirectional current flow through the circuit <NUM>.

Referring to <FIG>, each of the sub-banks of capacitors C<NUM>,. , Cn is charged to the same voltage as the main capacitor bank Cm. The switch Sm corresponds to the trigger switch <NUM> of <FIG>. When the switch Sm is closed, corresponding to the trigger switch being closed, the main capacitor bank Cm discharges current through the inductor Lm and into the cathode <NUM> to power the plasma arc. After a short, predefined time interval, such as a few tens to hundreds of microseconds, switch S<NUM> is closed, allowing current to flow from the sub-bank capacitor C<NUM> through the inductor L<NUM> to the cathode <NUM>. The inductor L<NUM> is present to ensure that the rise time of sub-bank C<NUM> is sufficiently small. Increasing the flow of current to the anode <NUM> increases the number of cathode spots that form on the surface of the cathode <NUM>. Increasing the number of cathode spots ensures that fewer surface ridges are formed on the upper surface of the cathode <NUM>.

After a further predefined time interval, switch S<NUM> is closed, allowing current to flow from the sub-bank capacitor C<NUM> through the inductor L<NUM> to the cathode <NUM>. This process is repeated for each of the switches S<NUM>,. , Sn, in turn. In one arrangement, the pulse control and termination system <NUM> of <FIG> controls operation of the switches S<NUM>,.

<FIG> is a schematic representation of a spacecraft <NUM> having a plurality of thrusters arranged in a thruster pod <NUM>. In the example of <FIG>, the spacecraft <NUM> is a satellite having a body portion <NUM>, solar panel arrays <NUM>, and the thruster pod <NUM>. In this example, the thruster pod <NUM> includes seven thrusters, wherein each thruster uses a PCA propulsion system of the type described herein. Each thruster includes a cathode-anode PCA propulsion system <NUM> and a magnetic nozzle <NUM>. The magnetic nozzle <NUM> can be used to control the direction of a plasma plume ejected from the respective thruster. It will be appreciated that the number of thrusters in the thruster pod <NUM> and the arrangement of those thrusters will depend on the particular application, including the size of the spacecraft.

The thruster pod <NUM> is used for orbital station-keeping, whereby one or more of the thrusters in the thruster pod <NUM> is activated as a thruster burn to place or keep the satellite <NUM> in an assigned orbit. Such thruster burns may be used, for example, to compensate for the gravitational forces from the Earth, the Sun, and the Moon, and atmospheric drag.

<FIG> is a schematic representation of a spacecraft <NUM> having multiple PCA propulsion system thrusters. In the example of <FIG>, the spacecraft <NUM> is a communications satellite <NUM> having a main body portion <NUM> and solar panel arrays <NUM>. The satellite <NUM> also includes first and second thrusters <NUM>, <NUM> longitudinally opposed at opposite ends of the body <NUM> of the satellite <NUM>. The first and second thrusters <NUM>, <NUM> are used for station keeping of the satellite <NUM> in a North/South direction. The satellite <NUM> also includes a third thruster <NUM> and a fourth thruster (not shown), which are transversely opposed about a middle portion of the body <NUM>. The third thruster <NUM> and fourth thruster are used for station keeping of the satellite <NUM> in an East/West direction.

The arrangements described are applicable to the space industries.

In the context of this specification, the word "comprising" and its associated grammatical constructions mean "including principally but not necessarily solely" or "having" or "including", and not "consisting only of". Variations of the word "comprising", such as "comprise" and "comprises" have correspondingly varied meanings.

Claim 1:
A pulsed cathodic arc propulsion system (<NUM>) comprising:
a solid conductive or semiconductive cathode (<NUM>);
an anode (<NUM>) having a potential difference relative to said cathode (<NUM>), said potential difference creating an electric field between said anode (<NUM>) and said cathode(<NUM>); and
a main capacitor bank (<NUM>), configured to dissipate its stored energy in the form of a pulse through the cathode (<NUM>);
the system further comprising
an insulated electrical trigger (<NUM>) comprising a trigger pin (<NUM>), a high voltage trigger power supply and an insulator between said trigger (<NUM>) and said cathode (<NUM>) adapted to trigger an arc discharge from a point on a upper surface of said cathode (<NUM>) in pulses, when said trigger (<NUM>) and cathode (<NUM>) are substantially in a vacuum, said trigger (<NUM>) being bounded within the cathode (<NUM>) so that the point at which the arc is triggered is located on the upper surface of said cathode (<NUM>), wherein the system is triggered by a high current surface flashover, wherein the insulated electrical trigger (<NUM>) is located at a trigger point on the upper surface of the cathode (<NUM>), said trigger point being within a perimeter of said upper surface and is adapted to create the high current surface flashover across said insulator between the trigger pin (<NUM>) and the upper surface of the cathode (<NUM>) to generate cathode spots (<NUM>) to create a plasma driven by the high voltage trigger power supply, generating a trigger pulse,
a pulse control and termination system (<NUM>), adapted to control grounding of the main capacitor bank (<NUM>) at the conclusion of the pulse to ensure a nominated pulse duration,
characterized by
a steering device to steer cathode spots (<NUM>) across the surface of the cathode (<NUM>), wherein the main capacitor bank Cm (<NUM>) is in series with an inductor Lm and switch Sm, wherein the main capacitor bank (<NUM>) is in parallel with a set of sub-banks of capacitors C<NUM>,.., Cn where each capacitor (Ci) is in series with an inductor Li and a switch Si, and wherein the pulse control and termination system (<NUM>) is adapted to control operations of the switches, and
the arc discharge pulses for producing the plasma are maintained between the cathode (<NUM>) and the anode (<NUM>) for a duration of between <NUM> to <NUM>.