High thrust-to-power ratio micro-cathode arc thruster

The Micro-Cathode Arc Thruster (μCAT) is an electric propulsion device that ablates solid cathode material, through an electrical vacuum arc discharge, to create plasma and ultimately produce thrust in the μN to mN range. About 90% of the arc discharge current is conducted by electrons, which go toward heating the anode and contribute very little to thrust, with only the remaining 10% going toward thrust in the form of ion current. A preliminary set of experiments were conducted to show that, at the same power level, thrust may increase by utilizing an ablative anode. It was shown that ablative anode particles were found on a collection plate, compared to no particles from a non-ablative anode, while another experiment showed an increase in ion-to-arc current by approximately 40% at low frequencies compared to the non-ablative anode. Utilizing anode ablation leads to an increase in thrust-to-power ratio in the case of the μCAT.


I. INTRODUCTION
The Micro-Cathode Arc Thruster (µCAT) is a scalable micro-propulsion sub-system developed by the George Washington University's (GWU) Micro-propulsion and Nanotechnology Lab for small spacecrafts, notably Cube Satellites (CubeSats.)A one unit (1U) CubeSat is a miniaturized satellite approximately 10 cm x 10 cm x 10 cm in volume and up to 1.33 kg is mass.CubeSat launches have been rapidly growing over the years, but despite the promising increase, maneuverability remains an issue.The majority of CubeSats cannot maneuver themselves, meaning mission functionality is limited and probability of collision is likely to increase with time.At the 65th International Astronautical Congress, it was stated that over a third of the CubeSats launched will stay in orbit for greater than 25 years. 1,2While currently only accounting for a small amount of actual space debris, CubeSats have approached other objects, within 5 km or less, over 360,000 times.Projections increase this to millions by 2043, with a few actually colliding. 1CubeSat lifetime is dependent upon the CubeSat's mass and orbit altitude, with altitudes less than 300 km lasting up to 100 days, and altitudes greater than 600 km lasting over 25 years. 3However, as of May 2015, the µCAT reached a NASA technology readiness level of seven (TRL7) with the successful launch of the United States Naval Academy's BRICSat-P.The µCAT sub-system was not only able to successfully fit into the BRICSat-P, it was also able to successfully de-tumble the CubeSat to less than 1 degree/minute.
The µCAT is based off of the vacuum arc process where an arc flows through a medium between two electrodes, a cathode and anode.In a vacuum environment, there is no medium for which electricity can travel, therefore, for a vacuum arc to occur it is necessary to introduce a medium by means of cathode evaporation.For this phenomena to occur, a high current (0.5 -300 A) 4 ), low voltage (20 − 50 V 5 ) arc is passed through the cathode, which, in turn, creates a high temperature, micrometer sized emitting area known as a cathode spot.The cathode spot ranges in lifetime between nanoseconds to microseconds 6 during which cathodic material will be expelled at high velocities normal to the cathode surface, producing quasi-neutral plasma, leaving behind a crater.Since the thruster will be in a vacuum environment before the ignition process begins, there needs to be an initial triggering method to create a medium to start the arc.In the past, triggering was accomplished through plasma gas injection, mechanical triggering, fuse wire explosion, or contact separation, however, these methods only add bulk and complexity.A"triggerless" method was developed by Anders et al. 7 whereby a thin conducting film is placed between the anode and cathode.The conducting film, carbon paint for the µCAT, creates a complete circuit between the anode and cathode, and as the applied current goes through the conducting film, it begins evaporating through thermal heating.As the conducting film evaporates, there is now a medium by which the arc can travel.This conducting film is only initially needed as ion bombardment will re-coat the surface allowing for the triggering process to start again if the thruster is turned off.The µCAT enhances the vacuum arc process by utilizing an external magnetic field to guide ions and allow for uniform erosion.
The current µCAT configuration, in the case of nickel propellant, has a thrust-to-power ratio of about 20 µN/W and an efficiency peaking around 15%.A major efficiency limitation stems from the fact that ions make up only about about 10% of arc discharge current while the remaining 90% goes to heating the anode through electrons. 8,9Since ions are the major contribution to thrust levels and the majority of this power is deposited into the anode, this means that 90% of the arc discharge current is not being utilized.It is theorized that by utilizing an ablative anode, or modifying the anode geometry, anode material can be expelled thereby increasing the flow rate.Since thrust can be expressed as T = ṁv e , it can be shown that an increase in flow rate will increase thrust levels if the exit velocity is maintained.In the thruster's acceleration region, flow is created though ions transferring momentum to neutrals through collisions, producing neutral flow, and through ions being accelerated themselves, creating ion flow. 10Injecting neutral anode material into this region will subsequently increase both neutral flow and ion-neutral flow.Some estimations 9 show that thrust can be enhanced by where L is the acceleration region and λ is the mean free path for ion-neutral collisions.Using the µCAT, 9 with an acceleration region of 1 cm and and an ion-neutral mean free path of 1 mm, the thrust-to-power level is expected to increase by a factor of three.Similar to the mechanics of the Hall Thruster it is possible that electrons may become trapped in the µCAT magnetic field, leading to further neutral ionization and an additional increase to thrust.Schematically this concept is shown in Figure 1.
The current µCAT thrust-to-power ratio may be expressed as where T is thrust, P is power, ṁ is the mass flow rate, I d is the discharge current, f is the ion current fraction (typically 0.08 -0.1), m i is the ion mass, V i is ion velocity, U d is discharge voltage, and e is the elementary charge.
Adding an ablative anode modifies the equation to where α is the ratio between mass fluxes from the anode to the cathode, σ is the collisional cross sectional area (10 19 m −2 ), n is ion density (10 21 m −3 ) and L is the length of the acceleration region (0.01 m).This means that if α is 1 to 2 than the thrust-to-power ratio can be expected to increase by a factor of 2 to 3.

II. EXPERIMENTAL SETUP
A preliminary set of experiments were setup to verify that the inclusion of an ablating anode could enhance thruster efficiency and thrust levels.The first experiment consisted of comparing ion-to-arc current from a µCAT with a titanium anode to that of an anode composed of Sn63/Pb37 solder at various frequencies.The second experiment included two 17 hour lifetime tests of the µCAT, one with an 18-8 stainless steel anode and one with the Sn63/Pb37 solder anode, where the anode mass loss was measured.During the lifetime test, a copper plate collector was placed in front of the µCAT during testing and was analyzed afterwards under a scanning electron microscope (SEM) to determine if any anode particles were present.

A. Ion Current Collection
To measure ion current, a copper flat plate biased to negative 82 Volts was placed over the µCAT.The vacuum chamber was brought to a pressure of 10 −5 Torr.As the µCAT expelled plasma, ions would be collected on the plate and a Tektronix 2004B oscilloscope would record both the µCAT arc discharge voltage and the collected ion voltage from the copper plate.To calculate the arc discharge current and the ion current, respective probes were placed over a 1 Ohm resistor and 0.2 Ohm resistor and Ohm's Law was used.The oscilloscope has an acquisition mode that collects several waveforms, in increments of 4, 16, 64 or, 128, and displays the average of them.This technique allows for the elimination of uncorrelated noise in the signal and is quicker than collecting single waveforms separately and later averaging them together.This technique was used to collect the average of 128 waveforms for µCAT pulse rates of 1, 2, 4, 8, 16, 32, 64 and 128 Hz.A diagram of the experimental setup is shown in Figure 2, with yellow circles representing ions and red circles representing electrons.

B. Anode Mass Loss
Before the µCAT lifetime testing began, five mass measurements of each anode were taken and averaged using a Sartorius CPA225D Semi-Micro balance.Testing ran for 17 hours, for each anode, FIG. 2. Experimental setup for ion current plasma discharge with an ablating anode.at a pulse rate of 10 Hz.The vacuum chamber was brought to a pressure of 10 −5 Torr.After each lifetime run, the anode was again measured five times and the final average mass was recorded.

C. SEM Analysis
During the 17 hour µCAT lifetime test, a piece of copper foil was placed 19.4 mm away from the head of the thruster to collect any plasma particles.Standard particles that were expected on the copper foil include: copper (from the foil itself), titanium (from the cathode), and carbon (from the carbon paint used between the anode and cathode.)However, with the theorized anode ablation, it is also expected that solder particles, both tin and lead, should be present as well.If the 18-8 stainless steel anode were to ablate, chromium, nickel, and iron particles should also be present.

A. Ion Current Collection
The results of ion collection experiment for various frequencies are shown in Figure 3.It can be seen that using an ablative anode, the ion-to-arc current for the solder anode proved to be 45% and 38% higher than the titanium anode at 1 Hz and 2 Hz, respectively.
As the pulse frequency increases, the ion-to-arc current for the titanium anode gets closer to the solder and eventually surpasses it.It is speculated that this observation is due to the creation of macro-particles in the solder anode.Macro-particles are mass droplets that break off consuming a large portion of propellant, contributing very little to thrust and lower the overall thruster efficiency.Figure 4 shows the solder anode before and after firing at a frequency of 128 Hz, where solder macro-particles can be seen condensing on the cathode.This can be problematic since not only did the thruster stop firing due to a massive loss in the anode, but macro-particles can land between the anode and cathode causing a short circuit and also stopping the thruster from firing.

B. Anode Mass Loss
Table I shows the initial mass, final mass and mass difference for the stainless steel and solder anode.
The solder anode lost 0.029566 grams, while the stainless steel anode actually gained mass.The mass loss and gain can be explained when the anodes were examined under the SEM.When When analyzing the copper collector the stainless steel anode showed only the expected elements of copper, titanium, and carbon, showing that there was no anode ablation.The solder anode copper collector showed the expected elements (carbon, copper, titanium, tin, and lead), in addition to tin and lead.This shows that the solder that was ablating was being expelled from the µCAT and most likely being converted into thrust.

IV. POTENTIAL THRUSTER DESIGN
In order to take advantage of the benefits that an ablative anode offers, it is required to have a system that will replenish the anode as it is eroded.Such a system is currently under development at GWU. Figure 7 shows a preliminary schematic.
The design will feature a cathode feed system as well as an anode feed system.The cathode and the anode segments, shown in black and red, respectively, will be fed with springs (shown in yellow).A solenoid located behind the Teflon casing will produce a magnetic field that will steer the arc spot in such a way as to produce a uniform erosion of the cathode.The springs will ensure that the material, for both anode and cathode, will be available for the discharge.The new thruster design will take advantage of the additional mass flow in order to have a higher thrust-to-power ratio when compared to similar devices.The cathode and anode system will allow the thruster to deliver a higher total impulse due to the increase amount of propellant available (both cathode and anode).Equation (3) estimates that the thrust-to-power level can increase by a factor of 2 to 3. Testing of the device will be performed using a thrust balance that is currently being designed in-house using two different configurations: a version with a stainless steel or titanium anode and the second version with the ablatable anode in order to measure the performance increase from the additional anode material mass flow.

V. CONCLUSION
The preliminary anode ablation tests were able to show that using a low melting point anode can increase ion-to-arc current at low frequencies.Higher frequencies lower the ion-to-arc current by increasing anode ablation and through the creation of more macro-particles.SEM images showed that a low melting point anode was found on a collecting plate further showing that the anode is expelling mass which could contribute to thrust levels.These results are the first experimental proof that anode ablation can lead to elevated thrust-to-power rations in the µCAT.Further testing needs to be done, namely thrust measurements and experimenting with other anode materials.
FIG.1.Concept for enhancing thrust through anode ablation.

FIG. 6 .
FIG. 6. SEM image of the (a) stainless steel anode before firing and (b) after firing.