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.
The ion energies and fluxes in the high power impulse magnetron sputtering plasma from a Nb target were analysed angularly resolved along the tangential direction of the racetrack. A reactive oxygen-containing atmosphere was used as such discharge conditions are typically employed for the synthesis of thin films. Asymmetries in the flux distribution of the recorded ions as well as their energies and charge states were noticed when varying the angle between mass-energy analyser and target surface. More positively charged ions with higher count rates in the medium energy range of their distributions were detected in +E × B than in −E × B direction, thus confirming the notion that ionisation zones (also known as spokes or plasma bunches) are associated with moving potential humps. The motion of the recorded negatively charged high-energy oxygen ions was unaffected. NbO x thin films at different angles and positions were synthesised and analysed as to their structure and properties in order to correlate the observed plasma properties to the film growth conditions. The chemical composition and the film thickness varied with changing deposition angle, where the latter, similar to the ion fluxes, was higher in +E × B than in −E × B direction.
Micropropulsion systems are rapidly gaining attention from the small satellite community as they can increase the mission lifetime and allow the satellite to perform complex maneuvers and precise attitude control. These systems need to be fully operational with the low power available on satellites. Various thruster concepts based on vacuum arcs are currently under development, predominantly in the pulsed regime due to the power constraints on small spacecraft. Pulsed vacuum arc thrusters are capable of efficiently producing highly-ionized supersonic plasma at very low average power. This Perspective article provides a critical analysis and a review of various aspects of electric propulsion technology based on vacuum arcs. Furthermore, we give a personal assessment of the present status and provide an outlook on the field, including the growing role in small satellites such as CubeSats. Vacuum arc micropropulsion systems could play an important role in mitigating the problem of space debris. Such a system could be integrated with a satellite so that, at the end of its mission and using metal components as solid fuel, it will lower the satellite’s orbit and accelerate reentrance into the atmosphere faster than by its natural decay rate.
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