Physics and performances of a low-power high-current vacuum arc thruster

The Vacuum Arc Thruster (VAT) is a spacecraft electric propulsion technology that is based on the use of a solid metal propellant. VATs are now considered a promising option for cubesats and microsats propulsion thanks to their small size and low power consumption. Recent VAT architecture rely on high-current and short pulse duration discharge to enhance performances. Research on high-current vacuum arcs is nevertheless relatively limited, especially in the context of space propulsion. For example, investigation of the way the plasma emission sites, so-called cathode spots, develop over the cathode surface is of major interest for the selection of architecture and choice of materials. A better understanding of the high current vacuum arc dynamics is therefore necessary to propose technological solutions with longer lifetime and optimized performance.

In this study, experiments were conducted with the PJP thruster, a pulsed 30 W VAT developed by COMAT. The thruster was equipped with a copper anode and several different metallic cathodes, allowing a maximum discharge current from 2 to 4 kA over 30 μs. Pictures taken with a long exposure time compared to the discharge duration allow to visualize the overall repartition of spots in the course of a plasma discharge. High-frequency recordings with a phantom TMX7510 high-speed camera reveal the dynamics of cathode spots during a single pulse. A home-made torsional thrust stand allows to compare the thrust produced by the different cathode materials. Analysis of the long exposure photographs, high speed images and thrust measurements was carried out for the different cathode materials.

A significant difference in spot distribution is observed among the various cathode materials. Examination of the high-speed images provides an explanation for phenomena observed in long-exposure images. The faster the spots propagate, the more surface they cover during the discharge as they rarely reignite in areas where they have already been active. The thrust measurements link the performance of the thruster to the spot dynamics. On average, we observe that richer spot dynamics result in higher thrust. Based on all observations, we can finally establish a link between the cathode material, the spot behavior, and the VAT thrust level.