# Plasma Expansion Characterization of a Vacuum Arc Thruster with Stereo Imaging
## 1 Introduction  
In many applications, a trend towards smaller and more flexible satellite systems can be observed. Mass and volume savings reduce launch and mission costs, making expeditions possible for smaller companies or research institutions. On the one hand, this miniaturization is possible due to innovatively developed electronics and, on the other hand, due to the use of smaller and lighter propulsion systems, which usually account for a considerable proportion of the satellite's total mass and are thus a major factor in high mission costs [1; 2; 3]. For microsatellites, the vacuum arc thruster (VAT) has established itself as a simple, low-mass and fairly robust propulsion system. The mode of operation is based on a pulsed vacuum arc discharge generated between two electrodes separated by an isolator. The VAT is a solid fuel thruster because it uses a solid cathode material (usually a metal) as a propellant. After ignition at so-called cathode spots, this material is vaporized and ionized, and the expansion of these spots creates a small impulse bit. This simplicity and the ability to easily scale the thrust up and down by simply changing the repetition rate make the VAT promising as an economically priced, compact propulsion system for microsatellites [4; 5; 6; 7]. There are two ways to initiate a discharge. There is the so-called triggerless mode, in which a low-resistance conductive layer exists between the cathode and anode on the insulator;alternatively, there is the trigger mode, which ensures a breakdown and thus a discharge by means of an applied high voltage between the two electrodes [8; 9; 10]. By applying an external magnetic field, which is operating as a magnetic nozzle, the thrust to power ratio of the thruster can be improved. The magnetic field interacts with the charged particles and accelerates them towards the nozzle exhaust. In previous studies, the influence of external magnetic fields on VAT performance has been demonstrated [5; 6; 11; 12; 13]. In order to gain a more detailed insight into the complex processes of a vacuum arc and to study its behavior, many simulations have already been carried out [14; 15; 16]. However, there are still analytic measurements necessary that have been used for the characterization of VATs in the past. Parameters such as plasma potential, ion currents and ion velocities are commonly measured via probes [9; 17; 18]. Ion or electron densities are determined by means of Faraday cups [19]. Moreover, there are optical diagnostics such as observing cathode spots or arc evolution [11; 11; 20]. The goal in the European Union's Horizon 2020 research and innovation program was the development of a commercially successful VAT integrated into the so-called plasma jet pack (PJP). For the development and improvement of this design, simulation as well as the acquisition of experimental data are necessary to obtain a better idea of the operating system and to locate improvement potential [17; 21]. The experimental data are also the basis for an attempted simulation of the system in the project. Numerous plasma diagnostic measurements from other project participants were supplemented with the results of the optical diagnostic investigated in this work. The goal of this study was to observe the expansion of the plasma plume and whether and how the PJP discharge can be manipulated by means of a magnetic nozzle. To obtain three-dimensional information about the plasma plume's expansion, tomographic measurement techniques have been used in previous research. This has been achieved by elaborate measurement setups and reconstruction algorithms [22; 23; 24; 25]. In addition, there have already been other approaches to observe the plume expansion [26; 27]. The idea of the present work was to use a less costly measurement setup to generate a three-dimensional reconstruction of the plume propagation. For this purpose, a pseudo stereo imaging system consisting of one camera with a biprism was assembled, giving us a temporally synchronized view from two directions.