# Thrust Measurements on the High Efficient and Reliable Vacuum Arc Thruster (HERVAT)
## 1 Introduction  
The aerospace industry is undergoing the biggest change since its beginning in 1950: new players in the field of launch services, aerospace technology and space hardware design are rushing into the old agency e.g., National Aeronautics and Space Administration (NASA) or European Space Agency (ESA), shaped business, making space technology available to more users worldwide. In contrast to pure military technology or science missions, the commercialisation of the space is the main focus of these new players like for example SpaceX or Amazon. One of the major projects of those companies are swarm based systems. These systems consist of hundreds or thousands of small satellites (e.g., CubeSats) which are linked together, increasing the performance and availability of the whole system. The use cases for those systems are the communication technology (e.g., internet of space), imaging technologies, science missions and many more. Normally, the satellites are brought into space by carrier rockets, which dispense several hundreds of satellites in a short period of time. In order to start their missions, a successful de-tumbling procedure and precise targeting of each satellite is essential. For these tasks, a low power, less than 2 W, low volume, low cost, highly reliable and efficient thruster technology is required [1]. Pulsed vacuum arc thrusters can fulfill these requirements and are under investigation since many years [2; 3; 4; 5]. Vacuum arc thrusters are based on the cathodic vacuum arc and use a solid cathode for thrust generation. Their basic working principle can be described in four steps:  
1. an electrical breakdown between an anode and cathode is created;
2. a vacuum arc establishes and creates a highly ionized plasma from the cathode spots;
3. the plasma is streaming away from the cathode's surface;
4. the mass flow and velocity of the plasma generates thrust. Although the principle of the vacuum thruster is simple and has been known since the seventies [6], the reliability in terms of maximum achievable pulse number is still not satisfactory for such commercial missions. While the efficiency of the system is well described [2; 7; 8], the long-term reliability is still an open question. The reliability issue results in the physical process of the thrust generation. During each pulse, the cathode material is slightly eroded and partly re-deposited on the thruster, which depends on parameters like the pulse energy, pulse length, cathode material, etc. At some point, this effect leads to an increased gap or a short circuit between the anode and cathode, which results in an ignition failure and makes the thruster inoperable. There are approaches using an extra cathode feeding device to extend the thrusters lifetime but it increases the complexity of the whole system [9]. A high voltage surface discharge ignition with an extra anode has also been investigated but not tested in a real thruster head [10]. To overcome the reliability issues, the Bundeswehr University Munich invented a novel vacuum arc thruster propulsion system to achieve high pulse numbers (high reliable) without an additional feeding system to generate thrust levels comparable to other vacuum arc thrusters with an optimized cathode erosion (high efficient). The so-called \"High Efficient and Reliable Vacuum Arc Thruster\" (HERVAT) uses a unique design for the thruster head and a novel pulse processing unit (PPU) instead of the usual inductive energy storage (IES) system [11]. The PPU was operated for more than \(5\times 10^{7}\) pulses in different experiments with variant frequencies (0.2 to 30 Hz) and pulse energies (0.2 to 5 J). During the tests no failure or degradation of the PPU was observed. The thruster head, together with the PPU, was operated for more than \(10\times 10^{6}\) pulses with a pulse energy of 0.35 J. The absence of a feeding mechanism lowers the overall mass of the system and increases the reliability due to the fact that no moving parts or additional power control lines are required.