# Thrust Measurements on the High Efficient and Reliable Vacuum Arc Thruster (HERVAT)
## 2 Experimental Setup  
### HERVAT Geometry  
The HERVAT has a concentric design with an auxiliary anode positioned at the central axis of the thruster and a toroidal main trigger anode (Figure 1). The main trigger anode is responsible for the vacuum breakdown and the main current conduction during the HERVAT operation. The auxiliary anode stabilizes the ignition behaviour and arc evolution in the case of cathode being substantially eroded toward the inside of the thruster. This phenomenon occurs because of an increased anode area available for the discharge. The cathode is manufactured from a titanium tube (99.97%) with an outer diameter of 10 mm and a wall thickness of 1 mm. The auxiliary anode is made out of a tungsten rod with a 1 mm diameter and mounted concentrically inside the cathode. It is fixed by a concentric insulator. The cathode is installed inside an Al\({}_{2}\)O\({}_{3}\) insulator with an outer diameter of 15 mm and a wall thickness of 2.5 mm. The anode geometry ensures a fixed gap of 0.4 mm at the front surface between the anode and cathode as illustrated on the right of Figure 1. For the magnetic field generation, an optional coil can be activated. The coil is made out of insulated copper wire with a 1 mm diameter and has an inductance of 40.2 uH.  
The thruster was optimized in terms of mechanical and electrical properties (e.g., auxiliary anode fixing, electrical connection and mounting points). It can achieve more than \(1\times 10^{7}\) pulses without a cathode feeding mechanism, while the ignition voltage is kept below \(10\,\mathrm{kV}\). In Figure 2, cathodes after thruster operations are shown. The cathodes were operated with a pulse energy of around \(0.35\,\mathrm{J}\) for \(1.27\times 10^{7}\) pulses and \(1.01\times 10^{7}\) pulses, respectively. Here, an unused cathode for reference is also shown.  
The thruster head is installed on the thrust balance into a 3D-printed mounting system to ensure electrical insulation with respect to the thrust balance setup and the vacuum chamber. The mounting system's material is PETG-polymer. It has an operation temperature up to \(100\,\mathrm{\SIUnitSymbolCelsius}\) and a very low tendency for water absorption, which is crucial for a good vacuum level. In order to prevent the melting of the printed mounting system during thrust measurements at higher power levels, cool down phases between the measurements are included. For all electrical connections, high voltage insulated wires with \(1.5\,\mathrm{mm}^{2}\) cross-section are used.  
Figure 1: Schematic of the high efficient and reliable vacuum arc thruster (HEVAT) setup. The anode’s geometry allows a good fitting of the insulator and a reproducible anode-to-cathode gap of \(0.4\,\mathrm{mm}\).  
Figure 2: Comparison of titanium cathodes used as fuel in the HERVAT thruster head. The left cathode (**a**) was operated for \(1.27\times 10^{7}\,\mathrm{pulses}\) while the middle cathode (**b**) was operated for \(1.01\times 10^{7}\,\mathrm{pulses}\). The cathode on the right (**c**) is unused and has an initial length of \(15\,\mathrm{mm}\) and a diameter of \(10\,\mathrm{mm}\times 1\,\mathrm{mm}\).