# Plasma Expansion Characterization of a Vacuum Arc Thruster with Stereo Imaging
## 2 Materials and Methods
### Periphery  
The periphery of the entire measurement setup consists of a vacuum chamber with suitable vacuum pumps, a high-speed camera for image recording, the PJP including a power supply and a control unit for triggering.  
A vacuum chamber with a diameter of 80 cm and a height of 70 cm, equipped with four optical inlets with a diameter of 21 cm on each side, was used. Furthermore, there  
Figure 1: The used PJP model mounted centrally in the vacuum chamber.  
Figure 2: Magnetic nozzle used: (**a**) exposure view of the nozzle system; (**b**) CAD model of PJP with mounted magnetic nozzle.  
are several electrical feedthroughs, to connect the VAT to the power supply and trigger system. A rotary valve pump and a turbomolecular pump were used, providing constant vacuum levels of approximately 10\({}^{-6}\) mbar. The required power of 30 W for the PJP was supplied by a Voltcraft vlp-1302a power supply, while the waveform generator LXI G5100A, manufactured by Picotest, Phoenix, AZ, USA, ensured the triggering of the ignition. For recording, a Dimax HS4 high-speed camera was used. Excelitas PCO GmbH, Kehlheim, Germany, manufactured the mentioned camera. Additionally, a pulse delay generator DG645 was integrated into the triggering system, manufactured by Scientific Instruments GmbH, Gilding, Germany.  
In Figure 3, the schematic drawings of the devices used and their connections are shown. The waveform generator triggers the PJP directly and the high-speed camera via the pulse delay generator. By means of the pulse delay generator, it is possible to set a desired value between the ignition of the thruster and the start of the recording of the camera. Thus, it is possible to take images at specific points in time of the discharge. This means that, additionally, the time course of the spatial evolution of the plasma plume can be observed. From this, the spatial expansion of the plasma can be obtained at desired points in time.  
As can be seen in Figure 4, the VAT was placed in the center of the chamber. To ensure recordings from two directions of observation, orthogonally from each other, the thruster was mounted with the electrodes facing upwards. Via two deflection mirrors and a biprism mounted immediately in front of the high-speed camera, synchronized imaging from two observation planes is possible. The used biprism was a 90\({}^{\circ}\) specialty mirror with 50 mm legs and a 70.7 mm hypotenuse. It was enhanced by an aluminum coating with an average reflectance of \(>\)95% at 450-650 nm and an angle tolerance of \(\pm\)2\({}^{\circ}\). The surface flatness amounted to \(\lambda\)/4 and the surface quality to 40-20. Both deflection mirrors were also enhanced with aluminum coating with an average reflectance of \(>\)95% at 450-650 nm at an angle of 45\({}^{\circ}\). The surface flatness amounted to 4-6 \(\lambda\) and the surface quality to 60-40. Both deflection mirrors had a size of 50 mm \(\times\) 75 mm. All mirrors were manufactured by Edmund Optics Inc., Barrington, NJ, USA. The CMOS image sensor of the used high-speed camera with a resolution of 2000 \(\times\) 2000 pixel had a maximum frame rate of 2277 fps at full resolution. The dynamic range of the camera amounted to 12 bits.  
Figure 3: Schematic of the controlling and triggering system. The red and blue lines are the power supply and the green line is the trigger.  
The entire optical setup, including all mirrors and the camera, was mounted on aluminum structural profiles and an optical rail system and was attached to the base of the vacuum chamber. To ensure a proper line for recording, all components were aligned in the center of the two windows. Care was also taken to ensure that the optical path was the same length in both directions of observation. The deflection mirrors and the biprism were mounted on specific holders so that fine adjustment could be made both vertically and horizontally. These kinematic mirror mounts were produced by OWIS GmbH, Staufen, Germany.