# Plasma Expansion Characterization of a Vacuum Arc Thruster with Stereo Imaging
## 4 Discussion  
The plasma plume expansion of a 30-W class vacuum arc thruster was investigated by means of high-speed 3D imaging. The VAT was operated with and without a magnetic nozzle (~1.3 T). The recordings were done by a pseudo stereo imaging system with a biprism, to reduce the system to one single camera, which simplifies the handling of the recording. By means of this imaging system, the VAT was observed from two observation directions, which were oriented orthogonally to each other. The areas of the expanded plasma plume were calculated and a simplified 3D model was computed to obtain information about the shape of the plasma plume.  
The results from calculating the areas of the plume expansion show that the PJP discharge is significantly influenced by the application of an external magnetic field. When the magnetic nozzle was mounted, the calculated plume area became significantly larger with time compared to the unmagnetized operation, as the results of recording 1 \(\upmu\)s and 10 \(\upmu\)s after thruster ignition show. For the points in time that are closer than 1 \(\upmu\)s to the ignition, the results for the setup with the magnetic nozzle indicate a smaller area than for the setup without the magnetic nozzle. This seems consistent with the results of previous works, where, with an increasing magnetic field, the axial plume expansion decreases. However, in these investigations, lower magnetic fields in the range <0.3 T and arc currents lower than 1 kA were used [5; 6]. The PJP operates in the same current regime only during the first \(\upmu\)s after ignition; an arc current of less than 1 kA is also detected within 1 \(\upmu\)s after ignition, while it increases exponentially in 10 \(\upmu\)s to approximately 4 kA [17]. Thus, in the time range <1 \(\upmu\)s, the PJP could be comparable to the thruster operated in previous works and the applied magnetic field is able to suppress the expansion of the plume. However,with increasing temporal distance, the plume expansion magnifies due to the abruptly increased discharge current, when the plasma follows the expanding field lines at larger distances from the PJP surface.  
The magnetic nozzle seems to support the more uniform formation of the plume, which can be seen by the calculated areas, which show lower deviations between the two perspectives, and furthermore by the reconstructed 3D models. The plume spreads further in the direction of the nozzle outlet in the case of the magnetic nozzle, while, without a magnetic nozzle, the expansion is lower and more horizontal regarding the nozzle outlet. The magnetic field is responsible for the consistent expansion, which was investigated in previous research [5,12].  
The more uniform expansion of the plasma plume when using the magnetic nozzle is advantageous, as it reduces the fluctuations of the plasma jet formation. Thus, the pulse-by-pulse varying discharge can be cushioned.  
For further research, it is recommended to investigate the influence of external magnetic fields on the PJP in more detail. The strength of the magnetic field as well as the position should be varied; hence, an electromagnet would be a good choice. Thrust measurements combined with ion current measurements should be used to validate the 3D investigations as this optical method is easy to apply and non-invasive.