# High-Speed Tomography--A New Approach to Plasma Bulk Velocity Measurement
## 4 Discussion
A possible explanation for the lower velocity values could also be the higher operating pressure and the invasive probe measurement setup, which unavoidably affects the plasma flow and ultimately the measured data. Velocities three times as high as in [9] were optically determined by [PERSON] et al. in [15] for a slightly smaller nozzle diameter. Considering also the values reported by [PERSON] et al. [16], which were determined non-invasively for a similar operating pressure as in [9; 15] but for a higher gas flow rate, the significant increase in velocity appears to be a consequence of the smaller nozzle diameter. The nozzle diameter used by the authors in [16] is identical to that used in this work. As a result, considering the difference in operating pressure, the velocities presented in this work seem plausible despite the lower gas flow and current. Compared to velocity measurements of arcjets, the velocities presented in this work are significantly lower despite a much higher gas flow. The reason for this is the difference in the diameter and geometry of the nozzle [21; 38]. Nonetheless, the velocity values of up to 2308 m/s reported [PERSON] et al. in [38] are in the same order of magnitude as the values determined in this work. It can be thus assumed that the plasma bulk velocity of an arcjet can be measured with the proposed method without significant modifications. A plasma bulk velocity measurement is of particular interest in electric propulsion applications, since the velocity enables the calculation of thrust. In order to resolve higher velocities, such as those presented by [PERSON] et al. also for an arcjet [21], the proposed image recording system requires an adaption and further parameter optimization. A higher frame rate can be achieved by using a shorter delay between each exposure. However, this would require a more powerful image intensifier. In addition, the rotation speed of the mirror has to be increased for a higher frame rate to separate the individual exposures from each other. In terms of parameter optimization, the influence of the number of viewing directions can be investigated. This could lead to an improved utilization of the camera chip, allowing for a higher number of exposures to be stored on the chip. In summary, it can be stated that the measured plasma bulk velocities of the DC plasma torch are in the order of magnitude of values reported in previous studies. Therefore, the presented diagnostic setup seems to be suitable for the determination of the plasma bulk velocity. Nonetheless, further research and validation regarding different operating parameters of the plasma torch is needed. In addition, the data processing and evaluation procedure can be optimized to reduce the standard deviation of the measurements. The proposed diagnostic method allows for the determination of the spatial dimensions of the measured object, which should be explored in future applications. By implementing this feature, the influence of the input parameters such as current or gas flow on the plasma plume expansion can be examined. The proposed setup could also be coupled with spectral filters and thus provide a rough indication of spatial and temporal distribution of the different species in the plasma jet.