# High-Speed Tomography--A New Approach to Plasma Bulk Velocity Measurement
## 4 Discussion  
The presented diagnostic method shows that it is possible to record and detect distinct fluctuations of a plasma jet. As previous studies show, a good reconstruction based on the ART algorithm can be realized with four directions of view [27; 28]. The two additional directions of view implemented in this study ensure an even more accurate reconstruction by providing more information without significantly increasing computation time. The velocity of the plasma bulk could be resolved locally and non-invasively by detecting the movement of characteristic fluctuations in the axial direction. Besides these advantages, one disadvantage of the proposed method is that both an image intensifier and a high-speed camera are required, depending on the measurement object. The proposed setup could be realized with just a high-speed camera if the following two conditions are met: the measurement object is bright enough, or conversely, the high-speed camera is sensitive enough to achieve a short exposure time, and at the same time, the frame rate is sufficiently high to resolve the fluctuations of the object. However, in the case of smaller plasma generators and especially electric propulsion thrusters, one of the above conditions cannot usually be met, and thus both devices are required. As a consequence, the spatial resolution, and to a certain degree the temporal resolution, are limited by the physical size of the image intensifier. The decay time of the phosphor screen of the image intensifier could be another limiting factor for the temporal resolution. Depending on the expected shape and fluctuations of the object to be analyzed, the temporal resolution can be increased by reducing the size of the imaged object. This can be done by shortening the focal length of the lens used. However, the spatial resolution should still be high enough so that distinctive features of the object under study can be detected. A compromise must therefore be found between spatial and temporal resolution. In the authors' experience, however, the proposed setup, as described in Section 2.1.3, has proven to be versatile enough to analyze plasmas of varying power. The results achieved for the DC plasma generator show that the velocity increases to a maximum value of 1061 m/s at a distance of three centimeters from the plasma outlet (see Figure 16). Further downstream, the velocity decreases non-linearly, showing a small dip at four centimeters before reaching a value of 919 m/s at seven centimeters. Since no external electric or magnetic field was applied, the deceleration observed at a distance of four centimeters from the nozzle outlet seems implausible. Therefore, it is assumed to be an outlier. For the sake of clarity, the velocities measured in this work, including the associated operating parameters, are compared with previous work in Table 2.  
Comparing the results with the velocities given by [PERSON] et al. [9], the values obtained in this work are twice as high. Although the used gas flow and current were lower, [PERSON] \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Reference** & **Measurement Setup** & **Max Velocity** & **Gas Flow** & **Current [A]** & **Nozzle Ø** & **Operating Pressure** \\ This work & DC plasma torch; optical measurement & 1061 & 35 & 40 & 6 & 20 \\ [PERSON] et al. [9] & DC plasma torch; probe measurement & 550 & 47.2 & 600 & 7.88 & 1013 \\ [PERSON] et al. [15] & DC plasma torch; optical measurement & 1600 & 45 & 520 & 7 & 1013 \\ [PERSON] et al. [16] & DC plasma torch; time of flight & 2200 & 60 & 600 & 6 & 1013 \\ [PERSON] et al. [21] & Arjet; doppler shift & 7000 & 6.7 & 20 & 1–2 & 10.3–100.7 \\ [PERSON] et al. [38] & Arjet; laser absorption & 2308 & 7.3 & 140 & 2 & 0.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of results presented in this work with those of previous work. et al. used a nozzle with a larger diameter.