# High-Speed Tomography--A New Approach to Plasma Bulk Velocity Measurement
## 1 Introduction
also combined probe and optical measurements and showed that a plasma jet from a DC torch operated at 800 A with argon and helium as plasma forming gases at atmospheric pressure under the assumption of local thermal equilibrium (LTE) can reach up to 1000 m/s [14]. [PERSON], [PERSON], and [PERSON] measured the plasma velocity optically by observing arc fluctuations [15; 16]. The authors examined a plasma torch fed with direct current in the range of 300-900 A and argon or nitrogen under atmospheric conditions. To measure the velocity, an image of the plasma jet was split into two planes so that a distance between them could be determined. Depending on the current, voltage, gas type, gas flow and nozzle shape, velocities up to 1600 m/s were measured [15; 16]. Plasma velocity can also be determined by means of laser-induced fluorescence [17; 18]. [PERSON] et al. used this method, for example, to examine an electromagnetic propulsion thruster in a vacuum. The authors recorded axial ion velocities of a straight cylindrical cusp field thruster in the range of 15.6 km/s [17]. The same diagnostics were also used to measure the ion velocity in a DC plasma sheath. Values of up to 21 km/s are reported in [18]. Another non-invasive technique, laser scattering, was used to determine the plasma velocity of a DC plasma torch operated with argon and current values between 300 and 900 A in ambient air. Values of up to 1100 m/s have been observed under these operating conditions [19]. Laser scattering was also used in another work involving a DC plasma generator operated at pressures lower than atmospheric. In [20], velocities of up to 2.6 km/s were achieved using argon and 600 A at 100 mbar operating pressure. In [21], on the other hand, laser absorption as well as fluorescence and emission spectroscopy were used to measure plasma velocity of a modified 4 kW cutting torch operated with argon and hydrogen in a vacuum chamber. The torch was used as a low-pressure plasma spraying source. Values of up to 7 km/s were reported by the authors [21]. An invasive approach was used by [PERSON] in [22] to measure the plasma velocity of a high power DC arc. The horizontal deflection of dropped ball bearings has been recorded, resulting in values in the range of 1500 m/s [22]. Tomography is used to detect the temporal behavior of the local emissions of the plasma jet, as done in [23]. In combination with spectroscopic diagnostics as in [24] or two-color pyrometry as in [25], tomography is also applied for temperature determination. In order to obtain a tomographic reconstruction, it is necessary to observe the studied object from a number of different viewing directions. This can be achieved with a single camera by rotating it around the object, as presented in [26], but at the same time this approach implies that the object does not change significantly during the recording. For temperature measurements, this assumption is usually acceptable due to the more statistical behavior. For other applications, it can be circumvented by using a set of mirrors that simultaneously image the object onto the camera's sensor plane, creating the required number of viewing directions. Such an approach provides an instantaneous image of the object and is therefore better suited to evaluate the temporal behavior of the object under study. For example, it was used to assess the asymmetries of a plasma jet in [27]. The spatial resolution of a tomographic reconstruction is determined by the number of images from different directions of view. The more complex the shape of the measured object, the more different viewing angles that are needed for an accurate reconstruction. Feasibility studies on how many directions of view are needed to reconstruct complex shapes were presented in [28]. According to the results, only four viewing directions are sufficient for a reconstruction calculation using the Algebraic Reconstruction Technique (ART) [28]. Four directions of view have also been used for a time-resolved temperature measurement of a thermal plasma jet [29].