# High-Speed Tomography--A New Approach to Plasma Bulk Velocity Measurement
## 1 Introduction  
In the field of thermal plasma diagnostics, a wide range of different measurement techniques exist that allow for a qualitative and quantitative description of the plasma under investigation. The techniques can be divided in general into invasive and non-invasive methods. The former provide quantitative results and are often used to determine electron properties or flow velocities. The latter can provide quantitative and qualitative results alike. Quantitative non-invasive methods are typically used for the identification of plasma constituents, the determination of species density or gas and electron temperatures. These methods include optical emission spectroscopy, photometry or laser-based diagnostics. Qualitative non-invasive methods are usually camera-based and include techniques such as high-speed imaging, schlieren optics, or emission tomography. Depending on the technique chosen, simplifying assumptions are often required for a quantitative description of the plasma under investigation [1; 2; 3; 4; 5; 6]. The presented high-speed tomography diagnostic combines two of the above non-invasive methods: high-speed imaging and emission tomography. While in previous works these tools have been used separately to determine the behavior or, in the case of tomography, to measure the temperature of the plasma, the techniques were combined in this work to assess the bulk velocity of the plasma. The bulk velocity is one of the key parametersfor determining the heat transfer from the plasma to the particles (relevant for thermal spraying applications) [7] and the fluxes of mass, thrust, and energy (relevant for electric propulsion applications) [8]. Previous studies have measured the plasma velocity invasively using electronic probes or optically using laser light scattering and spectroscopy [3; 4; 6]. In contrast to electronic probes, high-speed tomography offers the advantage of non-invasive diagnostics and a three-dimensional reconstruction of the object, which is not possible with other optical methods. A reconstructed model provides additional information, such as the shape and expansion of the plume, which can be used for further analysis. However, this is beyond the scope of this paper. The measurement of the plasma bulk velocity is the focus of this work. The plasma velocity of DC plasma spray torches has been measured using several different methods. Some authors have determined the velocity by means of electronic probes [9; 10]. Axial velocities of 100-550 m/s under atmospheric pressure at different currents and argon gas flow rates can be found in [9]. In [10], the velocities for different nozzle designs and pressure values of a DC plasma torch operated at 400 A with a gas mixture of argon and hydrogen were determined. The results presented show that the plasma jet velocity can reach up to 1200 m/s depending on the nozzle design and operating pressure. [PERSON] and [PERSON] used enthalpy probes to investigate the influence of the argon flow rate and current on the velocity of the plasma jet. The commercially available DC plasma torch used by the authors reached velocities in excess of 100 m/s while operating at atmospheric pressure [11]. A different approach was taken by [PERSON] and [PERSON] in [12], who determined the velocity of a super-sonic argon plasma stream in a low-vacuum environment by Langmuir probe measurements. Depending on the gas flow rate, velocities in the range of Mach number 1.12-1.18 were measured [12]. A comparison of probe and optical velocity measurement techniques for different kinds of plasma is presented in [13]. A pulsed DC non-equilibrium atmospheric plasma bullet generator operated with helium was used as a test object. Plasma plume velocities of about 50-60 km/s were assessed with probe measurements, while the optically determined velocities were about 80-85 km/s [13]. [PERSON] et al.