# A Simple and Compact Laser Scattering Setup for Characterization of a Pulsed Low-Current Discharge
## 5 Discussion of the Results
Almost identical electron density as well as heavy particle temperature values compared to those estimated in this work, were reported in recent investigations of gliding arcs constricted by tubes, referred to as gliding arc plasmatron by [PERSON] et al. [64] or tornado discharge by researchers collaborating with [PERSON] and [PERSON] [68]. A gliding arc is, in its simplest form, a discharge ignited in a transversal gas flow between two divergent electrodes. Due to the movement of the discharge along the electrodes caused by the gas flow, such discharge is often referred to as self-pulsating, even though it is driven with a direct current [69, 70, 71]. As presented in Table 1, [PERSON] et al. operated the plasmatron with nitrogen at a constant current of 230 mA [64]. The rotational and vibrational temperatures of about 6000 K were estimated by synthetic spectra fitting after measuring of the SPS \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Ref.** & **This Work** & [59] & [61, 62] & [64] \\ \hline Gas & \(N_{2}\) & – & Air & \(N_{2}\) \\ \(f\) (kHz) & 60 kHz & DC & 30 kHz & DC \\ \(i\) (A) & \(\sim\)1 (peak) & \(<\)50 & 30 (peak) & 0.23 \\ \(\overline{n}_{e}\) (m\({}^{-3}\)) & \(\sim\)2 \(\times\) 10\({}^{21}\) & \(<\)10\({}^{21}\) & 1.0 \(\times\) 10\({}^{21}\) & 1.6 \(\times\) 10\({}^{21}\) \\ \(\overline{T}_{e}\) (K) & 41,000 & \(<\)23,000 & 30,000 & – \\ \(\overline{T}_{h}\) (K) & 5900 & \(<\)5800 & 5300 & \(\sim\)6000 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of the scattering results with the most relevant reference values. \((0,0)\)- and FNS \((0,0)\)-transitions of molecular nitrogen. Similar to [61, 62], the authors also determined the reduced electric field based on spectroscopic measurements and used it to calculate the electron density with a collisional-radiative model of the discharge. The resulting \(n_{e}\) was estimated to be \(1.6\times 10^{21}\) m\({}^{-3}\)[64]. This value may be higher than reported by others for gliding arcs [69, 70, 71], but according to the observations of [PERSON] et al. [72], the increase in electron density is most probably caused by the constriction of the discharge. Both \(n_{e}\) and \(T_{h}\) values given in [64] are in good agreement with the ones determined in this work. [PERSON] et al. did not state any \(T_{e}\) values. The plateau width of 0.2 mm observed in the radial distribution of \(T_{h}\) in Figure 7 is considered to be the core diameter of the discharge channel. This assumption appears to be correct when the diameter is compared with other works. [PERSON] et al. estimated a diameter of about 0.3 mm for the nanosecond pulsed discharge discussed above [61]. This compares to the core diameter estimated by [PERSON] et al. of about 0.5 mm [64]. According to radial temperature profiles given by [PERSON] et al., who modeled a low-current high-voltage DC plasma jet operated with a constant current in the range of 0.5 A and an air flow rate of 25 L/min in [73], the diameter of the discharge channel at the nozzle outlet is about 0.4 mm. Apart from the references summarized in Table 1, several research groups analyzed plasma systems comparable to the one studied here [51, 53, 74, 75, 76, 77], though only few have determined the temperature of the effluent plasma jet using emission spectroscopy. [PERSON] et al. spectroscopically analyzed a comparable plasma system operated at a pulse frequency of 19 kHz, with nitrogen flowing at 30 L/min [74]. Based on the second positive \((0,0)\)-transition, the rotational temperature was estimated to be about 5000 K inside the plasma generator and between 3000 K and 4000 K at nozzle exit. For the same plasma system, also pulsed at 19 kHz but working with a compressed air gas flow rate of 76.6 L/min, [PERSON] et al. estimated a rotational temperature of about 1760 K at nozzle exit using the \((0,2)\)-transition spectra of the second positive system of molecular nitrogen [51].