Spectroscopic study of shock waves generated in a supersonic arcjet helium plasma

Recently, we found that the shock cell appeared in an arcjet plasma expanding through a conical-shaped supersonic nozzle. In order to understand the characteristics of the shock wave, the visible/UV emission spectroscopy was carried out. The arcjet plasma was generated between an anode (copper) and a cathode (Ce/W) with a gap length of 2.5 mm and then expanded through the anode nozzle (throat diameter: 1.0 mm) into low pressure region (expansion section). The discharge current voltage and gas pressure were 40 A $\sim$30 V and $\sim$1000 mbar respectively. A visible spectrometer (focal length: 1.0 m, grating: 2400 grooves/mm) was used to measure the plasma emission The electron temperature was evaluated by Boltzmann plot of He I 2p$^{3}$\textit{Pn}d$^{3}D$ series ($n=$6, 7, 8 and 9), whereas the density was determined by Stark broadening (He I 438.8 nm). It was found that the density significantly increased at the shock region, which can be expected by the simple gas dynamic theory. However, no distinct change of the temperature was observed.