Spectroscopic study of a helium plasma jet with hydrocarbon additives

The results of a spectroscopic study of the conversion of acetylene and methane in a helium plasma jet produced by a direct current plasma torch are presented. The operating mode of the plasma torch corresponds to conditions that provide a high yield of carbon nanostructures. In the emission spectra recorded during the transverse observation of a $20$-mm jet section following the outlet of the anode channel of the plasma torch, high-intensity Swan bands of the $\rm C_2$ molecule are the dominant component in the visible wavelength range. The spectra show atomic hydrogen lines of the $\rm HI$ Balmer series from $\rm H_{\alpha}$ to $\rm H_{\varepsilon}$, numerous $\rm CI$ carbon lines from ultraviolet $247.9$ nm to infrared $962$–$966$ nm, and a number of $\rm HeI$ helium lines. The axial temperature values of the plasma jet were spectrally determined by the observed ionized carbon $\rm CII$ lines $283.7$ nm, $392.0$ and $426.7$ nm. A joint analysis of the emission spectra and the mixture composition calculated in the Saha–Boltzmann approximation revealed the nature of the spatial heterogeneity of the studied $\rm He : \rm C : \rm H$ plasma jet. It is manifested by the difference in the electron temperature of the axial region of the jet, which is measured with ionized carbon lines $\mathrm{CII}\,(T_e(0) = 12000$–$14000$ K$)$ from the vibrational and rotational temperatures of $\rm C_2$ molecules $(T_V = T_R \cong 5000$ K$)$ that emit intensely at the jet periphery. The electron density measured along the $\rm H_{\beta}$ and $\rm H_{\gamma}$ line widths, which varies in the range of the observed jet region of $n_e = (4$–$2) \times 10^{16}$ cm$^{–3}$, corresponds to the ionization equilibrium in a plasma $\rm He : \rm C : \rm H$ mixture with an electron temperature close to $T_e$ measured by carbon-ion lines.