Spectroscopic Study of a Helium Plasma Jet with Hydrocarbon Additives

Abstract

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 С₂ molecule are the dominant component in the visible wavelength range. The spectra show atomic hydrogen lines of the HI Balmer series from H_α to H_ε, numerous CI carbon lines from ultraviolet 247.9 nm to infrared 962–966 nm, and a number of HeI helium lines. The axial temperature values of the plasma jet were spectrally determined by the observed ionized carbon 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 He : C : 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 CII (T_e(0) = 12 000–14 000 K) from the vibrational and rotational temperatures of C₂ molecules (T_V = T_R ≅ 5000 K) that emit intensely at the jet periphery. The electron density measured along the H_β and H_γ line widths, which varies in the range of the observed jet region of n_e = (4–2) × 10¹⁶ cm^–3, corresponds to the ionization equilibrium in a plasma He : C : H mixture with an electron temperature close to T_e measured by carbon-ion lines.