VUV Spectral Irradiance Measurements in H2/He/Ar Microwave Plasmas and Comparison with Solar Data

Abstract

Abstract Microwave plasmas with H2 and H2/rare gas mixtures are convenient sources of VUV radiation for laboratory simulations of astrophysical media. We recently undertook an extensive study to characterize microwave plasmas in an H2/He gas mixture in order to optimize a VUV solar simulator over the 115–170 nm spectral range. In this paper, we extend our investigation to the effect of the addition of Ar into H2/He plasma on the VUV spectral irradiance. Our study combines various optical diagnostics such as a VUV spectrometer and optical emission spectroscopy. Quantitative measurements of the spectral irradiance and photons flux in different mixtures are accomplished using a combination of VUV spectrometry and chemical actinometry. Results show that the Ar addition into H2/He plasma largely affects the predominant emissions of the hydrogen Lyα line (121.6 nm) and H2 (B1Σu–X1Σg) band (150–170 nm). While a microwave plasma with 1.4% H2/He is required to mimic the entire VUV solar spectrum in the 115–170 nm range, the combination with 1.28% H2/35% Ar/He is the best alternative to obtain a quasi-monochromatic spectrum with emission dominated by the Lyα line. The maximum of the spectral irradiance is significantly higher in the ternary mixtures compared to the binary mixture of 1.4% H2/He. Further Ar increase yielded lower spectral irradiance and absolute photon fluxes. Our measured spectral irradiances are compared to VUV solar data in the 115–170 nm range, emphasizing the use of microwave plasmas in astrophysical studies and laboratory simulations of planetary atmospheres.