Abstract
Microwave plasmas with H-2 and H-2/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 H-2/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 H-2/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 H-2/He plasma largely affects the predominant emissions of the hydrogen Ly alpha line (121.6 nm) and H-2(B-1 Sigma(u)-X-1 Sigma(g)) band (150-170 nm). While a microwave plasma with 1.4% H-2/He is required to mimic the entire VUV solar spectrum in the 115-170 nm range, the combination with 1.28% H-2/35% Ar/He is the best alternative to obtain a quasi-monochromatic spectrum with emission dominated by the Ly alpha line. The maximum of the spectral irradiance is significantly higher in the ternary mixtures compared to the binary mixture of 1.4% H-2/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.