Doppler-free, Stark broadened profiles at low plasma densities in helium

Abstract

This work utilizes Doppler-free saturation spectroscopy to measure Doppler-free, Stark broadened spectral profiles for the π-polarization of the HeI 21P → 61D transition in a low density (ne=7×1010−2×1012 cm−3) helium plasma. The measurements were performed in an electron-cyclotron resonance discharge at 23 mTorr with electron density being diagnosed using a combination of Langmuir probe and microwave interferometry techniques. The Doppler-free profiles were observed to be nearly symmetric at ne < 1011 cm−3 but markedly asymmetric above this transition point. Electron density is extracted from the spectral data via fitting to a spectral model based on quasi-static Stark broadening. The fit results are compared to ne measurements obtained using a combination of Langmuir probe and microwave interferometry techniques. The fit and measured ne are shown to agree within 20% on average. Finally, the quasi-static model is shown to be valid in the low-density regime for 21P → 61D helium transition.