Doppler-free absorption measurements of Stark broadening in a flowing hydrogen plasma

This paper presents measurements of electron number density in a low-densit y hydrogen arcjet plasma flow. A spatially translated Langmuir probe is use d to obtain these: data, and comparisons are made to measurements of electr on density using Doppler-free absorption spectroscopy (DFAS) of the Stark-b roadened spectral line of the atomic hydrogen Balmer-alpha (H-alpha) transi tion. A lineshape model is developed for the DFAS spectra which includes cr ossover resonances, and collisional coupling between the electronic states within the n = 2 atomic hydrogen level. The electron number densities extra cted from the Langmuir probe traces are found to be significantly less than those determined from the DFAS measurements. This difference is attributed to the averaging of the collected current over the extended probe surface. (C) 2000 Elsevier Science Ltd. All rights reserved.