Implementation of a spectroscopic neutral density diagnostic in a low temperature argon plasma*

Understanding the transition region between fully ionized and neutrally dominated plasmas is important to the study of the magnetosphere of the earth, the corona/chromosphere transition regions of the sun, and detached divertors in fusion devices. Determining the fractional ionization of a plasma requires accurately measuring the neutral density.  An absolute intensity calibrated spectrometer coupled with results from a collisional radiative model solver is used to measure  neutral density in the toroidal CTH and ALEXIS linear plasma device. Results will be shown for plasmas with electron temperatures ranging from 1 to 10 eV and electron densities ranging from 1 x 10⁸ to 1 x 10¹² cm^-3. A wide variety of plasmas with fractional ionizations ranging from 0.01% to 99.9% have been produced on CTH. The spectroscopic neutral density diagnostic on CTH shows a good match to pressure gauge measurements below 1 mTorr. Theoretical spectra demonstrate a good match to experimental spectra on both devices. Neutral density shows a linear decrease with an increase in power on CTH. Neutral density shows a linear increase with an increase in gas on ALEXIS. Both results support the validity of the neutral density diagnostic.