Spectral measurements of neutral density in argon plasmas with widely varying fractional ionization

Understanding the region between fully ionized and neutral dominated plasmas is important to the study of the magnetosphere of the earth, the corona/chromosphere transition region of the sun, and detached divertors in fusion devices. Determining the fractional ionization of a plasma requires accurately measuring neutral density. We use an absolute intensity calibrated spectrometer coupled with results from a collisional radiative model to infer neutral argon density. Results will be shown from benchmarking spectroscopic measurements of neutral argon density against pressure in an RF generated magnetized plasma column between 0.1 to 2.0 mTorr at 0.1% fractional ionization. Preliminary results show that spectroscopic neutral density measurements agree with pressure-based neutral densities below 0.5 mTorr but do not increase proportionally with pressure measurements above 0.5 mTorr. Results `will be shown from the neutral density diagnostic in a higher fractional ionization experiment ranging between 10% and 70%. The general use of the diagnostic for neutral density measurements in argon plasmas is also discussed.