Spectral measurements of neutral density in plasmas with varying fractional ionization*

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 the plasma requires accurately measuring the neutral density.  We use an absolutely calibrated spectrometer coupled with results from the collisional radiative solver ColRadPy to measure neutral density in two laboratory plasma experiments. Neutral density measurements are validated against pressure in the ALEXIS experiment an RF generated magnetized plasma column. Additionally, spectral measurements were collected from ECRH heated plasmas with varying fractional ionization in the Compact Toroidal Hybrid device. A triple probe and interferometer measure electron density ranging from 1 x 10¹⁷ m^-3 to 1 x 10¹⁸ m^-3  and temperature from 1 eV to 10 eV. Results will be presented from a study of fractional ionization within the plasma volume as well as the effects of metastables in both experiments.