Near-infrared collisional radiative model for Xe plasma electrostatic thrusters: the role of metastable atoms

Mestastable Xe atoms play an important role in the collisional radiative processes of dense xenon plasmas, including those of electric thrusters for space vehicles. Recent measurements and calculations of electron-excitation processes out of the 5p56s J = 2 metastable state (1s5 state in Paschen notation) have allowed for the development of a collisional radiative model for Xe near-infrared (NIR) emissions based on the population of the metastable level through 2p?1s5 radiative transitions, and based on depopulation through electron-impact excitation. A modified plasma radiative model incorporating newly computed electron-impact excitation cross sections using both relativistic distorted wave and semi-relativistic Breit?Pauli B-Spline R-matrix methods is presented. The model applies to optically thin, low-density regions of the thruster plasma and is most accurate at electron temperatures below 10?eV. The model is tested on laboratory spectral measurements of the D55 TAL and BHT-200 Hall thruster plasma NIR radiation. The metastable neutral fraction is determined to rise from 0.1 to slightly above 1% as the electron temperature increases from ~2 to 10?eV, reaching a maximum around 15?eV. Electron temperatures derived with the modified model are approximately 20% lower than a previous version of the model that used an approximate approach to account for metastable population and line intensity enhancement.