Passive optical diagnostic of electric propulsion Xe plasmas: the role of metastable atoms

Metastable Xe atoms play an important role in the radiation of xenon propelled electric thrusters. Experimental and theoretical cross sections for electron-excitation from the 5$p^{5}$6$s \quad J$=2 metastable level (1$s_{5}$ state in Paschen notation) to the lowest six 5$p^{5}$6$p$ (2p) levels have recently become available, thereby providing important rate coefficients for the depopulation of the 1$s_{5 }$state. Application of a collisional radiative model to near-infrared spectral intensities observed in the plume of a Hall thruster, however, demonstrates that additional depopulation paths are important. Present calculations show that the electron-induced excitation to 5$p^{5}$6$s$ J=1 (1$s_{4})$ and other 5$p^{5}$6$s$ levels, for which newly calculated cross sections are presented, can account for the additional de-excitation mechanisms in plasma regions with low electron temperatures.