Recombination, radiative energy loss and level populations in nonequilibrium cesium discharges

Abstract

The transition rate equations for the populations of cesium states in a homogenous, optically thin plasma have been solved for the excited-state populations, the effective ground-state ionization and recombination coefficients, and the effective electron kinetic-energy and radiative loss coefficients. A table of values is presented for the nonequilibrium steady-state plasma which is analogous to the Saha equation for the equilibrium plasma. Free electron densities from 10 12 to 10 15 cm -3 are considered, and a Maxwellian electron velocity distribution at temperatures from 1500 to 3000°K is assumed. These conditions are typical of those in thermionic and magnetohydrodynamic energy converters and other cesium discharges. The rate coefficients used for collision-induced and radiative atomic transitions are described. Molecular processes, diffusion, wall losses, etc., are discussed but not included explicitly in the model. The absorption of resonance radiation is simulated by reduction of the spontaneous emission coefficients. Other radiative absorption processes are neglected. The populations are seen to depart significantly from the Boltzmann distribution in steady state, and it is seen that many excited states participate in the ionization and recombination processes. The implications of these results for spectroscopic diagnostics, volume ion production, and energy loss in a cesium-discharge plasma are discussed.