MPD arcs as plasma sources for recombination lasers

Abstract

In this study the decay of argon plasmas ejected from a MPD thruster described in a previous paper is investigated. Various parameters, e.g. the electron densityN _e , the electron temperatureT _e , and the absorption coefficient α of the ArII-488 nm transition are measured by different experimental techniques. Axial and radial profiles ofN _e andT _e are determined, and used to decide on the relevant recombination mechanism. In spite of the fact that the dominant three-body recombination favours the population of the high-lying energy levels, population inversions have been observed even with the most sensitive method only in plasmas ejected from a reduced aperture of the MPD thruster. The theoretical analysis shows that the ranges ofT _e andN _e , in which recombination-lasing may be expected, are narrow. In addition, the mechanisms that limit the population inversions in discharge tubes of conventional Ar⁺-lasers restrict the dimension of the plasma perpendicular to the resonator axis. From these facts and the described measurements on population inversions we conclude that the initial diameter of the plasma has to be reduced. We therefore propose a new discharge configuration where extended regions of constant plasma parameters can be expected. With this arrangement it should be possible to reach population inversions required for laser oscillations in ArII.