Nonresonant optical tagging and ‘‘monopolar’’ transport in negative-ion plasmas

A diagnostic method has been devised and used to measure local transport velocities of negative hydrogen ions (${\mathrm{H}}^{\mathrm{-}}$) within a gas-discharge source chamber. The method is based on laser-pulse photodetachment of negative ions, inducing local changes in the electron and ion densities, whose space and time evolution are traced in detail by means of laser pulses and probes. The technique, which has general applicability, can be used in plasmas where conventional probe or spectroscopic methods are not practicable. A new plasma transport process—monopolar drift—in which particles with the same charge counterflow—is identified by means of this technique. This process can be dominant in multispecies plasmas, i.e., those containing negative as well as positive ions. A kinetic analysis of the plasma evolution in space-time, following localized photodetachment of the negative ions, is presented. The results are compared with the first experimental data obtained on ${\mathrm{H}}^{\mathrm{-}}$, demonstrating the principal modes of the diagnostic technique. The consistency between analysis and experiments indicates that the method is generally useful, and principles for its extension are laid out.