THE KINETICS OF NEGATIVE HYDROGEN-IONS IN DISCHARGES

The discovery of large (about 30%) concentrations of negative hydrogen ions in a simple hydrogen plasma and the requirement to produce intense H- ion sources for neutral beam heating of fusion devices has prompted a major effort to understand the kinetics of H- ions in discharges. Here a guide to the development of kinetic models of the hydrogen discharges in ion sources, from simple estimates of rate coefficients to large numerical codes is presented. The results of experimental studies to characterize ion source discharges are also discussed. The modelling and experiments provided the thoeretical basis for the successful tandem multi-cusp ion source. Current models generally assume that the primary H- production mechanism is through dissociative attachment of electrons to rovibrationally excited molecules. At present, the degree of agreement between measured and calculated quantities, and even between different models, is mixed. There is concern that the standard dissociative attachment model is inadequate and new mechanisms are being sought. It is apparent that nonperturbative measurement techniques, which can also provide spatially resolved information, are also required.