Plasma sheath in the presence of surface-emitted negative ions

Abstract

The need for negative hydrogen ion sources for heating in future fusion devices demands a full investigation of its production and interaction with plasma. To understand the interaction of emitted negative ions with plasma sheath, a one-dimensional collisionless kinetic model of a negative ion emitting electrode/grid in a low-density isotropic plasma is developed for conventional and the inverse sheath. The plasma electron and emitted negative ions are assumed to be half Maxwellian along with cold positive plasma ions for the conventional sheath and half Maxwellian for the inverse sheath. The influence of surface-produced negative ions, from floating and current-carrying electrode/grid, with varying temperatures on sheath structures, is analyzed for subcritical, critical, and supercritical emissions. The formation of potential well and inverse sheath is observed at high and very high emitted negative ion temperatures, respectively. The critical emission is observed at specific values of emitted negative ion temperature and number density, below which the solution does not exists. In critical and supercritical emission, the emitted negative ion number density remains low compared with plasma positive ions, but it is high in inverse sheath. The inverse sheath solutions for floating and current-carrying negative ion-emitting electrode/grid are also discussed, and a rough estimation between the experiment and this theory shows the existence of inverse sheath in currently existing negative ion sources, but for full understanding, we need further investigations.