Effects of cold electron emission on the plasma sheath

Abstract

The use of surfaces that emit cold electrons into a plasma has been shown to reduce the sheath potential drop by at least a factor of 2.6. This effect can be used to mitigate sputtering by causing a reduction of the mean energy of ions striking the emissive surface. The present study has been motivated by the severe sputtering problems that may occur at the divertor plate of a full power fusion reactor. In addition, secondary electron emission is a common occurrence at the bounding surface of a plasma, and so should be accounted for in the plasma analysis. Kinetic theory shows that, for a hydrogen plasma (singly charged, mean ion mass of 2.5 amu) with an electron temperature of Te and ion temperature of 0.1Te, emission reduces the sheath potential drop from 3.9Te to 1.5Te. This reduction occurs when the surface emits a cold electron current that is increased to nine times the incident ion current. For comparison with theory, thermionic electron emission has been studied in a hydrogen plasma (composed mostly of H3+) powered by inductive coupling from an rf coil. The plasma was in contact with an electrically isolated surface of tungsten impregnated with barium scandate. Plasma potential was measured with a Langmuir probe and with a mass and energy analyzer that sampled the energy distribution of ions passing through an aperture in the emissive surface. As the surface temperature was increased, the surface potential floated up to the plasma space potential so that the ion energy at the surface dropped from 5Te for temperatures below 570 °C to less than 1Te for temperatures above 680 °C. The Richardson equation, which determined emission current as a function of surface temperature, and the above kinetic theory, was used to predict that the reduction in potential drop should occur from 420 °C to 510 °C. The abrupt reduction in ion energy over a narrow range of surface temperature was observed experimentally as predicted; however, the sudden reduction began at a surface temperature of 600 °C rather than 420 °C. The delayed onset of emission was probably caused by the sputtering removal rate of the impregnate at the surface exceeding the replenishment rate from the bulk for disk temperatures below the normal operating range.