Effects of thermionic electron emission on the plasma sheath

Summary form only given, as follows. The use of surfaces which emit thermionic 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. In particular, this research has been motivated by the severe sputtering problems which may occur at the divertor plate of a full power fusion reactor. Time-independent kinetic theory shows that, for a hydrogen plasma (singly charged, mass 3 ions) with an electron temperature of T_e and ion temperature of 0.1 T_e, the sheath potential crop changes from 3.9 T_e to 1.5 T_e as the surface is heated. In addition. thermionic emission has been studied in a hydrogen plasma powered by inductive coupling from an RF coil. The plasma was in contact with an electrically isolated surface of tungsten impregnated with barium scandate. The surface was heated up to 800°C to emit an electron current which exceeds the incident electron current. Plasma potential was measured with a Langmuir probe and a mass and energy analyzer which 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 potential so that the ion energy at the surface dropped from 5 T_e for temperatures below 650°C to less than 1 T_e for temperatures above 700°C