B4C SOLID TARGET BORONIZATION OF THE MST REVERSED-FIELD PINCH

A solid rod of hot-pressed boron carbide is being used as the source of boron during boronization of MST. The most striking result of this procedure is the reduction in oxygen contamination of the plasma (0111 radiation, characteristic of oxygen at the edge, falls by about a factor of 3 after boronization.). The radiated power fraction drops to about half its initial value. Particle reflux from the wall is also lowered, making density control simpler. The rod (12.7 mm diameter) is inserted into the edge plasma of normal high-power RFP discharges. B4C is ablated from the surface of the rod and deposited in a thin film (a-B/C:H) on the walls and limiters. The energy flux carried by ''superthermal'' (not '' runaway'') electrons at the edge of MST appears to enhance the efficient, nondestructive ablation of the boron carbide rod.