Performance of the DITE bundle divertor

Conditions appropriate to reactor exhaust are achieved with the DITE MkII bundle divertor using high-power neutral-beam injection. A layer of high-temperature plasma in the exhaust carries a heat flux of 100 MW.m-2 and a particle flux of 1024m-2.s-1. A pressure of neutral gas in the divertor of 30x10-3 torr has been achieved. Plasma in the divertor ducts blocks the flow of neutral particles, allowing operation at high gas pressure in the divertor. Plasma in the high-temperature layer remains collisionless with supersonic flow even at high gas pressure. In a second layer, cool, marginally collisional plasma flows from the divertor to the torus for gas pressures above 3x10-4 torr. A 1-D kinetic model of plasma flow through the bundle divertor magnetic mirror and a 3-D version of the DEGAS neutral-particle code predict the main features of the exhaust. However, at high gas pressure, the amount of ionization in the plasma in the divertor is lower than predicted. This can be explained by the effects of atom-molecule collisions in the gas surrounding the plasma column. The low neutral-particle density in the plasma, combined with the low plasma density due to mirror action, inhibit the formation of a collisional, high-recycling divertor regime. Diverted discharges with good confinement are obtained when the m=2,n=1 mode amplitude is low. These discharges have confinement times at least as good as in limiter discharges, with a similar parametric variation. Confinement is seriously degraded when a stationary island is formed through the interaction of the m=2 mode with the divertor magnetic field. As yet, operation with good confinement at separatrix q less than 3 has not been achieved. (author)