DIVERTOR STUDIES IN THE L=3 URAGAN-3M TORSATRON

In the l = 3, m = 9 URAGAN-3M torsatron an open helical divertor is re alized. Plasmas are RF produced and heated. The time and space charact eristics of the plasma in the space between two coils of the helical w inding and on the outward side of the helical winding are studied by m icrowave and probe techniques. The experimental measurements are prece ded by numerical divertor field line tracing. As a result of the calcu lations and measurements, it is shown that the diverted plasma is form ed due to (i) plasma transportation from the confinement volume; and ( ii) neutral gas ionization by the near-antenna field. The flow of dive rted plasma transported from the confinement volume increases with RF power. This is considered as direct evidence of an increase in particl e loss with an increase in heating power, the effect having been deduc ed recently for a stellarator-type device under NBI and ECR heating co nditions. An intense RF discharge along open field lines crossing the operating antenna results in a greatly increased diverted plasma densi ty, in the appearance of a static positive plasma potential up to 150 V near the antenna, and possibly in significant additional RF power lo ss outside the confinement region.