Divertor studies in the l=3 URAGAN-3M torsatron

Abstract

In the l=3, m=9 URAGAN-3M torsatron an open helical divertor is realized. Plasmas are RF produced and heated. The time and space characteristics of the plasma in the space between two coils of the helical winding and on the outward side of the helical winding are studied by microwave and probe techniques. The experimental measurements are preceded by numerical divertor field line tracing. As a result of the calculations and measurements, it is shown that the diverted plasma is formed due to (i) plasma transportation from the confinement volume; and (ii) neutral gas ionization by the near-antenna field. The flow of diverted plasma transported from the confinement volume increases with RF power. This is considered as direct evidence of an increase in particle loss with an increase in heating power, the effect having been deduced recently for a stellarator-type device under NBI and ECR heating conditions. An intense RF discharge along open field lines crossing the operating antenna results in a greatly increased diverted plasma density, in the appearance of a static positive plasma potential up to 150 V near the antenna, and possibly in significant additional RF power loss outside the confinement region.