H-MODE IN THE TUMAN-3 TOKAMAK TRIGGERED BY EDGE PLASMA PERTURBATIONS

Several types of edge plasma perturbations in the TUMAN-3 tokamak [Proceedings of the 13th International Atomic Energy Agency Conference on Plasma Physics and Controlled Nuclear Fusion Research, 1990 (International Atomic Energy Agency, Vienna, 1991), Vo1. 1, p. 509] have been demonstrated to trigger the Ohmic H-mode transition. It is shown that three different methods, (1) radial electric field of either sign imposed by an electrode biased up to 500 V, (2) perturbation of the edge plasma density by strong gas puffing, and (3) LiD pellet injection, bring about the Ohmic H mode. In biasing experiments, the degree of improvement of particle and energy confinement depends on the polarity of the electric field and is higher for negative biasing. The evaporation of a LiD pellet (V approximately 150 m/sec, size approximately 0.3 mm, density perturbation approximately 50%) in the peripheral region of the plasma column can also lead to the H-mode transition. Experimental results are shown to be in reasonable agreement with the theory of radial electric fields in tokamaks.