Investigation on optimal limiter condition for stable sustainment of the potential confined plasma in GAMMA 10

In GAMMA 10, Electron Cyclotron Heating (ECH) is used for the formation of the axial confining potential (P-ECH) and for the bulk electron heating in central-cell (C-ECH). The paper describes the dependence of the diameter of the iris limiters on the plasma stored energy and the hydrogen recycling. In this experiment, it was observed that the plasma diamagnetism was degraded with the ECH injection and Hα emission was increased near the limiters at the same time. In the case that the diameter of the iris limiters shrunk to 350 mm, the plasma diamagnetism decreased with each ECH by the plasma-limiter interaction. On the other hand, in the case that the diameter of the iris limiters widened to 380 mm, the diamagnetism of the plasma increased with P-ECH, however the plasma collapsed with C-ECH. The above results indicated the close relationship between the iris limiters and the plasma stability. In this paper, we discuss the mechanism of the degradation of the plasma performance and the optimal limiter condition for effective operation of ECH. (author)