Formation of Plasma potential by means of ECH at divertor and peripheral region of plasma

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Stabilization of plasma has been attained via formation of plasma electric potential or control of pressure profile by means of electron cyclotron heating (ECH). ECH experiments on (1) inner region of plasma, (2) peripheral region and (3) divertor region were planned and done. The results are summarized as follows.(1) In the case of strong magnetic field and/or relatively low electron density, lower hybrid wave (LHW) can propagate into the inner side of plasma, where the tearing instability is suppressed by LH current drive (LHCD) and then a new MIHD instability is destabilized, The experimental results show that this instability is a pressure driven mode and pressure profile control by ECH is quite effective for suppression of the mode.(2) In the case of high electron density, toroidal rotation of plasma is slowed down by LHCD and the tearing mode is mode-locked to the vacuum vessel, and finally plasma disruption takes place. The slowdown of plasma rotation suggests the decrease of plasma potential, which may be ascribed to the loss of LHCD driven fast electrons at the plasma periphery. ECH is quite effective for unlocking of the mode lock and recovery of the plasma toroidal rotation. As a result, the plasma disruption can be avoided. These results suggests that ECH is effective for recovering of potential.(3) Since ECH by using Electron Bernstein waves is assumed to be quite effective for low electron temperature plasma near the divertor region, we have prepared a heating mm wave power source and designed the transmission line and injection antenna for the Bernstein wave heating. In this scheme, trapped electrons in toroidal ripple may be heated and injected into the main plasma via toroidal drift, and plasma potential may be formed at the j3lasma peripheral region.