Observations of the snakelike oscillation phenomenon on HT-7 tokamak

Abstract

Snakelike oscillation phenomenon has been frequently observed on HT-7 superconducting tokamak in various target plasmas. The longest lifetime snake, found in the pellet-fuelled discharge, can survive ten big sawtooth collapses and persist intermittently for a long lifetime of 53.7 ms more than three times the particle confinement time τP. The Ohmic spontaneous snake frequently occurs in the high-density discharge after wall siliconization or wall lithium-containing siliconization when the central line-averaged density is over a threshold value of 3.5×1013 cm−3, appears after the onset of the large sawtooth event and persists intermittently for a comparable long lifetime to that of the pellet-induced snake. In the ion Bernstein wave (IBW) heated plasma, while the antenna frequency ω is set at 30 MHz and the second harmonic deuterium cyclotron resonance layer is close to the sawtooth inversion radius, spontaneous snake oscillation easily occurs in the later phase of the IBW heated plasma or after switching off IBW power. In the low hybrid current drive plasma, negative snake appears and terminates silently. Suppression of the sawtooth activity and the impurity accumulation in the central region of the plasma are found to play a significant role in the occurrence of the spontaneous snake oscillation. In this paper, behaviours of the pellet-induced snakes have been summarized. Characteristics of various spontaneous snakes have also been presented and discussed.