Snake perturbations during pellet injection and LHCD in the HL-1M tokamak

Full text: Excitation of snake perturbations has been observed in the core region of pellet- fuelled HL-1M plasmas when the pellets cross surface with a q value of 1. It is observed that the snake oscillation has an m=1,n=1 helicity with quite a long lifetime.A detailed comparison has been made between the locations of q=1 surface and snake oscillation. Through measurements of plasma q-profile by means of multi-exposures with CCD camera during pellet ablation, and investigation on pellet ablation process, possible mechanisms for the formation of snake oscillation are discussed. In addition, a large, long-lived snake-like oscillation is frequently observed in lower hybrid current driven discharge in which the sawtooth has been stabilized early in the discharge. There is evidence that such a perturbation is due to impurity accumulation during sawtooth-stabilization, and good performance with peaking profiles after LHCD is limited by magnetohydrodynamic (MHD) instabilities including sawtooth and snake activities in HL-1M plasma. (author)