Destabilization Of Field-Line Localized Density Fluctuation With A 1/1 Internal Kink Mode In The East Tokamak

A high frequency (HF) density fluctuation, ranging from 100kHz to 400 kHz, is locally destabilized with an m/n = 1/1 internal kink mode before a sawtooth crash in the experimental advanced superconducting tokamak (EAST). The location is just around the region with a high electron pressure gradient due to the flux compression from the 1/1 mode. And this HF fluctuation can be triggered at both the low field side and high field side. This fluctuation has a poloidal wave number (k(theta)) from 3 to 5 cm(-1), corresponding to a poloidal mode number (in) of 36-60. A threshold of the local electron pressure gradient (vertical bar del p(e)vertical bar) exists for the stimulation of the HF fluctuation, and its amplitude increases with vertical bar del p(e)vertical bar. The results could be beneficial for the understanding of a sawtooth crash, and also physics in 3D equilibrium.