Observation of the poloidally asymmetrical density perturbation of sawtooth collapse on J-TEXT

The detailed density perturbations provided by the advanced polarimeter–interferometer system (Polaris) during sawtooth collapse on the Joint Texas Experimental Tokamak (J-TEXT) are reported in this article. During a sawtooth collapse and the crash of plasma pressure at the center, it is found that the increase in density in the region between the inversion radius and mixing radius is poloidally asymmetrical, while the increase in temperature is poloidally symmetrical. The poloidal location where the density increases is dependent on the phase of the precursory m/n = 1/1 kink mode. It is always out of phase with the hot core of the m/n = 1/1 mode. The behaviors of density perturbations during sawtooth collapse observed in J-TEXT are beyond the expectations of the standard model, and this can shed new light on the understanding of sawtooth collapse.