Experimental study of the tearing mode instability during the current ramp up stage in Sino-United Spherical Tokamak

The fierce magnetohydrodynamic (MHD) activity observed during the current ramp up stage of a tokamak discharge, which significantly limits the performance of the plasma, has been experimentally studied in Sino-United Spherical Tokamak (SUNIST). The mode structure extracted by magnetic measurements appears to be typically decreasing from to , where m and n are the poloidal and toroidal mode numbers. A phase reversal layer in the spatial distribution of the magnetic perturbation extracts the approximate position of the magnetic island chain, which gives insight on the evolution of the internal structure of the MHD activity. A matrix equation method that includes corrections for the shadowing effect of the probe array has been developed so as to derive a more accurate estimation of the current density profile. The resulting current profile evolution qualitatively explains the decrease in poloidal mode number and the spontaneous suppression of the MHD activity. Statistical results of the MHD response suggest that the deterioration of the plasma confinement is closely related to the phase inversion position of the poloidal magnetic perturbation.