Investigations of Magnetohydrodynamic (MHD) instabilities in the STOR-M tokamak

MHD modes in the STOR-M tokamak (R/a = 0 .46/0.12, B < 1 T, I < 50 kA) have been measured using 32 discrete external Mirnov coils. The data have been analyzed using (a) conventional Fast Fourier Transform (FFT) cross-correlation analysis, (b) direct Fourier components analysis, and (c) Singular Value Decomposition (SVD). During the nominal Ohmic discharge phase, the dominant poloidal mode numbers are 2, 3, and 4 and toroidal mode number n = 1. The oscillation frequencies are in the range 20-30 kHz. During the improved confinement phase triggered by compact torus injection, the Mirnov oscillations are strongly suppressed to a level lower than noise. The MHD activities reappear BEFORE the back transition from the H-mode-like to L-mode discharge phase. The reappearance of the MHD activities is led by an m=1 gong mode burst which is followed by a propagating m = 2 mode. (author)