Control of MHD Instabilities in the STOR-M Tokamak using Resonant Helical Coils

Full text: Previous studies on the MHD properties in the STOR-M tokamak (R/a =0.46/0.12 cm, Bt = 1 T, Ip < 50 kA) have shown that the dominant m = 2 instabilities and their interplay with other modes played a vital role in the H-mode like discharges triggered by a short current pulse, electrode biasing and compact torus injection, both for L-H transition and H-L back transition. Present studies concentrate on active control of the m =2 instabilities using a set of resonant helical coils (RHCs) in the m/n =2/1 configuration. Numerical simulation based on the STOR-M configuration indicates that a magnetic island's width decreases with increasing RHC current until it completely disappears and the island reappears and grows with further increases in the RHC current, consistent with the previous experimental observations which indicated MHD stabilization for moderate RHC current magnitude and destabilization for further increased RHC current. When an 800 A RHC current is applied during a 25 kA STOR-M discharge, significant reduction in magnetic fluctuation amplitude, particularly for the m = 2 mode, and change in frequency characteristics have been observed. Significant reduction (∼ 40%) in hydrogen line emission level and increase in global energy confinement time have also been observed. (author)