The resonant interaction between magnetohydrodynamic (MHD) instability modes and the externally applied helical magnetic field is demonstrated in the Saskatchewan Torus-Modified (STOR-M) tokamak. The study is conducted both numerically and experimentally using a 2D MHD equilibrium code in the former and an (l = 2, n = 1) helical coil carrying a short current pulse in the latter. It is shown numerically that the resonant helical current can efficiently suppress the magnetic islands resonating on the (m = 2, n = 1) magnetic surface when the value of the safety factor at the plasma edge is relatively low (⩽4). It is also found numerically that (2, 1) islands are induced when the applied helical current exceeds a threshold. The experimental study is performed in STOR-M during low-q ohmic discharges with high MHD activities. The amplitude and frequency of (2, 1) Mirnov fluctuations are significantly reduced after the activation of the resonant field. Lesser suppression in sideband islands is also observed. Moreover, a phase of improved plasma confinement, characterized by a reduction in Hα emission level, a reduction in loop voltage and an increase in the soft x-ray emission, is induced after application of the resonant field.
Pressure driven MHD instabilities in the intrinsic and externally enhanced magnetic stochastic region of LHD