Control of magnetic islands in the STOR-M tokamak using resonant helical fields

Abstract

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.

Understanding the effect of resonant magnetic perturbations on tearing mode dynamics

“…Another effect of rotation modulation by F em and F vs is that the island spends more time in the deceleration phase (0 < Φ < π) than in the acceleration phase (π < Φ < 2π), resulting in a net braking effect, that is why rotation reduction is usually observed associated with mode stabilization. 1,8,9 Based on the above understanding and numerical studies in Ref. 9, we conclude that the applied RMPs contributes a net and dominant stabilizing effect on TM, whether the TM can be stabilized depends on the plasma rotation and other plasma parameters.…”

“…3 Since the first RMPs experiment on PULSATOR, 4 many experiments related to RMPs have been carried out. 1,[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] The RMPs related experiments covers a broad spectrum of physics issues including the effect of RMPs on tearing mode (TM) dynamics, 1, 4-10 error field, 11,12 particle transport, 13,14 and mitigation or suppression of edge localized modes (ELMs). [15][16][17][18] One of the results is that RMPs with moderate amplitude can suppress Mirnov oscillations effectively.…”

“…1 Recently, it is found that the suppression of Mirnov oscillation by RMPs is accompanied by a reduction both in mode magnitude and mode frequency. 8,9 The RMPs effect on TM dynamics associated with mode stabilization has been investigated experimentally. 7 It is believed that there should exist some mechanisms which are responsible for the suppression of magnetic island by RMPs.…”

Research on the effect of resonant magnetic perturbations on disruption limit in J-TEXT tokamak

“…Most of the precursor MHD instability before disruption is verified to be the tearing mode (TM) [1], hence it is important to control the precursor TM for the disruption mitigation/avoidance. Previous experimental studies showed that RMPs can suppress rotating TM effectively, accompanied by a plasma rotation reduction [14][15][16][17]. Theoretical analysis developed by Fitzpatrick [18,19] reveals that the applied RMPs contribute a net stabilizing and braking effect on the TM and plasma rotation, respectively.…”

Power supply for generating frequency-variable resonant magnetic perturbations on the J-TEXT tokamak

“…Externally exerted dynamic resonant magnetic perturbation (DRMP) is an effective method used to control the TM [3][4][5][6][7][8]. On the J-TEXT tokamak, the phenomenon of the TM locking to the constant frequency DRMP has been observed [6].…”