Mitigation of Tearing Mode Instabilities by Radial AC Magnetic Fields in the STOR-M Tokamak

Abstract

Generating external radial magnetic fields and studying their effects on tokamak plasmas has been an ongoing topic in fusion research. Experimental studies have shown that externally applied radial magnetic fields can suppress plasma instabilities in tokamaks, particularly tearing mode instabilities. External radial magnetic fields have been successfully implemented in the STOR-M tokamak using (m = 2, n = 1) helical coils. However, this system can only produce static magnetic fields. The objective of this work is to design a new coil system capable of producing radial AC magnetic fields for improved tearing mode mitigation in STOR-M. The new system generates phase-varying rotating modes that can be tuned to match the natural frequency of tearing instabilities, which lies in the range of 10-30 kHz. The system consists of 8 toroidally distributed coil sets, each comprising trapezoidal and rectangular coils installed at different poloidal locations around STOR-M. Analytical modeling and numerical simulations have been conducted to determine the coil inductance, radial magnetic field distribution, and the resulting poloidal and toroidal mode spectra. Magnetic fields were calculated using the Biot-Savart law, and Fourier-based modal analysis was applied to identify dominant and sideband modes. A driving circuit consisting of an underdamped series RLC configuration and an H-bridge switching circuit was also analyzed to assess its capability to deliver stable high-frequency AC currents. The results show that the new coil system can generate radial magnetic fields with a dominant (2, 1) mode, accompanied by other sideband modes such as (1, 1) and (2, 7).