Disruption Control by the Helical Magnetic Field

The growth of m/n=2/1 tearing mode is studied numerically in a new kind of equilibrium magnetic configuration: a zeroth-order axisymmetric equilibrium field superposed with a small m/n=7/4 static helical field, where m and n are respectively the poloidal and toroidal mode numbers. The amplitude of the magnetic flux perturbation |φ_2/1| is found to be reduced as the magnitude of the m/n=7/4 helical field increases. |φ_2/1| can be reduced to zero when the m/n=7/4 magnetic island is large enough that it overlaps the q=2 flux surface. Oscillatory |φ_2/1| is also excited with appropriate the magnitude of the m/n=7/4 helical field. These results are of practical interest for tokamak reactor design.