EFFECT OF AN EXTERNAL HELICAL FIELD ON A ROTATING MAGNETIC ISLAND

Non-linear MHD calculations of the m/n = 2/1 tearing mode in a low beta cylindrical tokamak are carried out, taking account of the plasma rotation with the effect of the magnetic field induced by an external helical current. A rotating magnetic island is locked to the external helical field through the damping torque J(phi)h x B(r)h,where B(r)h is the radial component of the external helical field and J(phi)h is the plasma current induced by the helical field and the plasma rotation. When the rotating flow velocity is high enough, the damping torque produces a strong velocity shear near the resonant surface until locking of the magnetic island occurs. After mode locking, the width of the magnetic island becomes larger than that in the case without a helical field because of phase matching between the external helical field and the locked magnetic island. The stabilizing effect of the velocity shear can be sustained by keeping the velocity profile in a transient state by applying an external helical field with alternating directions. This method also ensures that the magnetic island does not become mode locked by other mechanisms such as interaction with a resistive shell or an error field, which is often followed by major disruptions.