On the stabilization of neoclassical tearing modes by electron cyclotron waves

Abstract

The control of neoclassical tearing modes in tokamaks by means of electron cyclotron current drive and heating is investigated. The nonlinear evolution of the amplitude in absence and in presence of the stabilizing terms of an auxiliary current inside the island and of the associate heating is solved self-consistently with the evolution of the rotation frequency for International Thermonuclear Experimental Reactor (ITER) reference magnetic equilibrium [ITER-JCT and Home Teams, Plasma Phys. Controlled Fusion 37, A19 (1995)]. It is shown that, unless the wall braking torque is neutralized by external means, neoclassical tearing modes in ITER will be locked in a very short time. On the other hand, for rotating islands, the beneficial effect of modulating the current source in phase with the island rotation is pointed out, after an analysis of the time scales of the relevant phenomena (time response of the driven current, island rotation frequency, power pulse duration, and inductive response of the plasma). Consideration is given to different effects that may reduce the efficiency of the control of the flux reconnection rate and to the benefits of wall stabilization associated to the island rotation frequency. A quantitative assessment of the EC (electron cyclotron) power required to keep the island width at a reasonable level is given, both in absence and in presence of wall stabilization.