On the effect of electron cyclotron waves on the evolution of neoclassical tearing modes in tokamak plasmas

In this paper we study the generation of current in a tokamak plasma in the presence of a magnetic island, using electron cyclotron (EC) waves in a process known as electron cyclotron current drive. We study some features related to the efficiency of the method, adopting a simplified model for a tokamak which incorporates the existence of a magnetic island associated with the occurrence of a neoclassical tearing mode instability. The study utilizes a self-consistent treatment of the interaction of EC waves with the plasma in the tokamak and the consequent current generation, taking into account the existence of the tokamak loop voltage and the occurrence of radial transport of particles, and also the existence of induced effects. The formalism also includes an approximate self-consistent description of the evolution of the width of the magnetic islands, which depends on the plasma current through the so-called modified Rutherford equation. The evolution of the islands depends on classical effects, which are driven mainly by the equilibrium current gradient, on neoclassical effects related to the perturbed bootstrap current and on the current generated by the radio waves. The results obtained in our numerical analyses confirm that the width of the islands can be substantially reduced by the use of EC waves, and lead to an estimate of the minimum amount of EC power required to be effective in reducing the width of magnetic islands. The results also show that, although the density of current generated by EC waves in the island region decreases along late time evolution, the total amount of plasma current is increased compared with the case in which the width of the island is assumed to remain constant.