Electron cyclotron current drive control of neoclassical nonlinear tearing modes

An important role of ECCD is that of maintaining MHD-stable operation by a well localized current driveattheq-2,312surfacestostabilizethem=2,n=l,m=3,n=2teaingmodes. Inparticular,we refer here to the Neoclassical Tearing Modes whose relevance for reactor-grade tokamaks has been pointed out as they could set the B limit of long pulse discharges well below the ideal limit. A way of controlling (or suppress) these modes could be to drive a non-inductive current in the island O-point replacing locally the missing bootstrap current and phasing the current relative to the rotating island by modulating the source. We address here this problem by retaining in the equation for island growth the stabilizing term of an auxiliary current and the associated heating. Evaluation of the driven current are made by a ray-tracing code as a result of the injection of focused multibeams. A quantitative assessment of the EC power required to keep the island width at a reasonable level is given. Consideration is given to different effects that may reduce the efficiency of the control and the benefits of the wall stabilization associated to the island rotation frequency.