Model for current drive stabilization of neoclassical tearing modes

A model derivation is presented for the effect of current drive on the saturated width of magnetic islands driven by the neoclassical tearing mode instability in axisymmetric plasmas. The derivation is carried out for continuous current drive as well as for modulated current that is driven at the same angle as the island O-point. The results of the derivation are implemented in a revision of the ISLAND module to compute saturated magnetic island widths. It is found that the greatest stabilizing effect of both modulated and continuous current drive on the island width is achieved when current is driven at the island center. In addition, narrow current drive is more effective at stabilizing the magnetic islands than wide current drive, for which more current falls outside the island. When modulated and continuous current drives are compared for equal total driven current, the modulated current is shown to be more effective, particularly as the offset from the island center increases.