Density scaling of the threshold for locked mode instability in the presence of toroidal field ripple in a tokamak

The nonlinear excitation of magnetic islands in a tokamak by external resonant magnetic perturbation is examined. The perturbation amplitude threshold for the appearance of a locked mode is determined for a tokamak with a rippled toroidal field. It is shown that the critical value of the magnetic field is increased by the ripple induced damping of toroidal rotation. Qualitative aspects of the experimentally observed reduced scaling of the threshold density for mode locking, are predicted in the regime of low viscosity and dominant plasma inertial effects. In such a case the scaling is shown to be close to ni∼Br1/2 rather than ni∼Br3/2 as predicted by the usual theory. copyright 1997 American Institute of Physics