Ponderomotive stabilization of external kink modes in tokamaks

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Abstract
It is shown that external kink modes in tokamaks can be stabilized by applied radio frequency (rf) waves in the ion cyclotron range of frequencies. This stabilization results from the work done by the plasma against the ponderomotive force exerted by the rf field. For a strongly evanescent surface field (such as the near field of an ion Bernstein wave coupler), the ponderomotive layer is confined to the vicinity of the plasma surface and, if sufficiently strong, influences kink stability in the same way as a close‐fitting conducting wall. This effect is illustrated by deriving an analytic condition for stabilizing a single (m,n) mode in the low‐beta straight tokamak model by means of a cylindrically symmetric E∥ rf field. The effects of poloidally or toroidally nonuniform rf fields are examined in a two‐mode coupling perturbation analysis. This stabilization mechanism may be useful in extending the beta limit of ion‐Bernstein‐wave‐heated tokamaks against both ideal and resistive modes.