THE AC RESPONSE OF A TOKAMAK PLASMA TO DRIVEN HELICALLY RESONANT RADIAL MAGNETIC PERTURBATIONS

The edge plasma response to an externally driven oscillating radial ma gnetic field perturbation is studied in the TEXT tokamak. The perturba tion has a dominant m = 7, n = 2 mode structure and is spatially reson ant with rational magnetic surfaces near the plasma edge. Non-overlapp ing islands with m = 5-8 are produced with a driving frequency between 1 and 8 kHz, and the plasma response is measured using numerically de signed magnetic sensing coils which match the mode structure of the pe rturbation. Experimental observations are compared with a theory by Bo ozer treating the resulting magnetic islands as resistive-inductive ci rcuit elements. As predicted, the plasma healing response scales as th e perturbing current to the 3/2 power, linearly with the perturbation drive frequency, and inversely with the resistivity at the resonant su rface. The magnitude of the response is, however, a factor of 2 to 3 l arger than that predicted by the Boozer theory, and consistent with is land widths ranging from 5 to 80% of the vacuum widths.