Dynamical plasma response to additional heating

Abstract

The response of a Tokamak plasma to an additional heating pulse is enigmatic. The paper presents results of the dynamical response on the electron temperature, deduced from the soft X-ray emissivity profile, to additional Alfven wave heating power. Most of the study used quasi-steady state sinusoidal modulation of the delivered r.f. power, but data are also included on the step-function response at the end of the r.f. pulse, and the impulse response to short periodic r.f. pulses. These different approaches all show similar behaviour of the radial profile of the response phase, which is dominated by a central flat region within the q=1 surface, with, at the higher frequencies used, a minimum phase of close to 180 degrees . Moving further outside the q=1 radius the phase increases extremely sharply. This basic character of the phase profile is not altered by varying the calculated power deposition profile but is still defined by the q=1 radius, demonstrated by a plasma current scan at constant toroidal field. In addition to the soft X-ray flux baseline modulation, the sawtooth period is modulated, but out of phase with r.f. power. These effects have been studied as a function of modulation frequency, plasma current, r.f. power and Alfven wave mode.