The dynamical response of the plasma as a tool for investigating transport mechanisms

The dynamical response of the soft X-ray emission profile to different external perturbations - gas feed, impurity injection, RF power, surface loop voltage - has been studied on the TCA tokamak and analysed using the same techniques. The frequency dependence of the response has been exploited to distinguish between the dominant transport processes. Remarkably similar phase response profiles were obtained with the different stimuli; they show a link with the sawtooth activity. The model which most plausibly explains these experimental observations requires diffusive transport with the diffusive coefficient locally modulated by the perturbation. (author) 15 figs., 27 refs