Transport analysis through heat waves driven at different radial positions

Abstract

In W7-AS, electron cyclotron resonance heating (ECRH) discharges sustained by three gyrotrons, one resonant on-axis and two off-axis, were investigated by a perturbative square-wave modulation of the on- and the off-axis deposited power, respectively. The absorption profile of the modulated ECRH power as well as the transport coefficients, were determined from the response of the amplitude and phase delay of the driven temperature perturbation measured during a scan in the modulation frequency . To explain the results obtained during perturbative off-axis modulation, the appearance of a convective term (not compatible with the results for on-axis modulation) is required. However, an alternative (and more conservative) explanation is related to the ECRH absorption physics. During off-axis modulation the inwardly propagating heat wave causes a temperature perturbation (of the order of few eV at the plasma axis). As the absorption coefficient depends on temperature, this temperature perturbation will also cause a modulation in the absorption of the (stationary) on-axis power. The phase of this `induced modulation' will depend on the phase of the inwardly propagating heat wave. Simplified modelling confirms that this effect may explain the experimental findings.