Dissipative saturation structure and transport effects of selfexcited microisland in tokamaks

The possibility of self-excitation in tokamaks of microislands (whose width is much smaller than the ion gyroradius and whose mode numbers m and n are large) by pumping energy from the plasma through the interaction of the ions with densely packed island chains has been suggested in recent years by Kadomtsev. A number of questions are addressed in this article in the context of this suggestion. The time dependent island evolution equations are analysed in the linear approximation in the presence of energy pumping and resistive damping. In the stationary case the non-linear partial differential equation describing the structure of the microislands is reformulated and solved numerically, elucidating how the topology of the islands is affected by non-linearity. The saturation conditions of self-excited microislands are discussed on the basis of the energy integral of the evolution equations, assuming that the pumping is balanced by damping arising from a resistive-like anomalous dissipation related to a resonant electron-island wave interaction. The saturated island width is expressed in terms of the physical and geometrical parameters of the tokamak. The consequences for plasma transport and the scaling of the energy confinement time with respect to these parameters are presented.