Modelling of pedestal transport during ELM suppression by external magnetic field perturbations

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Particle and energy transport in the edge transport barrier is analysed in the presence of magnetic field perturbations from external resonant coils successfully used recently for the mitigation of type I edge localized modes (ELMs). The modification of transport due to charged particle and heat flows along perturbed field lines in a small region near the separatrix, spanning from 2% to 4% of the total poloidal flux, where complete stochastization is provided by the overlap of the main magnetic islands, is taken into account. The observed reduction of the density in plasmas of low collisionality is explained by the generation of charged particle flows along perturbed field lines, the increase in the electron and ion temperatures in the barrier—by the reduction of the perpendicular neoclassical transport with decreasing density and non-locality of parallel heat transport. On the basis of the heat flux limit concept in a deeply collisionless regime, the parallel thermal conductivities are taken to be 17 times smaller for electrons and 7 times smaller for ions than from a standard free-streaming estimate. The model elaborated before is developed further by taking into account the radial variation of the inclination angle of stochastic field lines and convective energy losses including the acceleration of ions by the pressure gradient and ambipolar electric field. It is demonstrated that convection of parallel kinetic energy of ions gives greater losses than parallel thermal conduction in the outer 50% of the stochastic layer and its inclusion improves the agreement with experimental results. This modelling is performed by assuming in agreement with observations that the influx of recycling neutrals through the separatrix is not reduced with I-coils compared with its level between ELMs before the mitigation stage. By trying to match experimental profiles with this influx decrease, some enhanced thermal losses of another nature than that considered here are needed in order to mitigate the drop in the perpendicular thermal conductivities for the assumed density scaling. The impact of the neutral particle influx increase by gas puffing applied in order to restore the plasma density is investigated.