ELM-Induced Energy and Momentum Transport in ASDEX Upgrade

Heat and momentum transport play a key role in achieving high confinement in fusion plasmas.The ion and electron profiles were measured with sub-ms temporal resolution in deuterium and helium plasmas, thus allowing to gain insight into the dynamics of the pedestal recovery after an edge localized mode (ELM) crash.A local increase of Ti close to the separatrix is observed at the ELM onset, thus reducing the gradient in the pedestal.Shortly after the initial separatrix increase, the whole profile drops and the pedestal starts to build up again.The pre-ELM profile is fully recovered 3-4 ms after the ELM crash.This behaviour is observed in both deuterium and helium plasmas.Transport analysis of the ion energy reveals that the ion heat transport is at the neoclassical level before the ELM crash in the region where the edge ion temperature gradient is maximal.Further inwards, the ion heat transport is about a factor of 4-5 above the neoclassical level.The dynamics of the edge ion heat transport during the pedestal build-up phase after the crash is also consistent with neoclassical theory.Comparison to the electron profiles shows that the ion temperature gradient recovers on similar timescales as the electron density gradient (3 ms), while the electron temperature gradient takes twice as long to reach its pre-ELM values after the ELM crash.Two mechanisms responsible for this delay in the recovery are presented.The edge impurity toroidal rotation shows a dependence on collisionality, with negative (counter-current) values at low collisionality, and positive (co-current) values at high collisionality.Modelling of the edge toroidal rotation based on the toroidal torque balance equation including diffusion, pinch and external momentum sources indicates that diffusion and the external sources are the dominant players.The sign change of the impurity toroidal rotation observed at low collisionality can be explained by a negative edge torque combined with a large differential toroidal rotation, while the main ion toroidal rotation is almost unaffected.