The local nature of the plasma response to cold pulses with electron and ion heating at ASDEX Upgrade

In low density Ohmic heated tokamak plasmas a fast increase of the central electron temperature is regularly observed in response to a drop at the edge produced by the cold pulse of an impurity ablation. This observation has challenged the physical understanding and the theory-based local transport modelling for several decades. Recent experimental results in C-Mod and related modelling (Rodriguez-Fernandez et al 2018 Phys. Rev. Lett. 120 075001) have for the first time provided an explanation for this phenomenology, which results from the stabilization of trapped electron mode turbulence caused by the sudden flattening of the electron density profile as a consequence of the impurity ablation. Starting from this breakthrough result, recent cold pulse experiments in ASDEX Upgrade produced by laser ablation of carbon in deuterium plasmas demonstrate that the fast increase of the central electron temperature is only observed in the presence of dominant electron heating, whereas no fast increase is observed in dominantly ion heated plasmas, even at the same low density. The measured fast flattening of the density profile is shown to be large enough to produce a strong stabilization of the trapped electron mode turbulence in the case where the electron temperature largely exceeds the ion temperature, allowing the fast increase of the central electron temperature, consistent with the C-Mod modelling results. In contrast, it produces a destabilization of the electron temperature gradient modes when the electron and ion temperatures are closer, preventing the increase of the central electron temperature. Finally, modelling which also includes the dynamical evolution of the density profile with a local theory-based transport model shows that the impurity penetration transiently modifies the local turbulence causing a fast penetration of the particles, which explains the observation of a very fast flattening of the electron density profile. By this, the complex density and temperature behaviours in response to a cold pulse are demonstrated to be fully explained by multi-channel interactions within a local model.