Experimental investigation and gyrokinetic simulations of multi-scale electron heat transport in JET, AUG, TCV

Tokamaks dominated by electron heating like ITER could possibly suffer from theconsequences of electron temperature gradient (ETG) mode destabilisation, which coulddevelop a turbulent electron heat flux capable of setting an upper limit to the achievableelectron temperature peaking, resulting in a degradation of the fusion performance. An effortis carried out in this paper to collect and compare the results of dedicated plasma dischargesperformed during the last few years at three of the major European tokamaks, TCV, AUG andJET, by analysing the electron heat transport for cases presumably compatible with ETGsrelevance, given the actual theoretical understanding of these instabilities. The response of theelectron temperature profiles to electron heat flux changes is experimentally investigated byperforming both steady state heat flux scans and perturbative analysis by radio frequencyheating modulation. The experimental results are confronted with numerical simulations,ranging from simple linear gyrokinetic (GK) or quasi-linear runs, to very computationallyexpensive nonlinear multi-scale GK simulations, resolving ion and electron scales at the sametime. The results collected so far tend to confirm the previously emerging picture, indicatingthat cases with a proper balance of electron and ion heating, with similar electron and iontemperatures and sufficiently large ETG, could be compatible with a non negligible impact ofETGs on the electron heat transport. The ion heating destabilises ETGs not only by increasingthe ion temperature but also thanks to the stabilisation of ion-scale turbulence by a synergy offast ions and E × B shearing, which are in some cases associated with it. The stabilising effectof plasma impurities on ETGs is still under investigation by means of multi-scale GKsimulations, and also direct experimental measurements of density and temperaturefluctuations at electron scales would be needed to ultimately assess the impact of ETGs.