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 the consequences of an electron temperature gradient (ETG) mode destabilisation, which could develop a turbulent electron heat flux capable of setting an upper limit to the achievable electron temperature peaking, resulting in a degradation of the fusion performance. An effort is carried out in the paper to collect and compare the results of dedicated plasma discharges performed during the last years at three of the major European tokamaks, TCV, AUG and JET, by analysing the electron heat transport for cases presumably compatible with ETGs relevance given the actual theoretical understanding of these instabilities. The response of the electron temperature profiles to electron heat flux changes is experimentally investigated by performing both steady state heat flux scans and perturbative analysis by radio frequency heating modulation. The experimental results are confronted with numerical simulations, ranging from simple linear gyrokinetic or quasi-linear runs, to very computationally expensive nonlinear multi-scale gyrokinetic simulations, resolving ion and electron scales at the same time. The results collected so far tend to confirm the previously emerging picture indicating that cases with a proper balance of electron and ion heating, with similar electron and ion temperatures and sufficiently large electron temperature gradient, could be compatible with a non negligible impact of ETGs on the electron heat transport. The ion heating destabilises ETGs not only by increasing the ion temperature but also thanks to the stabilisation of ion-scale turbulence by a synergy of fast ions and ExB shearing which are in some cases associated to it. The stabilising effect of plasma impurities on ETGs is still under investigation by means of multi-scale gyrokinetic simulations, and also direct experimental measurements of density and temperature fluctuations at electron scales would be needed to ultimately assess the impact of ETGs.