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

Tokamaks dominated by electron heating like ITER could possibly suffer from the consequences of an elec-tron temperature gradient (ETG) mode destabilisation, which could develop a turbulent electron heat flux ca-pable of setting an upper limit to the achievable electron temperature peaking, resulting in a degradation of thefusion performances. An effort is carried out in the paper to collect and compare the results of dedicated plasmadischarges performed during the last years at three of the major European tokamaks, TCV, AUG and JET, byanalysing the electron heat transport for cases presumably compatible with ETGs relevance given the actual the-oretical understanding of these instabilities. The response of the electron temperature profiles to electron heatflux changes is experimentally investigated by performing both steady state heat flux scans and perturbativeanalysis by radio frequency modulation. The experimental results are confronted with numerical simulations,ranging from simpler 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 fartend to confirm the previously emerging picture indicating that only cases with a proper balance of electron andion heating, with similar electron and ion temperatures and sufficiently large electron temperature gradient, arecompatible with a non negligible impact of ETGs on the electron heat transport. The ion heating destabilisesETGs not only by increasing the ion temperature but also thanks to the stabilisation of ion-scale turbulence bya synergy of fast ions andE×Bshearing which are in some cases associated to it. The stabilising effect ofplasma impurities on ETGs is still under investigation by means of multi-scale gyrokinetic simulations, and alsodirect experimental measurements of density and temperature fluctuations at electron scales would be needed toultimately assess the impact of ETGs.