Edge turbulent transport toward the L–H transition in ASDEX Upgrade and JET-ILW

This work combines experimental observations from the ASDEX Upgrade and the JET-ILW tokamaks and related gyrokinetic simulations on the L-mode edge turbulence with different isotopes. The evolution of the edge logarithmic gradients and the edge radial electric field with increasing input power and the correlation between the two are studied. It is found that the edge ion temperature profile plays the dominant role in the evolution of the edge radial electric field in the considered cases. More input power and ion heat flux are needed in hydrogen to obtain values of Ti and RLT similar to the ones observed in deuterium. As a consequence, more power is needed in hydrogen to develop values of the edge radial electric field similar to those in deuterium plasmas. These observations point to a key role of the dependence of the edge turbulence on the main ion mass in determining the different L-H power thresholds with different isotopes. This dependence is found in gyrokinetic simulations to be connected to the parallel electron dynamics, i.e., to a different kinetic response of passing electrons with different main ion mass. The gyrokinetic simulations indicate different roles of RLT and in driving or stabilizing the edge turbulence at different wavelengths and indicate a strong role of the external flow shear in stabilizing the edge turbulence. The simulations indicate also that instabilities at finite values of kx play an important role in edge conditions.