Heat pulse propagation measurements and experiments with equal electron- and ion-scale turbulence drive on the optimized stellarator Wendelstein 7-X

The advanced stellarator Wendelstein 7-X (W7-X) [1] is optimized to have reduced neoclassical transport in comparison to a classical stellarator [2], and the electron heat transport in W7-X has been measured to be significantly higher than the neoclassical level during its first high-performance operation phase with an inertially cooled island divertor [3-4]. In experiments with significant electron-scale turbulence drive, where electron-ion temperature ratios are large, the experimental electron heat transport measured in the core of W7-X is comparable to that predicted from electron temperature gradient mode driven turbulence [5]. In this contribution, experiments with comparable ion- and electron-scale turbulence drives will be presented, and the electron heat transport measured through heat pulse propagation experiments will be compared to results from nonlinear gyrokinetic calculations. In these experiments, the electron temperature gradient was controlled by varying the power deposition of heating applied at 30% and 50% of the plasma minor radius, similar to previous tokamak experiments [6-7]. In W7-X, the electron heat flux scales linearly with the temperature gradient where the ion- and electron- temperatures are similar, and the stiffness in the electron heat flux increases where the two temperatures diverge. These findings are consistent with predictions that ion-scale turbulence is dominant in W7-X when the drive for ion- and electron-scale turbulence is similar while electron-scale turbulence can be significant in W7-X otherwise [8].