Turbulence in Stellarators and Tokamaks - Progress in the Dutch Fusion Program*

An overview of turbulence research at the Dutch Institute for Fundamental Energy Research and the Eindhoven University of Technology is given. In the Wendelstein 7-X stellarator, high-pressure configurations are susceptible to a non-ideal, sub-threshold kinetic-ballooning mode, which impacts zonal flows and substantially boosts transport well below the ballooning limit. A perturbative theory using an energy-dependent Krook operator is developed that captures growth-rate trends with collisionality for trapped-electron modes (TEMs) in this device. For tokamaks with shaped flux surfaces, it is found that a large negative or positive triangularity δ results in reduced TEM transport, an effect quasilinear models can capture only for negative δ due to changes in zonal-flow residual for δ > 0. In both tokamaks and reversed-field pinches, global gyrokinetic simulations with background currents capture tearing-mode physics and show complex interactions in the nonlinear state. Self-consistent multi-scale simulations how that TEM turbulence is affected by the presence of saturated tearing modes. Electron cyclotron current drive has been implemented in a gyrokinetic framework, and initial tests show that both ion- and electron-scale turbulence can be regulated through targeted current drive and heating.