Gyrokinetic microtearing turbulence in the stellarator Wendelstein 7-X

Following the optimization of the stellarator Wendelstein 7-X (W7-X) to reduce neoclassical transport [1], microturbulence prevails as the primary transport mechanism limiting its plasma confinement. While electrostatic turbulent modes – such as the ion-temperature-gradient (ITG) and the electron-temperature-gradient (ETG) modes – have been recognized as key drivers of turbulent transport across a wide range of W7-X operational scenarios [1, 2, 3, 4], this work presents the first demonstration that a type of electromagnetic turbulence, specifically driven by microtearing modes (MTMs), dominates in a different experimental regime. This scenario, characterized by only neutral beam injection (NBI) heating, displays a steep density gradient and moderate ion and electron temperature gradients in the inner core (ρ ≤ 0.5).

Using reasonably heat and particle flux-matched simulations with the gyrokinetic code GENE [5] including experimental profiles, collisions and electromagnetic effects, we show that MTMs are the primary contributors to turbulent transport in this scenario. This work highlights the importance of incorporating electromagnetic effects and collisions to reproduce experimental fluxes – parameters that were excluded in previous studies of density-gradient-driven regimes [6, 7]. In addition, our simulations show that density-gradient-driven trapped electron modes (TEMs) are not unstable. This stabilization is likely due to the maximum-J property of devices such as W7-X [8].