Gyrokinetic electrostatic turbulence close to marginality in the Wendelstein 7-X stellarator

The transition from strong (fluidlike) to nearly marginal (Floquet-type) regimes of ion-temperature-gradient (ITG) driven turbulence is studied in the stellarator Wendelstein 7-X by means of numerical simulations. Close to marginality, extended (along magnetic field lines) linearly unstable modes are dominant, even in the presence of kinetic electrons, and provide a drive that results in finite turbulent transport. A total suppression of turbulence above the linear stability threshold of the ITG modes, commonly present in tokamaks and known as the ???Dimits shift,??? is not observed. We show that this is mostly due to the peculiar radial structure of marginal turbulence, which is more localized than in the fluid case and therefore less likely to be stabilized by shearing flows.

Automated summary

The transition from strong to nearly marginal regimes of ITG driven turbulence in Wendelstein 7-X is studied, showing that linearly unstable modes dominate near marginally, contributing to finite turbulent transport. Unlike tokamaks, total suppression of turbulence above the linear stability threshold of ITG modes is not observed. This is mainly due to the peculiar radial structure of marginal turbulence, which is less likely to be stabilized by shearing flows.