Gyrokinetic global electrostatic ion-temperature-gradient modes in finite β equilibria of Wendelstein 7-X

Abstract

The standard scenario for the Wendelstein 7-X (W7-X) stellarator supposes equilibria with average β up to 4.3%. Here, the gyrokinetic description for full-radius ion-temperature-gradient (ITG) driven instabilities in three-dimensional configurations is used to investigate the linear stability and global structure of electrostatic ITG modes for the finite-β W7-X configurations. A massively-parallel initial-value particle-in-cell code has been modified to include the influence of a finite β on ITG modes due to equilibrium effects. We considered a sequence of W7-X equilibria with different plasma β, and found drastic changes in the structure and drive of the ITG instability but a minor influence on the value of the growth rate. The results presented are physically explained; differences between β-effects on ITG modes in a conventional tokamak and in W7-X are discussed, as well as the different structures and physical mechanisms of ITG modes for β = 0 and finite-β W7-X configurations.