The universal instability in optimised stellarators

In tokamaks and neoclassically optimised stellarators, like Wendelstein 7-X (W7-X) the Helically Symmetric Experiment, turbulent transport is expected to be dominant mechanism. Among electrostatic instabilities that drive turbulence, trapped-electron mode (TEM) has been shown both analytically in simulations absent over large ranges of parameter space quasi-isodynamic stellarator configurations with maximum- $J$ property. It proposed reduction linear TEM growth rate such may lead passing-electron-driven universal instability, which often subdominant TEM, becoming fastest-growing instability some range space. Here, we show through gyrokinetic using Gene code, a variety geometries typically occupied by but most consequentially devices possess beneficial stability properties W7-X, locally satisfies property for deeply trapped particles regions worst curvature. We find exists at long perpendicular wavelengths and, as result, dominates potential fluctuation amplitude nonlinear simulations. modes are found differ parallel structure from trapped-particle modes, impact turbulence localisation experiments.