Gyrokinetic simulations of the helically trapped electron mode in the W7-X stellarator

The first global gyrokinetic simulations of the helically-trapped electron mode (HTEM) in the Wendelstein 7-X stellarator are presented. Using the GTC code, simulations with a 3D equilibirum, kinetic electrons and a prescribed density gradient exhibit an unconventional trapped electron mode. The eigenmode extends along the field lines in the weak magnetic field region and shows a strong variation in the toroidal direction as it was the case in ion temperature gradient (ITG) simulations. However, HTEM is located in the inner side of the torus where the curvature becomes unfavorable in the so-called 'straight' section in W7-X. The HTEM is excited by helically-trapped electrons due to the W7-X magnetic configuration. In contrast to tokamaks, HTEM propagates poloidally in the ion diamagnetic direction. Further nonlinear simulations show that the zonal flows are a subdominant HTEM saturation mechanism. An inverse cascade in toroidal harmonics is observed during saturation which is enhanced by the excitation of low-n harmonics. HTEM can cause significant particle transport comparable to heat transport in ITG simulations with a similar normalized temperature gradient.