Multi-scale structures of electric current generated by collisionless trapped-electron-mode turbulence

The spatial structure and amplitude of the current induced by collisionless trapped-electron-mode (CTEM) turbulence are investigated by gyrokinetic simulations. It is shown that the barely passing electrons play a crucial role in determining the magnitude and direction of the current density. Two characteristic radial scales of the current density are found. The fine structure (a few ion Larmor radius) of the turbulence-induced current is observed near the rational surfaces. Furthermore, the mesoscale structure (tens of ion Larmor radii) of the turbulence-induced current related to the zonal flow shear is confirmed, especially for the high toroidal mode number (n) CTEM. For the strongly driven CTEM, the zonal flow shear effect on the turbulence-induced current is significant, while it is not visible for the weakly driven CTEM. We show that the magnitude of the CTEM turbulence-induced current density features a moderate local magnitude comparable to the bootstrap current density near rational surfaces, as demonstrated by nonlinear simulations with multi-n modes.