How non-adiabatic passing electron layers of linear microinstabilities affect turbulent transport
The response of passing electrons in ion temperature gradient and trapped electron mode microturbulence regimes is investigated in tokamak geometry making use of the flux-tube version of the gyrokinetic code GENE. Results are obtained using two different electron models, fully kinetic and hybrid in which passing particles are forced to respond adiabatically, while trapped are handled kinetically. Comparing linear eigenmodes obtained with these two models enables to systematically isolate fine radial structures located at corresponding mode rational surfaces, clearly resulting from the non-adiabatic passing-electron response. Non-linear simulations show that these fine structures on the non-axisymmetric modes survive in the turbulent phase. Furthermore, through non-linear coupling to axisymmetric modes, they induce radial modulations in the effective profiles of density, ion/electron temperature, and E x B shearing rate. Finally, the passing-electron channel is shown to significantly contribute to the transport levels, at least in our ion temperature gradient case. Also shown is that the passing electrons significantly influence the E x B saturation mechanism of turbulence fluxes.