How non-adiabatic passing electron layers of linear microinstabilities affect turbulent transport

Abstract

The response of passing electrons in ion temperature gradient (ITG) and trapped electron mode (TEM) 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 × B shearing rate. Finally, the passing electron channel is shown to significantly contribute to the transport levels, at least in our ITG case. Also shown is that the passing electrons significantly influence the E × B saturation mechanism of turbulence fluxes.

Intercode comparison of gyrokinetic global electromagnetic modes

“…However, corresponding evidence has already been found in local, fluxtube simulations. 25 An obvious extension of this work would hence be dedicated nonlinear simulations. While relaxation problems may be used for initial comparisons, gradient-driven simulations with carefully defined heat and particle sources and sinks will be prime choice.…”

Eliminating turbulent self-interaction through the parallel boundary condition in local gyrokinetic simulations

“…The time-averaged zonal E × B shearing rate (i.e. ∂ 2 φ/∂x 2 averaged over time and y) as a function of minor radial location within the flux-tube for slab simulations (though toroidal simulations with kinetic electrons look similar [10,12]). Data is shown for ion scale simulations using kinetic electrons with m i /m e = 3672 (black thick solid), kinetic electrons with m i /m e = 918 (red thick solid), and adiabatic electrons (gray thick dashed).…”

The effect of background flow shear on gyrokinetic turbulence in the cold ion limit

“…To investigate the nonlinear dynamics of turbulence, we will let k || = 0 in order to study two-dimensional turbulence. Strictly speaking, this is inconsistent with our assumption of adiabatic electrons [50]. However, the dynamics should be similar to a very small value of k || and setting k || equal to exactly zero facilitates benchmarking against gyrokinetic codes.…”