Decoupling of ion and electron heat transport via scale separation

Traditionally, turbulent transport is thought to be carried mainly by long-wavelength modes with $k_\perp\rho_i\sim 0.2$. While this seems to be generally true for the ion heat channel, there is experimental evidence that the electron heat fluxes behave differently -- both in transport barriers and beyond. Here, we will examine the spectral properties of heat transport in nonlinear gyrokinetic simulations with the {\sc Gene} code, focussing on contributions from shorter wavelengths than the ones mentioned above. Some simulations treat both ion and electron space-time scales fully self-consistently and are therefore very challenging from a computational point of view, requiring of the order of 100,000 CPU-hours or more. In pure ITG and TEM turbulence, the heat fluxes exhibit clear peaks around $k_\perp\rho_i\sim 0.2$, and fall off quickly for higher wavenumbers. However, in cases for which TE/ITG modes and ETG modes coexist, the spectral properties may change completely. Now, a wide range of wavenumbers, from ion scales all the way to electron scales, can contribute to the overall electron heat transport. This implies that the latter is dominated by high-wavenumber TEMs and ETG modes, while the direct contribution from ITG modes is relatively small. Applications to recent experiments at both DIII-D and NSTX will be discussed. Here, the concept of scale separation turns out to be crucial for the interpretation of experimental data.