Gyrokinetic Simulations of Microturbulence in TCV Electron Internal Transport Barriers

Using the Eulerian code GENE [1], gyrokinetic simulations of microturbulence were carried out under conditions relevant to electron-Internal Transport Barriers (eITB) in the TCV tokmak [2], generated under conditions of low or negative shear. For typical density and temperature gradients measured in such barriers, the corresponding simulated fluctuation spectra appears to simultaneously contain longer wavelength Trapped Electron Modes (TEM, for typically k_perprho_i < 0.5, k_perp being the characteristic perpendicular wavenumber and rho_i the ion Larmor radius) and shorter wavelength Ion Temperature Gradient modes (ITG, k_perprho_i > 0.5). The contributions to the electron particle flux from these two types of modes are respectively outward/inward and may cancel each other out for experimentally realistic gradients. This mechanism may partly explain the feasibility of eITBs. The non-linear simulation results confirm the predictions of a previously developed quasi-linear model [3], namely that the stationary condition of zero particle flux is obtained through the competitive contributions of ITG and TEM. Parameter scans of gyrokinetic microturbulence simulations were carried out with an attempt to identify combinations of density and electron/ion temperature gradients which not only cancel out particle fluxes but minimize electron heat fluxes as well.