The role of electron-driven microinstabilities in particle transport during electron Internal Transport Barriers

Experimental results obtained in TCV electron Internal Transport Barriers (eITBs) show a strong coupling between the plasma density and the electron temperature logarithmic gradients [1]. The plasma density is sustained by an inward thermodiffusive pinch whose theoretical understanding has not been achieved yet. The level of particle diffusivity, as es-timated in transient transport experiments, indicates anomalous transport as the main mech-anism to create this pinch. On the other hand, a clear improved heat and particle transport confinement is observed. Gyrokinetic calculations of the pinch coefficients are performed in the framework of quasi-linear theory, neglecting electromagnetic effects and assuming zero collisionality, limits which are reasonable for an eITB; core particle sources and impu-rities are also neglected. The role of non-adiabatic passing electrons is taken in to account and shown to be negligible. The coupling between density and temperature is provided via trapped electrons driven thermodiffusion which is maximized near the transition between ITG-dominated to TEM-dominated turbulence. This regime is obtained in a plasma charac-terized by large temperature and density logarithmic gradients through different stabilizing mechanisms discussed here