Rapid eITB formation during magnetic shear reversal in fully non-inductive TCV discharges

A rapid formation of an electron internal transport barrier (eITB) is observed during a slow evolution (∼200ms) from a centrally peaked to a hollow current density profile, while all external actuators remain constant. The time constant for the barrier formation appears to be shorter than the electron energy confinement time. The improved confinement associated with the barrier formation occurs first in a localized region off-axis. Then the effects propagate to inner and outer flux surfaces on a confinement time scale. The temporal and spatial localization of the barrier formation suggest a threshold in the magnetic shear profile, which triggers the onset of the eITB. The location of the barrier corresponds to the radial location of the zero-shear flux surface based on a Fokker-Planck code and, therefore, we assume that the inversion of the q-profile corresponds to the barrier formation in both time and space. A simplified model is presented that attempts to characterize the current profile evolution and correlates the onset of the barrier with the time at which the current profile becomes inverted. (author)