Electron ITB and the role of fast electrons in fully non-inductive reverse shear scenarios with large bootstrap fraction

The relation between the safety factor profile q, the electron transport and the bootstrap current is of crucial importance for predicting advanced scenarios in burning plasmas which will have dominant electron heating. In recent experiments in the Tokamak TCV, the full plasma current is sustained with only off-axis co-current drive. In this way an electron transport barrier is formed and the bootstrap current increases up to 50% of the total plasma current. The thermal energy confinement time improves significantly while no improvement is observed for suprathermal electrons. As soon as the EC power is turned on, the inductive current is set to zero and the current profile evolves from the ohmic profile to the one driven by the off-axis beams. Adding heating or counter CD in the center exhibits clear difference in the evolution of the electron temperature profile. With less current driven in the center, that is a more reversed q profile, the electron ITB formed is stronger and better energy confinement time is obtained. The time evolution and differences between co-CD and counter-CD in the center indicate that there is no power threshold to form an electron ITB. (author)