Control of the eITB formation and performance in fully non-inductively sustained ECCD discharges in TCV

The X2 ECH antennas on TCV are used to sustain and tailor the plasma current profile, forming either a centrally peaked or hollow profile. During the transition from peaked to hollow profile, an eITB is observed to form rapidly and in a localized region, which correlates with the appearance of a zero shear flux surface off-axis according to the ASTRA transport code. The barrier position can be controlled via the co-ECCD off-axis deposition location, and the barrier strength with central heating or counter-ECCD (increasing the depth of the hollow current profile), achieving H-factors of up to ≤6. In these discharges, the current in the ohmic transformer coil is held constant to avoid an inductively driven current contribution. After the eITB is created, a small amount of ohmically driven counter (co-) current has been added as a perturbative current source, transferring only a few kW of ohmic power compared to 1.4MW of ECCD. The ohmic current increases (decreases) the eITB performance demonstrating the clear dependence of the eITB on the current profile