Modulation of electron transport during Swing ECCD discharges in TCV

Generation of a swing electron cyclotron current drive (swing ECCD), i.e. driving alternated, symmetric, positive or negative local ECCD, during a single discharge and at constant total input EC power, was performed at the Tokamak a Configuration Variable (TCV). The electron temperature is observed to be modulated inside the deposition radius, implying modulation of the electron transport properties. The modulation of ECCD is the only actuator for the observed modifications in the electron transport properties. These exhibit inverted behaviors depending on the deposition location of the co- and counter-ECCD. At more on-axis depositions, swing ECCD results in a higher electron temperature during the co- ECCD phase, whereas at more off-axis depositions, the electron temperature is higher during the counter-ECCD phase. Transport modeling of these discharges shows that the local electron tranport behavior depends on the value of the modulated magnetic shear. The results are transport model independent, confirming the robustness of the magnetic shear modeling, and indicating that the main contribution is due to the ECCD. Moreover, the results are consistent with predictions from gyrokinetic simulations, that the local electron confinement is proportional to the magnetic shear at low shear and inversely at high shear values, s greater than or similar to 1

Studies of Electron Transport and Current Diffusion in Switched ECCD experiments on TCV

The aim of the present work is to provide a better insight on the magnetic shear profile modification in the Switched ECCD experiments. Modelling of the plasma current density is carried on by the ASTRA transport code employed in both predictive and interpretative modes, with two shear-dependent models for the calculation of the electron energy diffusion coefficient. In this context, the modulation of ECCD is the only actuator for the transport properties modifications. This study confirms the synergy between electron transport and magnetic shear, both of which are modulated around the EC deposition region. It also allows to completely decouple the effects of the current profile modification from those of slight plasma heating misbalance or non-constant plasma elongation, which are key concepts at the basis of Switched ECCD and can be a rather delicate experimental issue. The numerical results moreover validate a previous rough model based on electrodynamics calculations