Parameter dependence of the radial electric field in the edge pedestal of hydrogen, deuterium and helium plasmas

The characteristics of the edge radial electric field ( E r ) are studied in deuterium, hydrogen and helium plasmas at ASDEX Upgrade. The minimum of the E r well is analyzed as a function of pedestal parameters and the best correlation is found between the depth of the E r well and the ion pressure at the pedestal top. This result is consistent with E r being balanced by the main ion pressure gradient term. Studying the radial position of the E r minimum reveals that the E r well moves closer to the last closed flux surface the deeper the E r well is. This suggests that for deeper E r wells the distance between the steepest gradients in the ion temperature and ion density profile is reduced. The width of the E r well shows no significant variations despite changing the ion temperature, magnetic field and plasma particle species. At AUG, the E r well is on average 1.2 cm wide. A multi-machine comparison supports a machine size scaling of E r and indicates that the E r shear layer covers the outer 2% of the plasma minor radius independent of the size of the machine. Based on this scaling, the width of the E r well in ITER is estimated to ∼4 cm.