Evidence for the neoclassical nature of the radial electric field in the edge transport barrier of ASDEX Upgrade

Experiments have been performed on ASDEX Upgrade to clarify the nature of the radial electric field, E-r, in the edge transport barrier of tokamak plasmas. Highly resolved radial profiles of E-r have been diagnosed spectroscopically through the radial force balance of impurity ions. We show that in the fully developed, highly collisional edge pedestal the nature of the radial electric field is neoclassical. This requires, in particular, that the main ion poloidal rotation is at neoclassical levels. Both main ion and impurity ion poloidal rotation profiles have been measured in deuterium, hydrogen and helium plasmas with main ion pedestal top collisionalities, nu(*),(i), between 1.2 and 12. These profiles have been compared to a hierarchy of neoclassical models (from a conventional description to a comprehensive model including finite orbit width effects). They are found, in all cases, to be in good agreement demonstrating that inside the edge transport barrier the E-r well is sustained by the gradients of the main ion species.