An intrinsic source of radial electric field and edge flows in tokamaks

We propose a new mechanism for radial electric fields and edge flows in tokamaks that will also serve as an intrinsic momentum source in systems without an up–down symmetry. An essential feature of toroidal plasmas is that charge-dependent ∇B and curvature drifts would lead to a vertical polarization of the discharge if it were not for the Pfirsch–Schlüter currents that neutralize the resulting charge separation. However, in the presence of collisions, there is a residual vertical electric field that drives an E × B flow in the direction of increasing major radius, regardless of the orientation of the fields and currents. This flow is excluded from the hot core and is localized to the more collisional edge plasma. It has many features in common with the edge flows observed in tokamaks such as C-Mod. In an up–down symmetric geometry it carries no net toroidal angular momentum; however, its viscous interaction with asymmetric boundaries leads to a net momentum input to the plasma. Both this momentum input, and the residual vertical electric field, the source of these flows, may play an important role in the ∇B direction-dependence of the power threshold for the L–H transition.