On the Origin of Steep and Localized Radial Electric Field in the Transport Barrier at Plasma Edge

Theories to understand the steep and localized radial electric field in the edge of toroidal plasma, which appears in conjunction with H-mode, is revisited based on the electric field bifurcation model. Key elements in the models of the L-H transition (including the toroidal effects on the dielectric constant and the effects of the curvature of radial electric field on turbulence suppression) are assessed. Results are applied to tokamak and helical plasmas, for which data with high-resolution have been obtained recently. The status of quantitative tests on various mechanisms through comparison with experimental observations is also addressed.