The parametric dependence of the spatial structure of the radial electric field at the plasma edge in the DIII-D tokamak

Much recent theoretical and experimental work has focused on the mechanisms controlling the transition from L-mode to H-mode and the subsequent improvement in transport and confinement. Measurements of the radial electric field, Er, with high spatial and time resolution at the L-H transition have led to an understanding of the improved confinement of the plasma edge in H-mode plasmas based on the stabilization of plasma turbulence by sheared ExB flow. The radial electric field just inside the last closed flux surface (LCFS) changes dramatically at the L-H transition and a well-like structure in Er forms simultaneously at the transition. At present, there is no accepted theory which gives the spatial structure of Er near the plasma edge and which can predict its time evolution. Present theories propose mechanisms for the generation of a negative Er at the L-H transition, the bifurcation conditions for the transition and the stabilization of turbulence by sheared ExB flow. Although the various theories invoke different mechanisms to derive the negative Er (e.g., ion orbit losses, Stringer spin-up, nonlinear transport theory, self-regulating turbulence, they have as yet to come into full agreement with the observed experimental results and, in particular, the observed behavior of the poloidal rotation and pressure gradient of the main ions at the transition. Furthermore, these theories need to be extended to explain the formation of the spatial structure of Er at the transition and its temporal development into the H-mode. In order to help development of theories on the Er profile, we have examined the dependence of the radial profile of Er on different plasma parameters