Role of the radial electric field in the transition from L (low) mode to H (high) mode to VH (very high) mode in the DIII-D tokamak

The hypothesis of stabilization of turbulence by shear in the ExB drift speed successfully predicts the observed turbulence reduction and confinement improvement seen at the L (low)--H (high) transition; in addition, the observed levels of ExB shear significantly exceed the value theoretically required to stabilize turbulence. Furthermore, this same hypothesis is the best explanation to date for the further confinement improvement seen in the plasma core when the plasma goes from the H mode to the VH (very high) mode. Consequently, the most fundamental question for H-mode studies now is: How is the electric field Er formed? The radial force balance equation relates Er to the main ion pressure gradient ∇Pi, poloidal rotation vθi, and toroidal rotation vφi. In the plasma edge, observations show ∇Pi and vθi are the important terms at the L--H transition, with ∇Pi being the dominant, negative term throughout most of the H mode. In the plasma core, Er is primarily related to vφi. There is a clear temporal and spatial correlation between the change in ExB shear and the region of local confinement improvement when the plasma goes from the H mode to the VH mode. Direct manipulation of the vφi and ExB shear using the drag produced by a nonaxisymmetric magnetic perturbation has produced clear changes in local transport, consistent with the ExB shear stabilization hypothesis. The implications of these results for theories of the L--H and H--VH transitions will be discussed