Constraints on theories provided by fast time response Er measurements across the L-H transition on DIII-D

Results of experiments performed in DIII-D since the last H-mode workshop impose a number of constraints on theories. First, measurements demonstrate that both the main ion pressure gradient, ∇Pi, and poloidal rotation, νθi, are important in determining Er in the H-mode, with the ν x B term being the trigger for the transition. Accordingly, theories must include both ∇Pi and ν x B effects. Since the ν x B term appears to be the trigger, the physics of the power threshold must involve the physics of the perpendicular rotation. Second, the time sequence of changes in Er and the density and potential fluctuations are consistent with E x B shear causing the change in the fluctuations. Third, the sharp onset of the fluctuation suppression during this same time sequence strongly suggests a critical Er shear for fluctuation suppression. Fourth, Langmuir probe measurements show the need for a turbulence stabilization theory including multiple fluctuating fields and their relative phases. Finally, theories based on νθi predicted by standard neoclassical theory disagree with experiment. (author)