Recent measurements from the DIII-D tokamak indicate that shear in the group velocity of the edge density fluctuations is at least partly responsible for the factor 2 change in the H-mode power threshold that is observed when the direction of the ion ∇B drift relative to the X-point location is reversed. Spatially resolved edge density fluctuation measurements show a change in the poloidal group velocity of the fluctuations when the ∇B drift direction was changed, even though the edge profiles of density and temperature remained nearly the same. High (low) shear in the poloidal velocity is associated with a low (high) power threshold.
Comparison of L-H transition measurements with physics models