Evidence for the role of velocity shear on the L-H transition in DIII-D

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 delB 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 delB 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.