The plasma dynamics of enhanced confinement regimes in the TFTR core and the DIII-D core
and edge are compared in order to identify a common physics basis. Despite differences in transition
timescale and location, as well as the sign of the radial electric field Er, observations suggest that E × B shear effects on turbulence induced transport play a dominant role in governing barrier
dynamics in all cases. Fast confinement bifurcations are observed in the TFTR core enhanced reverse shear (ERS)
regime and in the edge DIII-D H mode. Both show spontaneous Er shear layer formation prior to the
confinement change and a negative Er well that persists as steep gradients form. These dynamics differ
from those of DIII-D negative central shear (NCS) plasmas. There, slow transitions are observed when
the applied torque from unidirectional beam injection is small, while faster development and more
dramatic confinement improvements occur at higher applied torques. Unlike the H mode and ERS
cases, the NCS core generally has a positive Er hill and no strong Er shear precursor. However,
similarity experiments performed on TFTR indicate that ERS, L mode and NCS-like regimes
can all be accessed in a continuous fashion by varying the E × B shear through changes in the
applied torque at constant power. As in the DIII-D NCS case, core confinement in TFTR reverse shear plasmas
improves slowly as co-rotation begins to dominate the determination of Er, no strong Er shear layer
develops prior to that improvement, and the plasma possesses a positive Er hill. Reductions in
transport with Er gradients of either sign are consistent with the picture of E × B shear
suppression and decorrelation of turbulence. At fixed input power, intermediate levels of confinement improvement
are achieved by varying the E × B shear with changes in the applied neutral beam torque. The
data suggest that control over the plasma pressure profile in a reactor may be possible if an external source of E × B shear, such as might be applied with RF techniques, is used to modify the shear
which otherwise occurs.