Dynamics of Transition to Enhanced Confinement in Reversed Magnetic Shear Discharges

A simple model of the transition to enhanced confinement in reversed shear discharges is presented. The proposed transition mechanism relies on a synergism between electric field shear suppression of turbulence aided by reduced curvature drive due to magnetic shear reversal or reduction. Profile structure and transport barrier propagation dynamics are predicted. A novel analytical theory for the time evolution of the barrier foot-point location is presented. The model predicts that the transition threshold has favorable dependence on pretransition temperature ratio $\left(T_i{/T}_e\right)$, in-out asymmetry in the $E\times B$ shearing rate (i.e., lower for larger Shafranov shift), density profile peakedness, and unfavorable scaling with density. Optimal confinement occurs in discharges where deposition is peaked within the magnetic shear reversal radius.