Effects of q-profile structures on intrinsic torque reversals

Changes in rotation have been observed in LHCD experiments. From these observations, reversals in intrinsic torque have been inferred. This paper identifies the mechanism for intrinsic torque reversal linked to magnetic shear ((s) over cap). Gyrokinetic simulations demonstrate that as compared to the normal (s) over cap case, the intrinsic torque reverses, for (s) over cap < <(s)over cap>(crit). Analysis shows that the reversal occurs due to the dominance of the symmetry breaking mechanism in residual stress due to the synergy of toroidal coupling and the intensity gradient. This mechanism is a consequence of ballooning structure at weak (s) over cap. Gyrokinetic simulation gives (s) over cap (crit) approximate to 0.3 for trapped electron modes (TEM) and (s) over cap (crit) approximate to 1.1 for ion temperature gradient (ITG) modes. The value of (s) over cap (crit) is consistent with results from the Alcator C-Mod LHCD experiments, for which (s) over cap > 0 in the whole plasma column and (s) over cap (exp)(crit) approximate to 0.2 similar to 0.3 (Rice et al Phys. Rev. Lett. 111 125003).