Effect of q-profile structure on intrinsic torque reversals

Intrinsic toroidal rotation plays an important role in mitigating macroinstability and regulating turbulent transport in ITER, where neutral beams are not sufficient to provide the requisite torque. Recent experiments on C-Mod with LHCD observed rotation reversal related to a change in the q profile [1]. In this work, we focus on understanding the physics of intrinsic rotation reversals in LHCD plasmas, using nonlinear, global gyro-kinetic simulations [2] and analysis of mode structure and spectrum symmetry breaking [3]. The sensitive dependence of turbulent residual stress on magnetic shear is identified and characterized. The basic residual stress is non-vanishing when the k-parallel spectrum symmetry is broken, e.g., by E x B shear induced radial shift, non-uniformity in turbulence intensity, etc. [3]. It is found that at low magnetic shear, the poloidal harmonics can shift strongly in the radial direction, as a feature of non-local effects, due to radial propagation and amplitude variation of the mode. This new symmetry breaking mechanism leads to a change in the sign of spectrum averaged parallel wave vector and thus the direction of intrinsic torque. Theoretical study [4] shows that the competition between magnetic drift and ion kinetic effects determines the non-local effects and the structure of the asymmetry. Specifically, it is found that the direction of the intrinsic torque changes from counter- to co-current in the core, when magnetic shear decreases through a critical value. A critical shear $\hat s_R=0.2\sim0.5$ for reversal of CTEM-induced intrinsic torque found by simulation is consistent with that from the LHCD C-Mod reversal experiments. In addition, simulations indicate $\hat s_R=1\sim2$ for the reversal of ITG-induced torque, a prediction which can be tested by experiments.\\[4pt] [1] Rice, J.E. et al. 2011 Phys. Rev. Lett. 107, 265002\\[0pt] [2] Wang W.X. et al 2006 Phys. Plasmas 13, 092505\\[0pt] [3] Diamond, P.H. et al. 2013 Nucl. Fusion 53 104019\\[0pt] [4] Lu, Z.X. et al. 2012 Phys. Plasmas 19, 042104