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Effects of q-profile structure on intrinsic torque reversals

Author: Zhixin Lu
Requested Type: Poster Only
Submitted: 2015-01-19 17:46:51

Co-authors: W.X.Wang, P.H.Diamond, G. Tynan, S. Ethier, C. Gao, J. Rice

Contact Info:
University of California at San Diego
9500 Gilman Drive M/C 0417
San Diego, California   92093
U.S.

Abstract Text:
Changes in rotation have been observed in LHCD experiments. From these observations, reversals in intrinsic torque have been inferred. This work identifies a new mechanism for intrinsic torque reversal linked to magnetic shear. Gyrokinetic simulations demonstrate that as compared to the normal magnetic shear case, the intrinsic torque reverses, when the magnetic shear is lower than a critical value. Analysis shows that the reversal occurs due to the dominance of a new symmetry breaking mechanism in the intensity-gradient induced residual stress. This mechanism is a consequence of ballooning structure at weak magnetic shear, related to the synergy of toroidal coupling and intensity gradient. Gyrokinetic simulation shows that for collisionless trapped electron modes (CTEM) and ion temperature gradient (ITG) modes, this critical magnetic shear is 0.3 and 1.3 respectively. The value of the critical magnetic shear is consistent with results from the Alcator C-Mod LHCD experiments, for which the magnetic shear is positive in the whole plasma column and the critical magnetic shear for torque reversal is 0.2~0.3. [J. Rice et al, Phys. Rev. Lett. 111, 125003].

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