Author: David R Hatch
Requested Type: Pre-Selected Invited
Submitted: 2017-03-17 17:49:46
Co-authors: R. Hazeltine, S. M. Mahajan, M. Kotschenreuther
Institute for Fusion Studies, University of Texas
2515 Speedway C1500
Austin, TX 78712-1
The suppression of turbulence by sheared flows is thought to be the main mechanism underlying the formation and sustenance of the H-mode pedestal.
A combined analytic and computational gyrokinetic approach is used to address the question of the scaling of pedestal transport with ExB shear in regimes spanning the weak and strong shear limits. Due to strong gradients and shaping in the pedestal, the instabilities of interest are not curvature-driven. Rather, the growth rates exhibit very weak variation with ballooning angle resulting in modes that are insensitive to the toroidal effects upon which many theories of shear suppression are based. Consequently, despite the major computational challenges inherent in pedestal gyrokinetic simulations, the earliest decorrelation theories [1-3], which are completely unaware of geometry and the details of instability drive, are found to be most relevant. We present detailed comparisons between gyrokinetic simulations and a theory closely related to , which is valid for arbitrary shear rates and accounts for the self-consistent variation of fluctuation amplitude with nonlinear diffusivity. The gyrokinetic simulations entail scans of ExB shear rate for a range of scenarios starting with local simulations with constant shear rate and culminating with global simulations using non-monotonic flow profiles. With the flexibility of only a single order-unity free parameter, the analytic theory reproduces the scaling of the gyrokinetic simulations—often quantitatively—across the strong to weak shear limits in all cases studied. In light of the expected decrease of ExB shear rates over the transition to low rho* burning plasma regimes, the implications of this work are substantial.
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