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Author: Emily Belli
Requested Type: Poster
Submitted: 2017-03-17 16:19:52

Co-authors: J. Candy

Contact Info:
General Atomics
3550 General Atomics Court
San Diego, California   92121
United States

Abstract Text:
The gyrokinetic solver CGYRO has been upgraded to include sonic toroidal rotation for comprehensive studies of gyrokinetic stability including heavy impurities, such as from the tokamak wall. Sonic toroidal rotation, which arises in tokamaks from torque due to neutral beam injection, produces a strong centrifugal force that pushes the ions toroidally outward, causing them to redistribute non-uniformly around a flux surface. As a result of quasi-neutrality, a poloidally-varying electrostatic potential is generated by the electrons to balance the density asymmetry. In CGYRO, the poloidal asymmetry in the equilibrium density is computed via solution of the nonlinear, zeroth-order (in rho*) Poisson equation. Full centrifugal drifts and trapping, as well as the Coriolis drift and parallel velocity shear, are implemented in the nonlinear electromagnetic gyrokinetic equations according to the formulation of Sugama [1]. Because of their complexity, these effects, which are second-order in the main ion Mach number, are ignored in most gyrokinetic codes. Subtle modifications to the gyrokinetic Maxwell equations are also included. For simple test cases, we find that, while the ITG mode is primarily affected by the stabilizing Coriolis drift for low ion Mach number (Mi < 1), at large Mi the dominant effect of sonic rotation is on the electron dynamics. Though the centrifugal force is small for electrons due to their small mass, enhancement of the mirror force leads to an increased effective electron trapped fraction. This has a strong destabilizing influence on TEMs and leads to mode transition from ITG to TEM for increasing Mi. Resolving this effect requires high poloidal and pitch-angle resolution. The impact on high-Z impurities and the interaction with detrapping induced by collisions are also explored. This work supported in part by the U.S. DoE under DE-FG02-95ER54309.
[1] H. Sugama, Phys. Plasmas 5, 2560 (1998).

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