Abstract Details
Abstracts
Author: Jacobo Varela
Requested Type: Consider for Invited
Submitted: 2023-03-17 03:21:25
Co-authors: D. Spong, L. Garcia, Y. Ghai, D. Zarzoso, H. Betar, J. Ortiz, D. C. Pace, M. A. Van Zeeland and X. Du
Contact Info:
University Carlos III of Madrid
Av. Universidad
Leganes, Madrid 28911
Spain
Abstract Text:
NBI driven Alfvén instabilities can significantly enhance fast ion transport above classical rates, leading to degraded heating efficiency and wall damage. These instabilities also lead to complex nonlinear and self-organized phenomena; the goal of this study is to analyze such effects for a range of NBI operational regimes in NBI-heated DIII-D plasmas. The analysis is done using the linear and nonlinear versions of the gyro-fluid code FAR3d [1]. A set of parametric studies is performed modifying the nonlinear simulation EP β (linked to the NBI injection power), EP energy (associated with the NBI voltage) and the radial location of the EP density profile gradient (connected to the NBI radial deposition). The analysis indicates a transition from the soft to the hard MHD regime if the simulation EP β ≥ 0.02, leading to global plasma relaxation as bursting MHD activity caused by n=3 and n=6 AEs overlapping as well as a large decrease of the EP density in the inner plasma linked to an enhancement of the EP transport. In addition, shear flows and zonal current are generated that modify the flux surfaces and q profile, respectively. The evolution of the equilibrium profile in the hard MHD regime causes a decrease of the AE frequency, an effect that may explain the frequency down-shift observed for some AEs along DIII-D discharges [2]. Reducing the EP energy in the nonlinear simulations leads to a weakening of the plasma perturbation although an enhancement if the EP energy increases, consistent with DIII-D experimental observations [3]. Nonlinear simulations for off-axis NBI configurations indicate a lower plasma perturbation as the EP density gradient is located further away from the magnetic axis. In addition, the zonal currents induced by the AEs in the hard MHD regime for off-axis NBI configuration reinforce the reverse shear region, although on-axis configurations tend to reduce the deep of the reverse shear region.
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