Abstract Details
Abstracts
Author: Elena V Belova
Requested Type: Poster
Submitted: 2025-03-10 14:26:52
Co-authors: E.D. Fredrickson, C. Liu, N.A. Crocker
Contact Info:
PPPL
100 Stellarator Road
Princeton, NJ 08540
United States
Abstract Text:
Self-consistent simulations of beam-driven AEs in the TAE frequency range have been performed for National Spherical Torus experiment (NSTX-U) using full-orbit kinetic beam ion option of the HYM code. Simulations have been carried out using experimental plasma parameters and profiles for the NSTX-U discharge #204707, where the beam ion density and beta were relatively large, resulting in the strong instability drive. Experimentally, the AEs were observed for low toroidal mode numbers n=1-7. Numerical simulations for n= 3 - 4 show instabilities with mode frequencies in the TAE gap, and a mixed TAE / reversed shear Alfvén (RSA) characteristics. Nonlinear simulations show saturation of the instability, the nonlinear frequency chirping, and the appearance of the lower frequency mode later in the simulation. Comparison of the normal plasma velocity perturbation with the experimentally obtained displacement profile shows a good agreement in mode location and structure. However, the simulations also show a very large compressional component of magnetic field perturbation, comparable in amplitude to the shear component. The compressional perturbation is localized close to the peak of the beam ion density gradient, reducing sharply near the edge and making the mode polarization at the edge the shear Alfvén-like. Additional simulations have been performed by reducing either the thermal plasma pressure, or the beam density by half. In both cases the n=3 mode was unstable with frequency close to the TAE frequency, but the amplitude of the compressional magnetic field component was noticeably reduced compared to the original case. This indicates a strong effect of plasma beta on mode polarization. Simulations using the M3D-C1-K code for the NSTX case also show that AEs in the TAE frequency gap have a very large compressional magnetic field perturbation.
Characterization: 1.0
Comments: