Abstract Details
Abstracts
Author: Elena V Belova
Requested Type: Consider for Invited
Submitted: 2024-04-04 16:13:34
Co-authors: E. D. Fredrickson, N. A. Crocker
Contact Info:
PPPL
100 Stellarator Road
Princeton, NJ 08543
United States
Abstract Text:
Alfvén eigenmodes (AEs) in the sub-cyclotron frequency range have been frequently observed during neutral beam injection (NBI) in the National Spherical Torus Experiment (NSTX/NSTX-U) and other fusion devices such as MAST, DIII-D, JT-60U and ASDEX Upgrade tokamak. In NSTX, correlation between strong activity of sub-cyclotron frequency AEs at high beam power and flattening of the electron temperature profile have been reported, therefore these instabilities can have important implications for future fusion devices. A set of nonlinear simulations has been performed to study the nonlinear evolution of unstable global Alfvén eigenmodes (GAEs) in the NSTX-U. Results of the single toroidal mode number simulations are compared with a full nonlinear simulation (all toroidal harmonics included). Nonlinear simulations show that unstable GAEs can very efficiently redistribute the beam ions, reducing the anisotropy in the pitch distribution in the resonant region of the phase space, and accelerate the fast ions to energies above the injection energy. It is shown that these effects are especially strong for super-Alfvénic injection velocities and more perpendicular injections. Changes of the beam ion distribution correspond to a wider spread in the pitch parameter distribution, and reduction of the instability drive. The excited GAEs can very efficiently transfer energy from the perpendicular to the parallel component (for the particles driving the instability) or vice-versa (for the particles near the injection energy, which are taking energy from the mode). A smaller fraction of the energy goes into the excitation of the mode itself. Effects of GAEs on the beam ion radial redistribution are studied and compared with low-frequency instabilities. It is shown that strong GAE instability can lead to an increased NBI losses, caused mostly by the adiabatic changes in the perpendicular energy of the particles close to the prompt loss boundary.
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