Nonlinear simulations of TAEs in NSTX-U

Self-consistent simulations of beam-driven AEs in the TAE frequency range have been performed for the National Spherical Torus experiment (NSTX-U) discharge #204707 using the full-orbit kinetic beam ion option of the HYM code. Numerical simulations for n = 3 - 4 show instabilities with frequencies in the TAE gap, with TAE and reversed shear Alfvén (RSA) characteristics, 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. The simulations also show that the compressional component of the perturbed magnetic field is 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. In both cases the n=3 mode was unstable with a frequency close to the TAE frequency, but the amplitude of the compressional magnetic field component was noticeably reduced compared to the original case. HYM simulations of TAEs are compared with the M3D-C1-K simulations for NSTX and NSTX-U