Nonlinear simulations of GAEs for NSTX and NSTX-U

Nonlinear simulations of subcyclotron-frequency global Alfven eigenmodes (GAEs) has been performed for National Spherical Torus Experiment (NSTX/NSTX-U) conditions to study dependence of saturation amplitude on beam parameters and compare with experimental scaling. GAEs are frequently excited during neutral beam injection (NBI) in the NSTX(-U), as well as other beam-heated devices such as MAST and DIII-D. These modes are driven unstable through the Doppler shifted cyclotron resonance with the NBI ions and can be excited in ITER due to super-Alfvenic velocities and strong anisotropy of the beam ions. Numerical simulations using the HYM code have been performed to study the excitation and the saturation of GAEs for a single toroidal mode number, and also investigating the nonlinear evolution in the presence of multiple unstable modes. The scaling of the saturation amplitude with the linear growth rate and the enhancement of the saturation level due to resonance overlapping in the presence of multiple unstable modes are shown. The nonlinear simulations show the evolution of the beam ion distribution function towards reduction of anisotropy; the effect is stronger for larger normalized NBI injection velocity.