First gyrokinetic simulations on fast ion driven TAEs in MAST/-U and comparison with multiple diagnostics measurements*

Linear gyrokinetic (GK) simulations were conducted for the first time using the Gyrokinetic Toroidal Code (GTC) [1] to investigate Toroidicity-driven Alfvén Eigenmodes (TAEs) driven by neutral beam injection (NBI) induced fast ions in spherical tokamaks (STs). A TAE case observed in MAST discharge #26887 with on-axis NBI power of ~1.5MW and plasma current ~800kA exhibited frequency chirping, with the radial structure obtained from the tangential soft X-ray (SXR) camera array peaking near the |q| ~ 1.5 surface. Linear simulations were performed using various excitation methods in GTC, including antenna excitation of GK thermal ions and the use of analytic Maxwellian GK fast ion distributions. The simulations with antenna excitation closely matched the measured TAE characteristics such as the spatial structure and frequency before chirping, and were also consistent with results from ideal MHD simulations MISHKA and NOVA. Simulations with an analytic GK fast ion Maxwellian distribution exhibited similar radial mode structures, but with frequencies approximately 15 to 20 kHz lower than the measurements and ideal MHD simulations. Current efforts are focused on simulating TAEs with analytic GK fast ion slowing down distributions, which provide a more realistic representation of the behaviors of fast ions excited by neutral beam injection.

 

This research project represents a significant advancement in improving the predictive capabilities of beam-driven instabilities in STs. Once validated against experimental measurements, the GTC simulations can be extended to investigate other instabilities that cannot be effectively simulated using ideal MHD codes. Furthermore, the simulation results obtained from GTC can be used as inputs for reduced energetic particle transport models, such as the kick model [2], in the future to estimate the fast ion losses associated with these instabilities.

  

[1] Z. Lin, T. S. Hahm, W. W. Lee, W. M. Tang, and R. B. White. Science 281, 1835(1998)

[2] M. Podestà et al., PPCF 56 055003 (2014)