Linear Gyrokinetic and Reduced Transport Simulations of TAE in MAST*

For the first time, linear gyrokinetic (GK) simulations using the Gyrokinetic Toroidal Code (GTC) [1] have been performed to investigate Toroidicity-driven Alfvén Eigenmodes (TAEs) driven by neutral beam injection (NBI) induced fast ions in the Mega-Amp Spherical Tokamak (MAST). A specific TAE case in MAST discharge #26887, with on-axis NBI power of ~1.5 MW and plasma current ~800 kA, exhibited frequency chirping, with the radial structure from the tangential soft X-ray (SXR) camera array peaking near the |q| ~ 1.5 surface, close to the expected location of the n = m = 1 TAE. Various excitation methods were used in GTC linear simulations, including antenna excitation of GK thermal ions and the use of analytic fast ion Maxwellian and slowing-down distributions. The radial structures from these GK simulations closely match measurements and calculations by the NOVA ideal MHD code. The mode frequencies from these GK simulations match the measured frequency of the chirping mode at peak amplitude but are approximately 10 kHz (11.3%) lower than the ideal MHD and measured initial mode frequencies, likely due to various kinetic and non-perturbative effects. The simulations measured the damping and growth rates due to various mechanisms, such as continuum damping, radiative damping, ion Landau damping, and fast ion pressure drive, demonstrating GTC's capability to realistically examine the mechanisms and behaviors of fast ion-driven TAE in spherical tokamaks. Reduced fast ion transport model (ORBIT-Kick [2]) calculations were also conducted using results from GTC, NOVA, and experiments, showcasing the use of GK simulation results for fast ion transport calculations that cannot be predicted by linear ideal MHD codes, such as mode-associated parallel electric field perturbations (𝜹E_||), saturated mode amplitudes, and frequency chirping, thus enhancing the predictive capabilities of fast ion transport associated with beam-driven instabilities in spherical tokamaks.

 

[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)