Nonlinear dynamics and effects of fast-ion driven instabilities in HL-2A NBI heating high-βN H-mode plasmas

High-performance plasmas had been obtained with hybrid scenarios on HL-2A in past several years, and the quality factor $G=H_{89}{\beta }_N/q_{95}^2$ is around 0.3-0.4. In these high-${\beta }_N$ H-mode plasmas with ${\beta }_N>2.5$ and $H_{98}>1.2$ Chen et al. (2022) [24], there are multiple energetic-ion driven instabilities, such as toroidal Alfvén eigenmode (TAE) and beta-induced Alfvén eigenmode (BAE), and there are complex magnetohydrodynamic (MHD) dynamics, e.g. nonlinear mode-mode and mode-particle interactions. The TAE amplitude increases explosively, and the frequency is fast chirping in the range of $f=90-150$ kHz. The TAE toroidal mode-number is $n=3-4$, and they localize in outer plasma region. The BAE amplitude also increases explosively, and the BAE frequency is fast chirping in the range of $f=50-90$ kHz. The toroidal mode-number of dominant BAEs is $n=2$, and these modes localize at q=2 rational surface where there is a large radius internal transport barrier (ITB). Generally, the energetic-ion fraction is low in outer plasma region. It indicates that thermal ions at large radius ITB location possibly provide additional drive and destabilize the BAE in company with energetic-ions. These TAEs/BAEs occur regularly in high-${\beta }_N$ region and before large type-I ELM burst. To further illuminate the characteristics of these Alfvénic modes, the 2D and radial mode structures had been calculated by using a well-benchmarked general Alfvénic mode eigenvalue code (MAS). The experimental results suggest that these Alfvénic modes may have important effects on the pedestal transport and ELM-onset.