Distinguishing electron driven from fast-ion driven Alfvénic instabilities

Modes with frequencies below the toroidal Alfvén eigenmode (AE) are often unstable in existing toroidal plasmas and cause appreciable fast-ion transport, so understanding their stability is vital to predict their impact on alphas in future devices. Through experiment and theory, we now understand that fast ions play an important role in driving beta-induced AEs (BAE), while another mode, the recently identified low-frequency Alfvén mode (LFAM), relies on electron temperature T_e. Reversed shear DIII-D plasmas with simultaneous neutral beam injection (NBI) and electron cyclotron heating (ECH) have unstable BAEs and LFAMs. The eigenfunctions of both modes peak near q_min but BAEs typically have higher frequencies and lower mode numbers [1,2]. The BAEs rapidly disappear when the NBI is notched off, then rapidly reappear when it resumes, demonstrating that BAEs are driven by resonances with high-energy fast ions [1]. In contrast, as long as T_e is sustained, LFAMs persist throughout the beam notch and are even observed in high T_e discharges without any NBI [2]. A large database shows that BAEs are more unstable in plasmas with beta poloidal β_p>0.5 but LFAMs are unstable most often when T_e>2.1 keV but β_p is small. Theoretical analysis that identifies the LFAM as a reactive-type kinetic ballooning mode instability with dominant Alfvénic polarization successfully predicts all of the experimental properties [3], including stronger instability in hydrogen plasmas [4]. Gyrokinetic simulations [5] reproduce most but not all of the BAE and LFAM properties. Calculations of ITER stability are underway.

[1] W.W. Heidbrink et al., NF 61 (2021) 066031.

[2] W.W. Heidbrink et al., NF 61 (2021) 016029.

[3] R. Ma et al., PPCF 64 (2022) 035019.

[4] W.W. Heidbrink et al., NF 61 (2021) 106021.

[5] G.J. Choi et al., NF 61 (2021) 066007.