Observation and simulation of energetic electron driven beta-induced Alfvén eigenmode on J-TEXT

Abstract Energetic electron-driven beta-induced Alfvén eigenmodes (e-BAEs) around 20 kHz are observed in J-TEXT electron cyclotron resonance heating (ECRH) plasmas, and their frequency and intensity are positively correlated with the ECRH power. Experimental measurements indicate that the e-BAE propagates in the electron diamagnetic drift direction, with poloidal and toroidal mode numbers m / n = −4/−1. Statistical analysis and Alfvén continuum simulations imply that the mode frequency is positively correlated with the Alfvén velocity, being consistent with the general fishbone-like dispersion relation, and locates in the BAE gap of the Alfvén continuum. Further calculation evaluation entails that this e-BAE is most likely to be excited by 5–9 keV trapped energetic electrons around q = 4 rational surface, via precession resonance. Numerical simulations using the FAR3d code successfully reproduce the e-BAE, showing good agreement with experimental results in both mode number, frequency and growth rate. Simulation results show that the growth rate of e-BAE increases linearly with the energetic electron beta, as well as the frequency displays a positive correlation with the energetic electron pressure and a negative correlation with the characteristic trapping length of energetic electrons. The simulated eigenfunction of the e-BAE also presents that the mode is localized to a narrow region around the q = 4 rational surface, close to the plasma edge. This paper reports the first application of the FAR3d code to simulate energetic electron-driven instabilities, demonstrating its potential for studying such physical phenomena.