Toroidal Alfvén eigenmode driven by energetic electrons during high-power auxiliary heating on HL-2A

Abstract

Toroidal Alfvén eigenmodes (TAEs) driven by energetic electrons have been observed first during high-power electron cyclotron resonant heating and neutral beam injection on HL-2A. The mode frequencies are in a wide range of 170–370 kHz and are proportional to the Alfvén velocity. The toroidal mode number is confirmed as n = 4. According to the calculated results from the Alfvén mode code, there exists a core localized TAE mode with poloidal mode numbers m = 4 and m = 5 at ρ∼0.35, and the calculated frequency is close to the experimental observation. Besides, the radial mode structures are obtained. Therefore, the modes are identified as TAEs. The TAEs propagate in the electron diamagnetic drift direction poloidally. The energy distribution of energetic electrons is obtained by the relativistic shift-down of the 3-rd harmonic frequency effect of the rough non-thermal spectrum from electron cyclotron emission. Therefore, TAEs should be driven by the energetic electrons with energy in the range of 150–230 keV.