Destabilization of Internal Kink Mode by Energetic Electrons on the HL-2A Tokamak

Strong burst of the internal kink mode has been observed during ECRH on the HL-2A. It has been experimentally identified that the energetic electrons, which deviate from Maxwell velocity distribution, excite the mode, which was so called electron fishbone (e-fishbone). The energy distribution of the electrons is indirectly measured by a hard X-ray detector (CdTe) with the pulse height analysis (PHA).When the counts of the energetic electrons with 35-70keV increase to a higher level, the mode can be observed obviously. The e-fishbone can be excited during off –axis ECRH deposited both the high field side and low field side. The modes propagate toroidally parallel to the precession velocity of deeply trapped ions which is in the same direction as the plasma current (co-current) and poloidally parallel to the electron diamagnetic drift velocity. In order to further identify with e-fishbone mode, the resonance condition of wave-particle has been investigated. Comparing with experimental results, the calculation analyses show that the mode frequency is close to the precession frequency of the barely trapped electrons (BTEs) or the barely circulating electrons (BCEs) when the magnetic shear is very weak or negative.