Fishbone instability excited by the supra-thermal circulating electrons in tokamaks is investigated. It is found for the first time that the precession of all the circulating electrons is in the ion diamagnetic drift direction if magnetic shear is neglected. The circulating electrons, which experience both the high field side and the low field side, play a bigger role on the modes than the barely trapped electrons. The analyses show that the mode frequency is close to the precession frequency of circulating electrons comparable with experiment observations. The correlation of the theory with experiments is discussed.
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