Destabilization of beta-induced Alfvén eigenmodes by energetic trapped electrons in tokamak

The stability of high torodial mode number (n) beta-induced Alfvén eigenmodes excited by precessional resonance of magnetically trapped energetic electrons in low magnetic-shear tokamak plasma is investigated. The dispersion relation of the energetic-electron beta-induced Alfvén eigenmodes (e-BAE) is derived and solved analytically and numerically. It is found that the contribution of the trapped energetic electrons to the e-BAE dispersion relation is limited to the ideal MHD structure of the BAE. Numerical results show that, for typical equilibrium parameters, the e-BAE can be destabilized by the energetic electrons, and the mode frequency is consistent with the experimental observation. Varying the background plasma parameters can lead to transitions between e-BAEs and energetic particle modes. Moreover, the dependence of the e-BAE frequencies and growth rates on energetic electron parameters shows that the growth rates monotonically increase(decrease) with the energetic electron density(the normalized energetic electron density scale length), and the frequencies are not much affected. The frequency and growth rate are sensitive to the energetic electron temperature, and there exists a maximum growth rate.