Unified theory of Mercier-ballooning and Alfvén eigenmodes in positive-shear tokamaks with large-orbit energetic ions

A unified theory of the Mercier-ballooning and the compensating-electron Alfvén eigenmodes (CEAEs) in positive-shear tokamaks with large-orbit energetic ions is developed. It is shown that the cross-field drift effect of electrons compensating the electric charge of energetic ions (the compensating-electron effect) leads to rotation of the Mercier-ballooning modes. If the Mercier stability criterion is satisfied, the ballooning modes rotate in the direction of compensating-electron frequency, while in violation of this stability criterion the unstable modes rotate against this frequency. The compensating-electron effect also results in decreasing the growth rate of ballooning modes, though their instability condition is unchanged. The Mercier and ballooning effects influence both rotation and decay rate of the CEAEs, the ballooning effect being smaller than the Mercier effect. As a result, rotation and damping of CEAEs increases∕decreases in the case of magnetic well∕hill.