Trapped particle induced fishbone mode in spherical tokamaks

The stability of the fishbone mode in the presence of trapped energetic ions in plasmas of spherical tokamaks is investigated. It has been shown that, when plasma b is sufficiently high to result in a magnetic valley in the equilibrium magnetic field, the fishbone mode is stable due to the inversion of direction of the toroidal precession. This result is valid for both high frequency and low frequency fishbone branches. It is based on general qualitative analysis and calculations involving a suggested analytical expression for the equilibrium magnetic field and calculated bounce and precessional frequencies of the energetic ions. Stability conditions are obtained which agree with experimental observations.