Mechanisms of stochastic diffusion of energetic ions in spherical tori

Stochastic diffusion of the energetic ions in spherical tori is considered. The following issues are addressed: (i) Goldston–White–Boozer diffusion in a rippled field; (ii) cyclotron-resonance-induced diffusion caused by the ripple; (iii) effects of nonconservation of the magnetic moment in an axisymmetric field. It is found that the stochastic diffusion in spherical tori with a weak magnetic field has a number of peculiarities in comparison with conventional tokamaks; in particular, it is characterized by an increased role of mechanisms associated with nonconservation of the particle magnetic moment. It is concluded that in current experiments on National Spherical Torus eXperiment [M. Ono et al., Nucl. Fusion 40, 557 (2000)] the stochastic diffusion does not have a considerable influence on the confinement of energetic ions.