Superbanana orbits and redistribution of marginally trapped fast ions during sawtooth crashes

Abstract

The work represents the first step in studying the effect of sawtooth crashes on the “resonant” fast ions, i.e., the ions that are trapped or marginally untrapped with respect to the helical perturbation associated with the sawtooth crash. The simplest case is considered, when the particles in the absence of perturbations are marginally trapped in the tokamak magnetic field and characterized by narrow orbits. It is shown that in the presence of perturbation the guiding centers of the banana orbits oscillate in the toroidal and radial directions, forming “superbanana” orbits. The radial width of such “superbananas” constitutes a significant part of the sawtooth mixing radius, which implies that sawteeth can strongly redistribute these particles.