Small-Action Particles in a Tokamak in the Presence of an $\mathit{n}=1\mathrm{}$ Mode

It is found that an $m=n=1\mathrm{}$ mode with amplitude exceeding a certain threshold can lead to stochastic motion of energetic ions in tokamaks, the large orbit width particles (potatoes) being most easily affected. An $n=1\mathrm{}$ mode can redistribute particles also in the absence of stochasticity but only when the perturbation is quickly switched on/off, e.g., during a sawtooth crash. In the latter case, the perturbation results in a regular motion of particles around a certain helical orbit, at which a resonance driven by the mode but having no amplitude threshold takes place.