Conservation of canonical angular momentum is shown to require that all trapped particles drift towards the magnetic axis with velocity cE_φB_θ (E_φ is the toroidal electric field; B_θ the poloidal magnetic field). This property, plus an amplification process for the number of trapped particles, will explain the relaxation oscillations which occur for $q<3$. In addition, there is experimental evidence that it is an important contribution to the good containment when $q>3$.
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A. A. Ware (1970) studied this question.