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In a magnetic field, transitions between classes of guiding-centre motion canlead to cross-field diffusion and escape. We say a magnetic field is isodrasticif guiding centres make no transitions between classes of motion. Therefore,this is an important ideal for enhancing confinement. First, we present a weakformulation, based on the longitudinal adiabatic invariant, generalisingomnigenity. To demonstrate that isodrasticity is strictly more general thanomnigenity, we construct weakly isodrastic mirror fields that are notomnigenous. Then we present a strong formulation that is exact forguiding-centre motion. We develop a first-order treatment of the strong versionvia a Melnikov function and show that it recovers the weak version. The theoryprovides quantification of deviations from isodrasticity that can be used asobjective functions in optimal design. The theory is illustrated on some simpleexamples.

Isodrastic Magnetic fields for suppressing transitions in guiding-centre motion