Resonant normal form and asymptotic normal form behaviour in magnetic bottle Hamiltonians

Abstract

We consider normal forms in ‘magnetic bottle’ type Hamiltonians of the form (second frequency ω2 equal to zero in the lowest order). Our main results are: (i) a novel method to construct the normal form in cases of resonance, and (ii) a study of the asymptotic behaviour of both the non-resonant and the resonant series. We find that, if we truncate the normal form series at order r, the series remainder in both constructions decreases with increasing r down to a minimum, and then it increases with r. The computed minimum remainder turns to be exponentially small in , where ΔE is the mirror oscillation energy, while the optimal order scales as an inverse power of ΔE. We estimate numerically the exponents associated with the optimal order and the remainder's exponential asymptotic behaviour. In the resonant case, our novel method allows to compute a ‘quasi-integral’ (i.e. truncated formal integral) valid both for each particular resonance as well as away from all resonances. We applied these results to a specific magnetic bottle Hamiltonian. The non-resonant normal form yields theoretical invariant curves on a surface of section which fit well the empirical curves away from resonances. On the other hand the resonant normal form fits very well both the invariant curves inside the islands of a particular resonance as well as the non-resonant invariant curves. Finally, we discuss how normal forms allow to compute a critical threshold for the onset of global chaos in the magnetic bottle.