Chaotic particle motion and halo formation induced by charge nonuniformities in an intense ion beam propagating through a periodic quadrupole focusing field

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Abstract
A test-particle model is used to investigate the charged-particle dynamics in an intense matched ion beam with nonuniform density profile propagating through an alternating-gradient quadrupole focusing field in the space-charge-dominated regime. It is shown that self-field nonlinearities due to the transverse nonuniformity in the beam density profile not only can result in chaotic ion motion but also can cause halo formation by the ejection of particles from the beam core. The structure of the particle phase space is studied. It is shown that the process of halo formation can occur on a fast time scale—on the order of a few lattice periods. The halo size is found to be determined by a Kolmogorov–Arnold–Moser (KAM) surface on a time scale much shorter than that of Arnold diffusion.

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