Equipartitioning and halo due to anisotropy

Equipartitioning and certain aspects of halo formation in high-current linac beams are explained in terms of collective multipole oscillations in x-y geometry. For strong space charge tune depression and anisotropy (emittance and/or focusing strength) some eigenmodes can-in principle-become unstable leading to emittance exchange. It is shown that for parameters of practical interest in linac design beams can be un-equipartitioned without risk of instability. The effect of (stable) mismatch core oscillations on the halo is briefly discussed.