Effects of Resonances on Halo Formation in High Intensity Rings*

Numerical calculations for the Spallation Neutron Source accumulator ring indicate that lattice resonances excited by the space-charge potential can increase a mismatch significantly by deforming the beam distribution in phase space. Hence increased mismatch leads to enhanced envelope oscillations that are driving the 2:1 parametric resonance leading to halo formation, even for initially matched beams. We have observed this behaviour for the 2νx−2νy=0 resonance and for the 4νy=23 resonance [1]. This mechanism for halo formation peculiar to rings through resonance driven mismatch is very sensitive to the tunes, which emphasises the importance of a careful choice of operating point in tune space.