Effects of resonances on halo formation in high-intensity storage rings

Numerical calculations for the Spallation Neutron Source accumulator ring i ndicate that lattice resonances excited by the space-charge potential can i ncrease a mismatch significantly by deforming the beam distribution in phas e space. Hence increased mismatch leads to enhanced envelops oscillations t hat are driving the 2:1 parametric resonance leading to halo formation, eve n for initially matched beams. We have observed this behavior for the 2 v(x ) - 2 v(y) = 0 resonance and for the 4 v(y) = 23 resonance. This mechanism for halo formation peculiar to rings through resonance driven mismatch is v ery sensitive to the tunes, which emphasizes the importance of a careful ch oice of operating point in tune space. [S1063-651X(99)10012-6].