Observations and simulation of a fourth order resonance with space charge

A benchmarking experiment with a high intensity bunched beam stored for 1.2 s in a nonlinear lattice has been performed at the CERN Proton Synchrotron (PS). Beam emittance and beam intensity have been measured for several working points at different distances from a lattice-induced resonance. We found a regime of emittance growth for machine tunes far from the resonance and a regime of emittance shrink/beam loss for working point close the resonance. We compare the observations with 3D simulations and in the blow-up regime we find good agreement. We interpret these results in terms of space charge induced trapping and de-trapping on the lattice resonance. We show that this mechanism is responsible of halo formation which depends on the distance from the resonance and discuss the interplay of dynamic aperture and halo size as main mechanism to explain beam loss in the experiment.