Studies of loss mechanisms associated with the half integer limit on the Isis ring

Copyright © 2014 CC-BY-3.0 and by the respective authors. ISIS is the spallation neutron source at the Rutherford Appleton Laboratory in the UK. The facility centres on an 800 MeV rapid cycling proton synchrotron, which provides 0.2 MW of beam power operating at high levels of transverse space charge (peak incoherent tune shift ~ 0.5), but with low loss. Half integer resonance is considered to be a main driver for loss that limits the intensity in high power, medium energy proton rings like ISIS. However, the detailed mechanisms causing loss as the half integer limit is approached are not well understood, particularly in the context of a real machine. In this paper we report progress on experiments on the ISIS synchrotron inducing half integer loss, comparing with detailed simulations, and attempts to relate these to simplified theoretical and simulation models. Studies here concentrate on 2D coasting beams, with a view to extending later to the more complicated 3D, bunched beam case of an operational machine.