Beam halo from Touschek scattering in the KEK Accelerator Test Facility

Abstract: Beam halo is one of the most crucial issues limiting the machine performance and causing radioactivation in high-intensity accelerators. From the diagnostics point of view, the beam halo is of low density and challenging to observe. For high-luminosity colliders, modern light sources and spallation neutron sources, a clear picture of beam-halo formation is of great importance for successfully suppressing the undesired beam loss by employing a dedicated collimation system. The Accelerator Test Facility~(ATF) of KEK has been constructed to study the feasibility of producing the low-emittance beam required at a linear lepton collider and nanobeam focusing \& control techniques. On the other hand, it supplies an excellent opportunity for the experimental investigation of beam-halo formation in a GeV storage ring. This article presents numerical and experimental studies of transverse and longitudinal halos in the KEK Accelerator Test Facility. The general consistency between predictions and observations in various conditions indicates that the Touschek scattering is the dominant mechanism forming the horizontal and momentum halos.