Beam halo from Touschek scattering in the KEK Accelerator Test Facility

Beam halo is one of the most crucial issues limiting the machine performance and causing radioactivation in high-intensity accelerators. Understanding halo formation is vital for high-luminosity colliders, advanced light sources, and spallation neutron sources to effectively mitigate beam loss through machine optimizations and tailored collimation systems. This article details numerical and experimental investigations of transverse and longitudinal halos at the KEK Accelerator Test Facility. Halo distributions are modeled incorporating operational beam parameters, synchrotron radiation, beam-gas scattering, and Touschek-scattering processes. A newly developed yttrium aluminum garnet/optical transition radiation monitor, positioned at the extraction line, enables direct imaging of transverse and longitudinal halo distributions. 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.