Tracking studies of top-up safety for the Advanced Photon Source

The Advanced Photon Source (APS) is a 7-GeV, third-generation synchrotron radiation source. To provide more stable beam for users, we are pursuing a new operating mode called "top-up". In this mode, the beam current is not allowed to decay as it normally would, but instead is maintained at a high level through frequent injection. A safety question with top-up mode is, during injection with photon shutters open can injected beam ever exit a photon beamline? This might happen, for example, due to full or partial shorting of a dipole coil. Extensive, detailed tracking studies were performed to assess the possibility of such an accident given the planned safety measures. We discuss the safety philosophy, the scenarios simulated, and the advanced computational techniques employed. A companion paper discusses analytical estimates of top-up safety.