Top-up operation experience at 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, in September 1998 we began commissioning a new operating mode called "top-up." In this mode, the beam current does not decay but is maintained at a high level using frequent injection, while photon shutters are open and photon beams are delivered to users. The hardware, software, and safety requirements for top-up are reported. Safety issues related to injection with open photon shutters are covered in companion papers in this conference. Present operational experience includes testing aspects of top-up injection and delivering beam to X-ray users for a few hours with fractional current stability of 10/sup -3/. We expect to run several top-up operation shifts in Spring 1999. Issues of importance are orbit and emittance transients during the injection and scheduling of injection pulses for the convenience of users.