Advanced Photon Source - Upgrades and Improvements ∗

Here a typical fill pattern of the APS storage ring is shown with a fill to 100 mA every 12 hours. In this case the lifetime of the fill is 33.2 hours before the particle beam decays to approximately 50 mA. With every fill the accelerator components, in particular the vacuum chambers and the beamline user equipment including the target samples, undergo a thermal cycle. In contrast to this operational mode, top-up provides a thermally more stable environment that benefits the users of the APS. In this mode the APS storage ring is filled every 2 minutes to 102 mA, which keeps the beam current at that level with an accuracy of ± 0.1 mA creating a virtually infinite lifetime. Figure 2 shows the current history when operating the APS in top-up mode. Fig. 2 APS storage ring beam current history in top-up mode A characteristic for top-up operation is that during the fill the beamline safety shutters remain open. Before top-up could be implemented at the APS the Accelerator Physics Group performed many particle-tracking studies [1] for several failure modes to show that no particle beam can be extracted via a beamline. Certain criteria have to be met for this to be true. One of these criteria assumes the positional stability of the storage ring magnets, vacuum chambers, and beamline safety shutters. For that reason we are required to measure the position of the storage ring components in two-year time intervals. On the