POSSIBLE LONG-TERM IMPROVEMENTS TO THE ADVANCED PHOTON SOURCE ∗

Although third-generation light sources like the Ad-vanced Photon Source (APS) already deliver very high per-formance, there are many user experiments that could ben-efit from further improvements. Improved brightness, im-proved beam stability, higher single-bunch current, more beamlines, and shorter bunches are all of interest to certain user groups. In this paper, we outline some possibilities to meet some of these needs. Presently, the APS has an effective horizontal emit-tance of 3.0 nm-rad, achieved by allowing dispersion in the straight sections. Top-up allows us to go lower in spite of the poor lifetime, which we could do by adding pole-face windings to the dipoles to reduce the emittance to 1.8 nm-rad. A more dramatic effort would involve shortening the dipoles, which would have the added benefit of increasing the straight-section length from 5 m to 9 m. Beyond this, several much more speculative ideas have been explored. One of these is to use the APS ring as part of an energy recovery linac facility, with single- or multi-turn circulation of a high-brightness linac beam in the existing ring. We present some preliminary simulations of the per-formance of this option. Another possibility is replacement of the APS storage ring with a next-generation ring capa-ble of ultra-low emittance and making full use of top-up. We present a concept for a facility that promises 75 pm-rad horizontal emittance and 10m-long straight sections