Possible long-term improvements to the Advanced Photon Source

Although third-generation light sources like the Advanced Photon Source (APS) already deliver very high performance, there are many user experiments that could benefit from further improvements. Improved brightness, improved 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 emittance 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 nmrad. 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 performance of this option. Another possibility is replacement of the APS storage ring with a next-generation ring capable 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.