Impedance and the single bunch limit in the APS storage ring

The single-bunch current limit and tune shift with current have been documented over time in the 7-GeV Advanced Photon Source (APS) storage ring as a function of lattice, chromaticity, and number of small-gap insertion device (ID) chambers. The contribution to the machine coupling impedance of the 8 mm-gap ID chambers was reported earlier by Sereno et al. (1998). One 5-mm-gap ID chamber was installed in December 1997. This required changing the lattice to preserve the vertical acceptance. The new lattice reduced the average vertical beta function at the 5-mm chamber as well as at all the other ID chambers and so has also lowered the effect of the vertical coupling impedance. As additional 8-mm and 5-mm chambers are planned, a more detailed characterization of the impedance is essential. This includes separating the effects of the transitions between the small-gap chambers and the standard chambers from the resistive wall impedance of the small-gap chambers themselves. In this paper, we report on the transverse instabilities and thresholds observed in the vertical and horizontal planes. From these observations, various contributions to the coupling impedance are derived.