Investigation of resistive wall instability in the 7-GeV APS storage ring

The Advanced Photon Source (APS) storage ring is a 7-GeV light source with 40 straight sections. Intense X-ray beams will be delivered by 34 insertion devices installed in these straight sections. The vacuum chamber for the insertion devices has an elliptical cross section with the gap equal to 8 mm. With this narrow gap, we estimate that the transverse impedance of the ring at the revolution frequency could be as high as 36 MΩ from the resistive wall. By increasing the (unnormalized) chromaticity to 7, we cure the head-tail modes of order up to m=1 for all 60 coupled bunch mode patterns around the ring. Tracking results show that the increased sextupole strength resulting from a higher chromaticity does not significantly reduce the dynamic aperture. Since increased chromaticity alone cannot cure all the head-tail modes, the APS storage ring will have a feedback system to damp the rigid-bunch modes