Influence of resistive wall impedance on the VSX light source

We describe the calculated resistive wall impedance of the vacuum chamber and the corresponding growth rate of coupled-bunch instabilities for the VSX light source, a Japanese third generation VUV and soft X-ray source with a beam energy of 2 GeV. The impedance calculation was performed for the normal Al vacuum duct and the SiC duct, a higher-order mode (HOM) absorber of the damped RF cavity. The impedance of the Al vacuum duct with a narrow vertical aperture is sufficiently high at low frequencies for exciting a transverse coupled-bunch instability. On the other hand, the SiC duct does not cause a serious problem about coupled-bunch instabilities and it gives only a small contribution to the broad-band impedance related to single-bunch instabilities