Transverse impedance distribution measurements using the response matrix fit method at APS

The Advanced Photon Source storage ring has a number of narrow-gap insertion device vacuum chambers, and there is a plan to further increase their number. These vacuum chambers contribute to the impedance of the machine and to the impedance limits the single-bunch current threshold. In order to understand the contribution of the vacuum chambers and its dependence on the gap, the distribution of the vertical transverse impedance around the circumference of the APS storage ring has been determined using the response matrix fit method. This method allows us to find focusing errors around the storage ring and was originally used to determine and correct the linear model of the APS. The high accuracy of the method enables us to measure the variation of betatron phase advance around the ring with beam current. Results of these measurements are reported and the impedance of different parts of the storage ring is presented.