Measurement of sextupole orbit offsets in the APS storage ring

Horizontal orbit errors at the sextupoles in the Advanced Photon Source (APS) storage ring can cause changes in tune and modulation of the beta functions around the ring. To determine the significance of these effects requires knowing the orbit relative to the magnetic center of the sextupoles. The method considered here to determine the horizontal beam position in a given sextupole is to measure the tune shift caused by a change in the sextupole strength. The tune shift and a beta function for the same plane uniquely determine the horizontal beam position in the sextupole. The beta function at the sextupole was determined by propagating the beta functions measured at nearby quadrupoles to the sextupole location. This method was used to measure the sextupole magnetic center offset relative to an adjacent beam position monitor (BPM) at a number of sextupole locations. We report on the successes and problems of the method as well as an alternate method.