Diagnosis of optical errors with a precision BPM system at CESR

In the Cornell Electron Storage Ring (CESR) multibunch beams of electrons and positrons share a common vacuum chamber and guide field. A new precision beam position monitor system, with bunch-by-bunch measurement capability, is now in operation in one sector of the CESR ring including the interaction region (IR). Two of our position monitors are located 67 cm on either side of the interaction point (IP) within the IT experimental solenoid. By measuring the relative positions of electron and positron bunches at these detectors we can obtain information about the displacement of the beams at the collision point with an anticipated resolution on the order of the vertical beam size. The measurement of the dependence of beam trajectories at these IR beam position monitors (IRBPMs) on changes in the ring steering elements provides a powerful diagnostic of guide field errors in the interaction region.