BEAM-BASED OFFSET CALIBRATION OF THE PLS BPM

There are 108 beam position monitors in the PLS for the orbit measurement and closed orbit correction. As the BPM pickup electrodes are assembled directly on the 10m-long vacuum chamber, calibration of the electrical center of BPMs have been done with the external rf transmission-reflection method. Since the closed orbit distortion is still larger than 500 mm rms, we are developing the beam-based calibration method for the BPMs in PLS to improve the accuracy of the absolute position reading. In this report we present the method of beam-based calibration of BPMs and its experimental results.