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Advances in orbit drift correction in the Advanced Photon Source storage ring

The Advanced Photon Source storage ring is required to provide X-ray beams of high positional stability, specified as 17 /spl mu/m rms in the horizontal plane and 4.4 /spl mu/m rms in the vertical plane. We report on the difficult task of stabilizing the slow drift component of the orbit motion down to a few microns rms using workstation-based orbit correction. There are two aspects to consider separately-the correction algorithm and the configuration of the beam position monitors (BPMs) and correctors. Three notable features of the correction algorithm are: low-pass digital filtering of BPM readbacks; "despiking" of the filtered orbit to desensitize the orbit correction to spurious BPM readbacks without having to change the correction matrix; and BPM intensity-dependent offset compensation. The BPM/corrector configuration includes all of the working BPMs but only a small set of correctors distributed around the ring. Thus only those orbit modes that are most likely to be representative of real beam drift are handled by the correction algorithm.