Estimation and measurement of effective beam position monitor offsets by using a stored beam

In general, a closed orbit distortion is observed as the sum of beam orbit and various noises such as beam position monitor (BPM) setting errors, BPMʹs electric response errors, etc. In order to obtain a beam with high performance in a low emittance ring with strong sextupole magnets, it is important to pick up information only of the beam orbit and perform orbit corrections by using this information. For this purpose we developed a new method of estimating BPM offsets to suppress the noise in the orbit data, based on the concept of “two-stage magnet alignment with common girders”. This alignment scheme, adopted in an 8 GeV electron storage ring at SPring-8, reduces high harmonic Fourier components of the orbit which results from magnet misalignment. Then, the main part of high harmonic components of a measured closed orbit is the artificial noise, and these harmful high harmonic components can be extracted as “effective” BPM offsets by filtering a measured closed orbit. This method was applied to the SPring-8 storage ring and found to be effective for reducing residual orbit distortions and a leakage of the dispersion function. To check the reliability of the resulting effective offsets, a beam-based measurement was also carried out at some BPMs and the beam position was measured with respect to the center of a nearby quadrupole magnet. The results were compared with the effective offsets and a reasonable agreement was found.