Scheme for precise correction of orbit variation caused by dipole error field of insertion device

Abstract

We developed a scheme for precisely correcting the orbit variation caused by a dipole error field of an insertion device (ID) in a storage ring and investigated its performance. The key point for achieving the precise correction is to extract the variation of the beam orbit caused by the change of the ID error field from the observed variation. We periodically change parameters such as the gap and phase of the specified ID with a mirror-symmetric pattern over the measurement period to modulate the variation. The orbit variation is measured using conventional wide-frequency-band detectors and then the induced variation is extracted precisely through averaging and filtering procedures. Furthermore, the mirror-symmetric pattern enables us to independently extract the orbit variations caused by a static error field and by a dynamic one, e.g., an error field induced by the dynamical change of the ID gap or phase parameter. We built a time synchronization measurement system with a sampling rate of 100Hz and applied the scheme to the correction of the orbit variation caused by the error field of an APPLE-2-type undulator installed in the SPring-8 storage ring. The result shows that the developed scheme markedly improves the correction performance and suppresses the orbit variation caused by the ID error field down to the order of submicron. This scheme is applicable not only to the correction of the orbit variation caused by a special ID, the gap or phase of which is periodically changed during an experiment, but also to the correction of the orbit variation caused by a conventional ID which is used with a fixed gap and phase.