Undulator field correction by in-situ sorting

A novel field-correction method for insertion devices called in-situ sorting has been developed. It is based on a rearrangement of magnet blocks with an analysis of the magnetic data measured for the total device after assembly instead of that for each magnet block before assembly. The field integral at each magnet pole obtained from the magnetic distribution determines how to exchange or flip magnet blocks to reduce both the phase error and multipole components of the field integral. The practical results of the in-situ sorting used for a field correction of an in-vacuum undulator are shown to reveal the validity of the in-situ sorting.