Octupole based current horn suppression in multistage bunch compression with emittance growth correction

Abstract

High brightness electron beams are critical for x-ray free-electron laser performance. To achieve higher peak current, strong longitudinal compression often produces electron beams with peaks in the head and/or tail of the current profile. These current horns are formed after bunch compression due to nonlinear correlations in the longitudinal phase space and the higher order optics of the compressor. It has been suggested that this higher order compression can be corrected by inserting an octupole magnet near the center of a bunch compressor. However, this scheme provides a correlated transverse kick leading to growth of the projected emittance. We present here a method whereby octupole magnets are inserted into two sequential bunch compressors. By tuning a π betatron phase advance between the two octupoles, the correlated transverse kick from the first octupole can be corrected by the second, while providing a cummulative adjustment of the higher order compression.