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 $\ensuremath{\pi}$ 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.
Highlights
Advancements in the production of high peak brightness electron beams have revolutionized the field of ultra-fast science through the advent of the x-ray Free Electron Laser (FEL)
The ability to reach femtosecond level x-ray pulse durations at a growing number of FEL facilities allows for the study of molecular and atomic scale structures as well as femtosecond scale dynamical processes [1]
The total compression is limited by non-linear correlations in the electron beam longitudinal phase space. These correlations stem from RF curvature [2], longitudinal space charge (LSC) [3], coherent synchrotron radiation (CSR) [4,5], and resistive wall wakefields [6,7]
Summary
Advancements in the production of high peak brightness electron beams have revolutionized the field of ultra-fast science through the advent of the x-ray Free Electron Laser (FEL). These current horns can produce significant CSR in the bunch compressor causing further distortions in the longitudinal phase space This correlated longitudinal energy variation can result in a correlated transverse kick leading to growth in the projected emittance, reducing FEL performance as portions of the beam will not be matched ideally to the transverse focusing lattice. It was shown in [23,24,25], that the growth of current horns can be suppressed by adjusting the higher order compression with an octupole magnet inserted in a bunch compressor This scheme relies on placing an octupole at a point of significant dispersion, providing a transverse kick correlated with the longitudinal beam coordinate.
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