Current-horn suppression for reduced coherent-synchrotron-radiation-induced emittance growth in strong bunch compression

Abstract

Control of coherent synchrotron radiation (CSR)-induced emittance growth is essential in linear accelerators designed to deliver very high brightness electron beams. Extreme current values at the head and tail of the electron bunch, resulting from strong bunch compression, are responsible for large CSR production leading to significant transverse projected emittance growth. The Linac Coherent Light Source (LCLS) truncates the head and tail current spikes which greatly improves free electron laser (FEL) performance. Here we consider the underlying dynamics that lead to formation of current spikes (also referred to as current horns), which has been identified as caustics forming in electron trajectories. We present a method to analytically determine conditions required to avoid the caustic formation and therefore prevent the current spikes from forming. These required conditions can be easily met, without increasing the transverse slice emittance, through inclusion of an octupole magnet in the middle of a bunch compressor.

Inhibition of Current-spike Formation Based on Longitudinal Phase Space Manipulation for High-Repetition-Rate X-ray FEL

“…These electrons experiencing overcompression will overlap with other ones and dramatically enhance the local charge density. Based on it, the current spike formation is analogous to the caustic phenomenon in the optics, and the fundamental mechanism has been studied analytically, readers with interest are kindly referred to [28,57,58].…”

“…The corrugated structure was placed to provide a passive wakefield for energy modulation upstream the bunch compression chicane in order to reduce the high-order energy spread in longitudinal phase space [25,26]. Moreover, the nonlinear optics of the magnetic chicane can be adjusted by inserting an optics corrector like sextupole or octupole [27,28], similar optical linearization method is applied to remove the quadratic energy chirp in a compact FEL design [29]. Nevertheless, achievement of this target is always troublesome, especially for XFEL facility operating at the frequency of 1 MHz, which involves more complicated beam dynamics properties evolution along the beamline transmission [25,30].…”

Bunch Shaping in Electron Linear Accelerators

“…The introduction of higher-order longitudinal dispersion (up to third order) was also proposed to suppress current spikes arising from collective effects in the LCLS accelerator (Charles et al, 2017). At LCLS, extreme (England et al, 2008).…”

“…Extension of this technique to control higher-order distortion is straightforward: correction of third order nonlinearties to, e.g., suppress current spike "horns" sometimes encountered in bright-beam injectors, was demonstrated via numerical simulations (Seok et al, 2019a) as an alternative to the technique described in Section IV.B.2; see also Ref. (Charles et al, 2017). From (Seok et al, 2019b).…”

Machine Learning-Based Direct Solver for One-To-Many Problems on Temporal Shaping of Electron Beams

“…For the 325 flat-top profile, horns occur at the head and the tail of the obtained bunch, which cannot be completely eliminated due to the nature of bunch compression with a single chicane compressor. The horns may be further flattened with an additional bunch compressor [54] that is, how-330 ever, beyond the scope of this study. Nevertheless, the FWHM of the horns is very narrow compared with the flat part of the bunch.…”