Electron beam energy chirp control with a rectangular corrugated structure at the Linac Coherent Light Source

Abstract

Electron beam energy chirp is an important parameter that affects the bandwidth and performance of a linac-based, free-electron laser. In this paper we study the wakefields generated by a beam passing between flat metallic plates with small corrugations, and then apply such a device as a passive dechirper for the Linac Coherent Light Source (LCLS) energy chirp control with a multi-GeV and femtosecond electron beam. Similar devices have been tested in several places at relatively low energies (∼100 MeV) and with relatively long bunches (> 1 ps). In the parameter regime of the LCLS dechirper, with the corrugation size similar to the gap between the plates, the analytical solutions of the wakefields are no longer applicable, and we resort to a field matching program to obtain the wakes. Based on the numerical calculations, we fit the short-range, longitudinal wakes to simple formulas, valid over a large, useful parameter range. Finally, since the transverse wakefields-both dipole and quadrupole-are strong, we compute and include them in beam dynamics simulations to investigate the error tolerances when this device is introduced in the LCLS.

Wakefield potentials of corrugated structures

“…The frequency of this mode is 105.5 GHz or the wave length of this mode is 28.4 mm which is in agreement with calculations in Ref. [4]. However, as we can see from the plot, this fit does not give a good approximation at small distances: in the bunch region.…”

“…We calculate the wake potential for the PAL experiment [2] and found better agreement with the measured results than the single dominated mode assumption that was used to design the experiment. Based on our calculations, the single mode approximation [2][3][4] begins to break down when a bunch length becomes smaller than the period of the corrugated structure. In the next steps we plan to analyze the wake function for an offset trajectory in the dechirper.…”

“…Although they referred to our 1997 publication [5], where we found a similarity to the wakefields in corrugated structures with the fields in a tube with a thin dielectric layer, the authors do not fully analyze the applicability of the single mode approach to describe wakefields of very short bunches. A corrugated wall structure is planned for use in the device that causes an additional energy loss along the very short bunches of the linac coherent light source (LCLS) [4]. It now becomes very important to check how well a single mode approach can describe the fields, that are excited in corrugated structures and perhaps another description must be used.…”

“…The references to these publications can be found in Refs. [3,4]. These formulas are derived from the assumption of a single dominating mode and the Green's function for the wake potentials is described as a damped cosine function.…”

Generation of ultra-large-bandwidth X-ray free-electron-laser pulses with a transverse-gradient undulator

“…By using these methods, or combining them, the bandwidth of the XFEL pulses can reach the few-percent level. A possibility to further increase the energy chirp of the beam is given by exploiting the wakefield of dedicated beamline elements such as corrugated pipes and dielectric structures (Craievich, 2010;Emma et al, 2014;Antipov et al, 2014;Zhang et al, 2015). For practical reasons the energy chirp of the electron beam can only be increased up to a few percent, otherwise dispersive and chromatic effects will prevent the amplification of the XFEL process.…”