Dechirper Wakefields for Short Bunches

TL;DR

This paper derives simplified analytical models for wakefields in flat structures like dechirpers for short electron bunches, enabling quick estimates of energy loss, transverse kicks, and emittance growth in X-ray FELs.

Contribution

It introduces generalized short-bunch wake approximations valid for any flat structure with surface impedance, extending previous work to arbitrary transverse offsets.

Findings

Derived analytical short-range wake approximations for flat structures.

Validated approximations against full-length wake calculations for short bunches.

Provided practical estimates for energy loss, transverse kicks, and emittance growth.

Abstract

In previous work [1] general expressions, valid for arbitrary bunch lengths, were derived for the wakefields of corrugated structures with flat geometry, such as is used in the RadiaBeam/LCLS dechirper. However, the bunch at the end of linac-based X-ray FELs--like the LCLS--is extremely short, and for short bunches the wakes can be considerably simplified. In this work, we first derive analytical approximations to the short-range wakes. These are generalized wakes, in the sense that their validity is not limited to a small neighborhood of the symmetry axis, but rather extends to arbitrary transverse offsets of driving and test particles. The validity of these short-bunch wakes holds not only for the corrugated structure, but rather for any flat structure whose beam-cavity interaction can be described by a surface impedance. We use these wakes to obtain, for a short bunch passing through a dechirper: estimates of the energy loss as function of gap, the transverse kick as function of beam offset, the slice energy spread increase, and the emittance growth. In the Appendix, a more accurate derivation--than is found in [1]--of the arbitrary bunch length wakes is performed; we find full agreement with the earlier results, provided the bunches are short compared to the dechirper gap, which is normally the regime of interest. [1] K. Bane and G. Stupakov, Phys. Rev. ST Accel. Beams 18, 034401(2015).