Dechirper wakefields for short bunches

In previous work (Bane and Stupakov, 2015 [I]) 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 a function of beam offset, the slice energy spread increase, and the emittance growth. In the Appendix, a more accurate derivation than that is found in Bane and Stupakov (2015) [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. (C) 2016 Elsevier B.V. All rights reserved.