Development of a quadripartite wakefield structure as dechirper for free electron laser

TL;DR

This paper introduces a quadripartite wakefield structure for FEL dechirpers that suppresses quadrupole wakefields, reduces emittance growth, and improves performance through advanced simulation and tracking methods.

Contribution

The design of a four-plate wakefield structure that fully suppresses quadrupole wakefields while maintaining strong longitudinal wakefields is novel.

Findings

Significantly reduced projected emittance growth with the quadripartite design.

Achieved 25% shorter structure length compared to planar structures.

Nonlinear wakefield effects can cause slice emittance growth for large beams.

Abstract

Wakefield structures are critical for beam manipulation in free-electron lasers (FELs), particularly when serving as dechirpers, where beam-induced longitudinal wakefields compensate the energy chirp introduced during beam magnetic compression. However, conventional planar structures also generate time-dependent quadrupole wakefields due to their asymmetric geometry, which can cause beam mismatch and projected emittance growth. To address this limitation, we propose a quadripartite wakefield structure comprising four identical corrugated plates, able to fully suppress quadrupole wakefields while preserving strong longitudinal wakefields. To accurately evaluate its performance, we calculate wake potentials based on the Panofsky-Wenzel theorem using three-dimensional simulation software and extract the corresponding wake functions by deconvolution. We further adopt a particle-to-particle (P2P) tracking method incorporating these wake functions, which is capable of accounting for higher-order components and nonlinear effects that are typically neglected in standard tracking codes. Simulation results confirm that the quadripartite geometry offers significantly reduced projected emittance growth and a 25% shorter structure length compared with the planar design. The tracking method also reveals that the nonlinearities of three-dimensional wakefields induce noticeable slice emittance growth for large transverse beam sizes, which may in turn affect lasing performance. In addition, the tracking method enables analysis of various types of assembly error and indicates that misalignment along the direction of plate motion may severely degrade the emittance via dipole wakefields. Such misalignment can be mitigated through beam-based alignment and precise plate adjustment using high-resolution servo motors.