A self-consistent refractive index model for fast simulation of free-electron lasers

TL;DR

This paper introduces a fast, self-consistent refractive index model for simulating free-electron lasers, capturing key transverse dynamics efficiently and accurately in the linear regime.

Contribution

It presents a novel second-order approximation method that models FEL transverse dynamics using optical fiber analogies, improving simulation speed and accuracy.

Findings

The method is both fast and highly accurate.

It can model transverse offsets in the x-ray and electron beams.

Applicable to various FEL configurations including SASE and seeded FELs.

Abstract

Modern x-ray free-electron lasers (XFELs) produce x-ray pulses of exceptional transverse coherence. This is due largely to the process of optical guiding by which the radiation is both refractively guided by the bunched electron beam and gain guided by the preferential amplification of on-axis radiation. These effects may be summarized by an effective index of refraction, which has been used in the past to study the transverse dynamics of the FEL process with significant simplifications and approximations, but never fully self-consistently. We present here a self-consistent method for studying high gain FELs in the linear regime by approximating the FEL equations to second-order in the lateral displacement from the nominal electron beam axis. This is made possible by casting the FEL equations in the language of optical fibers with an appropriately chosen refractive index. We demonstrate that this approach is both fast and highly accurate, indicating that the most important FEL dynamics are inherently second-order. In its full form our method can capture the effects of transverse offsets in both the x-ray beam and the electron beam, making it a versatile tool for studying non-ideal effects in seeded FELs, regenerative amplifier XFELs, and even self-amplified spontaneous emission (SASE) FELs.