# Fast longitudinal beam dynamics optimization in x-ray FEL linear accelerators

## TL;DR

This paper introduces a fast, simplified model for optimizing the longitudinal phase space of electron beams in x-ray FEL linacs, enabling rapid design improvements with results surpassing original configurations.

## Contribution

A novel lumped parameter model for quick optimization of beam dynamics, validated against detailed simulations, and applied to enhance LCLS-II linac performance.

## Key findings

- Achieved higher core peak current in optimized linac design.
- Model shows good agreement with detailed multi-particle simulations.
- Enables faster optimization compared to traditional tracking methods.

## Abstract

A high peak current, flat longitudinal phase space electron beam is desirable for efficient x-ray free electron laser (FEL) radiation in next generation light sources. To attain such a beam requires the extensive design of the linear accelerator (linac) including both linear and nonlinear effects. In this paper, we propose a lumped longitudinal beam dynamics model for fast optimization of the electron beam longitudinal phase space through the accelerator. This model is much faster than available tracking programs and also shows good agreement with the fully three-dimensional element-by-element multi-particle simulations. We applied this model in a parallel multi-objective differential evolution optimization program to an existing LCLS-II superconducting linac design and obtained an optimal solution with significantly higher core peak current than the original design.