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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.
Highlights
Coherent x-ray radiation from a free electron laser (FEL) light source provides an important tool for scientific discovery in physics, chemistry, biology and other fields
The high brightness electron beam used in an x-ray FEL light source typically comes from a linear accelerator beam delivery system [1,2,3,4,5,6]
This system consists of a photoinjector to generate an initial high brightness electron beam, a rf linac to accelerate the beam to multiple GeV energy and to compress the beam to hundred or thousand Ampere peak current, and transport beam line to deliver the beam to different undulators
Summary
Coherent x-ray radiation from a free electron laser (FEL) light source provides an important tool for scientific discovery in physics, chemistry, biology and other fields. In the LCLS-II design, the final correlated energy spread of the electron beam is almost completely removed by the resistive wall wakefields before entering the undulator [9,10] Including those collective effects in the longitudinal beam dynamics model is crucial for the high peak current accelerator design. To include those effects, one normally resorts to detailed multiparticle element-by-element tracking simulations. Using a weighted macropaticle method, given the initial current profile and correlated energy profile in longitudinal phase space, a small number of macroparticles (from a few hundreds to a thousand macroparticles) that corresponds to the longitudinal slice coordinates are needed in the simulation All these make the longitudinal beam dynamics simulation very fast.
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