Modelling elliptically polarised Free Electron Lasers

TL;DR

This paper develops a model for elliptically polarised Free Electron Lasers, incorporating harmonic interactions and averaging techniques, revealing enhanced harmonic coupling and differences in radiation evolution compared to unaveraged models.

Contribution

It introduces a generalized Bessel function scaling for elliptically polarised FELs and compares averaged and unaveraged models, highlighting new effects of polarization on harmonic coupling.

Findings

Enhanced harmonic coupling in elliptical undulators.

Good agreement between averaged and unaveraged models in the linear regime.

Different evolution of harmonic radiation near FEL saturation.

Abstract

A model of a Free Electron Laser operating with an elliptically polarised undulator is presented. The equations describing the FEL interaction, including resonant harmonic radiation fields, are averaged over an undulator period and generate a generalised Bessel function scaling factor, similar to that of planar undulator FEL theory. Comparison between simulations of the averaged model with those of an unaveraged model show very good agreement in the linear regime. Two unexpected results were found. Firstly, an increased coupling to harmonics for elliptical rather than planar polarisarised undulators. Secondly, and thought to be unrelated to the undulator polarisation, a signficantly different evolution between the averaged and unaveraged simulations of the harmonic radiation evolution approaching FEL saturation.