The Generation of Variable Polarization States in Terawatt X-Ray Free-Electron Lasers

TL;DR

This paper explores the potential of terawatt x-ray free-electron lasers (XFELs) to generate high-power harmonics at shorter wavelengths and higher energies with variable polarization, using simulations to demonstrate significant harmonic powers.

Contribution

It introduces a detailed simulation study of harmonic generation in TW XFELs with variable polarization, analyzing effects of undulator types and tapering on harmonic power output.

Findings

Peak powers of 180 MW at the fifth harmonic are achievable.

Harmonics saturate before the fundamental in tapered undulators.

High harmonic powers enable advanced applications in x-ray science.

Abstract

Terawatt x-ray free-electron lasers (XFELs) represent the frontier in further development of x-ray sources and require high current densities with strong transverse focusing. In this paper, we investigate the implications/potentialities of TW XFELs on the generation of harmonics at still shorter wavelengths and higher photon energies with variable polarization. The simulations indicate that significant power levels are possible at high harmonics of the XFEL resonance and that these XFELs can be an important coherent source of hard x-rays through the gamma ray spectrum. For this purpose, we use the MINERVA simulation code which self-consistently includes harmonic generation. Both helical and planar undulators are discussed in which the fundamental is at 1.5 {\AA} and study the associated harmonic generation. While tapered undulators are needed to reach TW powers at the fundamental, the taper does not enhance the harmonics because the taper must start before saturation of the fundamental, while the harmonics saturate before this point is reached. Nevertheless, the harmonics reach substantial powers. Simulations indicate that, for the parameters under consideration, peak powers of the order of 180 MW are possible at the fifth harmonic with a photon energy of about 41 keV and still high harmonics may also be generated at substantial powers. Such high harmonic powers are certain to enable a host of enhanced applications