Abstract

Compact coherent x-ray sources have been the focus of extensive research efforts over the past decades. As a result, several novel schemes like optical and nano-undulators for generating x-ray emissions in ‘table-top’ setups are proposed, developed, and assessed. Despite the extensive efforts in the past decades, there exists no operational FEL based on optical or electromagnetic undulators. By combining the particle confinement capability of optical cavities with wiggling motion inside an optical undulator, this paper proposes a new concept for making a compact coherent x-ray source. The full-wave solution of first-principle equations based on finite-difference time-domain and particle-in-cell is performed to simulate inverse-Compton scattering (ICS) off both free and confined electrons. It is shown that the strong space-charge effect in a low-energy electron beam (5 MeV) is the main obstacle in acquiring coherent gain through the ICS mechanism with a 10 micrometer laser. Subsequently, it is shown that by confining the electron beam at the field nodes of an optical cavity, the space-charge effect is compensated, and additionally, the ultrahigh charge density enables high FEL-gain at confinement spots. The full-wave numerical simulations predict enhancement of about three orders of magnitude in the radiation efficiency when ICS is carried out with confined electrons compared to free electrons. These theoretical results show promising potential as a new scheme for implementing a compact coherent x-ray source.

Highlights

  • Conventional compact x-ray sources include bremsstrahlung tubes [1], channeling radiation [2], triboluminescence [3], and inverse Compton scattering (ICS) sources [4,5,6,7,8]

  • Some examples are meeting the lasing condition < λx/4π with being the electron beam emittance, preserving the transverse coherence of the beam, delimiting the longitudinal energy spread, and circumventing the space-charge effects. These effects are safely covered in the theoretical studies only if full-wave solvers based on first-principle equations are used for simulation of the radiation phenomenon

  • A new scheme based on inverse-Compton scattering (ICS) off low-energy electrons transversely confined inside fields of an optical cavity is presented

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Summary

August 2021

Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Keywords: compact x-ray sources, computational physics, free-electron lasers, electron beam, radiation physics

Introduction
Full-wave simulation of FELs
Challenges in lasing of optical undulators
Confinement of electron beams traversing optical cavities
Inverse Compton scattering off a confined electron beam
Coherence properties of the radiation
Radiation energy scaling
Conclusion and outlook
Data availability statement
Full Text
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