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Design study of a high-brightness high-repetition rate thermionic injector for free-electron laser application. II. Gun technology

Vitaliy Goryashko1,2,*, Kazuaki Togawa1,†, and Anatoliy Opanasenko2,1,3

  • *Contact author: vitaliy.goryashko@physics.uu.se; vitgor06@gmail.com
  • †Contact author: togawa@spring8.or.jp
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Phys. Rev. Accel. Beams 28, 110101 – Published 13 November, 2025

DOI: https://doi.org/10.1103/srht-5hmq

Abstract

The SPring-8 Angstrom Compact free-electron LAser (SACLA) linear accelerator utilizes a dc gun with a thermionic cathode, known for delivering high beam quality, excellent operational stability, and low maintenance requirements. In the second part of our design study, we present a comprehensive beam dynamics analysis of the SACLA dc electron gun and outline a roadmap toward achieving submicrometer (0.1  μm) beam emittance. We provide both analytical and numerical estimates of four key sources contributing to emittance growth: (i) image = charge effects on the cathode, (ii) nonlinear space-charge forces arising from imperfect beam edge geometry, (iii) aberrations in the accelerating field, and (iv) solenoid field aberrations due to longitudinal–transverse momentum exchange. The analysis yields a closed-form expression for the superposition of correlated emittance growth mechanisms, highlighting an “interference” term that amplifies the net emittance when space-charge and solenoid effects coexist. To address the challenge of emittance growth, we propose a compact hybrid magnet that ensures strong longitudinal confinement of the magnetic field. This confinement allows placement of the hybrid magnet in close proximity to the gun exit, thus mitigating emittance growth effectively, while having a negligible magnetic field on the cathode. The hybrid magnet design is achieved by symmetrically positioning permanent ring magnets on either side of a solenoid, enabling fine control over the field distribution and “squeezing in” it longitudinally. Our findings offer new insights applicable to low-voltage continuous-wave very high-frequency guns and superconducting radio-frequency electron guns.

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Design study of a high-brightness high-repetition rate thermionic injector for free-electron laser application: The architecture

Vitaliy Goryashko, Kazuaki Togawa, Peter Salén, and Anatoliy Opanasenko
Phys. Rev. Accel. Beams 28, 091601 (2025)

Article Text

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Phys. Rev. Accel. Beams 28, 110101– Published 13 November, 2025
  • Received 8 July 2025
  • Accepted 10 October 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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