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

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 ( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mn>0.1</a:mn> <a:mtext> </a:mtext> <a:mtext> </a:mtext> <a:mrow> <a:mi mathvariant="normal">μ</a:mi> <a:mi mathvariant="normal">m</a:mi> </a:mrow> </a:math> ) 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 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.