Lasing of a cavity-based X-ray source

Area of Science:

• X-ray optics
• Accelerator physics
• Materials science

Background:

• Lasers revolutionized visible light optics, but extending this to X-rays faced challenges with gain media and mirrors.
• Current hard X-ray free-electron laser (XFEL) facilities produce high brightness but suffer from noisy, multi-spiked temporal and spectral profiles.
• Cavity-based XFELs (CBXFELs) were proposed to improve spectral purity by recirculating filtered X-ray pulses in a synchronized cavity.

Purpose of the Study:

• To demonstrate lasing with multi-pass gain in a cavity-based XFEL setup.
• To validate the use of diamond Bragg optics for X-ray resonators in an accelerator environment.
• To establish the feasibility of CBXFELs for generating spectrally pure X-ray pulses.

Main Methods:

• Utilized a 132.8-m round-trip diamond-based Bragg cavity at the European XFEL.
• Synchronized the cavity to the 2.23-MHz bunch spacing of the superconducting accelerator.
• Maintained stringent length and angular stability requirements for the optical cavity.

Main Results:

• Achieved lasing with multi-pass gain at 6.952 keV.
• Observed a 'ring-up' of X-ray pulses across successive electron bunches within the cavity.
• Produced spectrally pure, microjoule-level X-ray pulses.

Conclusions:

• Established the feasibility of CBXFELs in a real accelerator environment.
• Validated diamond Bragg optics as suitable for X-ray resonators.
• The demonstrated spectral purity offers a pathway to next-generation X-ray science requiring coherent, stable sources.