Speaker
Description
Classical optical lasers provide intense, coherent light in the visible and infrared regions, but extending this concept to X-rays has been particularly challenging due to a lack of suitable gain media and mirrors. Current hard X-ray free-electron laser (XFEL) facilities overcome this difficulty using a relativistic high-peak-current electron bunch as gain media for a self-amplified spontaneous emission in a single pass. This implies the use of long lines of undulators, delivering a very high brightness beam but with a noisy, multi-spiked temporal and spectral profile. Cavity-based XFELs (CBXFELs) were thought to close this gap by recirculating spectrally filtered X-ray pulses in a cavity, similarly to what is done in optical lasers. The first-ever lasing of one of these configurations, the XFELO, was realized at European XFEL in May 2025. The diamond-based Bragg cavity was tuned at 6.952 keV in a 132.8 meter round-trip and a ring-up across successive bunches was observed, producing spectrally pure, microjoule-level pulses.
In this talk, we show more details about the setup used for such a demonstrative result, with insights into the commissioning and the challenges that were overcome during the alignment and the tuning of the cavity until lasing was achieved. An insight into the next steps will also be illustrated.
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