2025/10/01 by Nanshun Huang, Huang, Nanshun, Hanxiang Yang +3
Engineering · Medicine · #Accelerator Physics (physics.acc-ph) #Adrenal and Paraganglionic Tumors #FOS: Physical sciences #Geophysics and Sensor Technology #Optics (physics.optics) #Particle Accelerators and Free-Electron Lasers
paper · pdf · doi:10.48550/arxiv.2510.00640
openalex publication_date 2025/10/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
Precise spectral control in the hard X-ray regime remains a long-standing challenge that limits applications in atomic-scale science and ultrafast spectroscopy. We present an actively mode-locked cavity-based X-ray free-electron laser that achieves deterministic spectral programmability with phase-locked pulse trains and comb-like spectra, by coherently modulating the electron-beam energy. Three-dimensional time-dependent simulations predict \SI700\micro\joule total energy, \SI30\giga\watt peak power, and frequency-comb spacing of \SI1.55\electronvolt set by the modulation frequency. We further develop selective single-line amplification via undulator tapering and absolute frequency positioning through modulation-laser tuning with better than 2 × 10-5 relative precision. Importantly, stable mode-locked operation persists under >80% peak-to-peak cavity-reflectivity variations, substantially relaxing requirements on X-ray optics. These results establish active mode locking as a practical route to fully coherent, spectrally agile hard X-ray sources and enable new opportunities in time-resolved core-level spectroscopy, X-ray quantum optics, and precision metrology.