Cavity-based X-ray free-electron lasers present a promising path toward fully coherent, high-brightness X-ray sources with enhanced stability and spectral purity. By using Bragg-reflecting crystal cavities to recirculate and amplify an X-ray seed pulse over multiple passes, CBXFELs offer the potential for orders-of-magnitude improvements in coherence and brightness compared to single-pass FELs. This talk will present an overview of the CBXFEL concept and the proof-of-principle experiment currently under development at SLAC. Recent progress will be presented, along with ongoing efforts in beam–X-ray overlap diagnostics and cavity alignment. The talk will also address the key technical challenges ahead for CBXFELs and briefly explore alternative cavity-based XFEL designs as promising paths forward.

We report on the commissioning results of the newly implemented Beam Overlap Diagnostic (BOD) instrument, known as Station F, at the hard X-ray line of the Linac Coherent Light Source (LCLS). As part of the CBXFEL project at SLAC, Station F is designed to facilitate alignment between the relativistic electron beam entering the undulator hall and the X-rays returning from the CBXFEL cavity via the return line. The station features two interchangeable targets: (1) a diamond screen for direct imaging of the LCLS electron beam, enabling measurement of its transverse size and position; and (2) a YAP:Ce scintillator for detecting faint returning X-rays when the diamond’s sensitivity is insufficient. Emission from either target, whether generated by electron-induced cathodoluminescence or X-ray-induced scintillation, is captured using a fast-gated optical/UV camera. We present results from recent commissioning runs, including direct electron beam imaging, beam size and position characterization in both single- and two-bunch modes, and observations of coherent radiation linked to early microbunching, with implications for the laser heater configuration.