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.

Shaping ultraviolet (UV) laser beams is critical for optimizing photoinjector performance for applications in free-electron lasers (FELs). It has been shown that a 50% truncated Gaussian beam can achieve the lowest emittance via space charge compensation at LCLS-I. However, conventional shaping techniques to prepare this beam are limited by significant power losses or are not adapted for UV light. Here we report a high-precision transverse-shaping technique based on custom fused-silica phase plates with >99 % transmission at 253 nm. This approach enables spatial beam profile tailoring and significantly enhances beam stability at the photocathode. Using IMPACT-T simulations, we predict a 33% (from 0.67um to 0.45um) reduction in normalized emittance for a 250 pC bunch at LCLS-I. Experimental implementation at FACET-II demonstrated a 37% emittance reduction (from 5.4um to 3.4um) at 1.6 nC. These results establish phase-plate beam shaping as a high-fidelity, low-loss approach for high-brightness photoinjectors. Implementation at LCLS-II which will enable stable operation at megahertz repetition rates is underway.