The cavity-based x-ray free-electron laser (CBXFEL) R&D project utilizes a low-loss x-ray cavity (65.5 m long) to provide circulating monochromatized x-ray seeding for electrons from the Cu-linac at SLAC. The project aims to demonstrate the two-pass gain in x-ray regenerative amplifier and XFELO modes by 2024. Here, we report on the design, manufacture, and characterization of x-ray optical and diagnostic components for this project. The low-loss wavefront-preserving x-ray optical components include high-reflectivity C(400) diamond crystal mirrors, drumhead diamond crystal with thin membranes, beryllium refractive lenses, channel-cut Si monochromators, and exact-Backscattering C(440) diamond crystal. The x-ray diagnostics are designed to ensure the accuracy of beam alignment and to characterize and optimize CBXFEL performance. These include different types of x-ray beam position and profile monitors and x-ray beam intensity monitors, and a meV-resolution x-ray spectrograph. All x-ray optical and diagnostic components have been fully characterized with x-rays, and the mechanical installation of these components is expected to be finished soon.

Cavity-based XFEL, or CBXFEL, is a future photon source concept under intense development at SLAC. It is considered a path towards full 3D coherence at angstrom wavelength, delivering another 2-3 orders of magnitude leap in source brightness compared to current XFELs configurations. In a first phase of the project, one of the goals is to demonstrate the regenerative amplification by returning and amplifying the seed pulse from 7 LCLS Hard X-ray Undulators (HXUs) with a rectangular crystal cavity. In this paper, we report on the recent measurement of early stage XFEL lasing characteristics at 9.831 keV photon energy by using 7 LCLS HXUs under e-beam conditions close to those chosen for the first phase of CBXFEL gain demonstration.