Cavity-based XFEL systems will potentially offer much higher spectral quality of the hard x-ray beam compared to traditional XFEL SASE and self-seeded sources. A promising cavity-based concept is the population inversion x-ray laser oscillator, dubbed XLO, where the SASE beam is used as a pump, and a transition metal serves as a gain medium. We will report on the progress in design and construction of the XLO, using LCLS as an x-ray pump, being developed by a SLAC, CFEL, University of Hamburg, University of Wisconsin, and UCLA collaboration. Initially, XLO will be demonstrated at the Coherent X-ray Imaging (CXI) LCLS end-station, as a two pass Regenerative Amplifier operating at the Copper Kalpha1 photon energy of 8048 eV. In the later phase of the project, it will utilize LCLS multi-bunch mode, with up to 8 x-ray pulses. Finally, XLO will generate fully coherent transform limited pulses with about 50 meV FWHM bandwidth. We expect the XLO will pave the way for new user experiments, e.g. in inelastic X-ray scattering, parametric down conversion, quantum science, X-ray interferometry.

Polarization is a fundamental property of light used in experiments to probe various properties of matter such as the chirality of molecules and crystal structures. There is increasing interest in generating bespoke radiation pulses for experiments with increasingly complex structures of polarization. At short wavelengths, free electron lasers offer an avenue to control the polarization structure at the point where the radiation is emitted through manipulation of the electron beam, removing the requirement for polarizing optics not readily available at x-ray wavelengths. This talk discusses a method for manipulating the polarization of FEL generated light based on temporal intensity modulation of radiation emitted in orthogonally polarized undulators. Simulations demonstrate the method can produce radiation that switches between orthogonal polarization states at attosecond timescales. Implementation of this ultra-fast polarization switching would provide a valuable new tool to the scientific community.

Operation of a Quantum Free-Electron Laser (QFEL) could provide fully coherent X- and gamma-rays in a compact setup. Imperative to experimental realization is allowing for decoherence of either spontaneous emission or space-charge to take place, having opposing constraints [1]. Here, for the first time, we discuss a comprehensive QFEL model that takes into account both decoherence effects. Then, we use this model to investigate the ultracold electron source (UCES) [2] as a potential QFEL electron injector. The UCES, based on near-threshold photoionization of laser-cooled and trapped atomic gas, has the unique property of allowing highly charged electron bunches to be extracted while maintaining ultralow transverse emittance. We find that the ultracold electron bunches meet the stringent requirement for potential QFEL operation with commercially available laser systems.

Quantum diffusion is caused by the recoil effect that a particle experiences when it emits a photon [1]. Quantum diffusion due to the synchrotron radiation in high-energy electron and positron circular accelerators defines the main parameters of the beam: its energy spread and hence the bunch length, as well as the horizontal emittance. It is calculated as a single particle effect assuming incoherent radiation. This assumption is not valid in FELs where the radiation is coherent. In this work, we develop theory of the quantum diffusion in coherent radiation and show that it leads to the energy diffusion of the particles that is correlated between the different positions in the bunch.