Cavity-based X-ray Free Electron Lasers (FELs) such as the X-ray regenerative amplifier FEL (XRAFEL) [1] and the X-ray FEL oscillator [2] have drawn great interest as a means of producing high-brightness, fully coherent and stable hard x-ray pulses for high-repetition rate FELs [3]. However, high efficiency outcoupling of the stored cavity x-ray radiation remains challenging. Here we present a novel XRAFEL design to achieve efficient cavity outcoupling or Q-switching by introducing energy chirp in the electron beam while leaving the high-quality X-ray optics intact. During the FEL interaction, electron beam with an linear energy chirp can be slightly compressed or decompressed by the undulator, which leads to a gradual shift of radiation frequency outside the bandwidth of the Bragg crystal for efficient outcoupling. Our simulation results show that substantial power can be outcoupled from the X-ray cavity driven by chirped electron beams at 100 kHz repetition rate. We also discuss parameter tradeoff in such an XRAFEL scheme and a practical way to achieve the desired fast chirp control by a small, normal-conducting RF station in the LCLS-II [4]. [1] Z. Huang and R. D. Ruth. PRL96, 144801 (2006). [2] K.-J. Kim, Y. Shvyd'ko, S. Reiche, PRL100 244802 (2008). [3] G. Marcus, et al., PRL125, 254801 (2020). [4] M. Nasr, et al., in proceedings of IPAC'16 (Busan, Korea,2016).

Electrons in a X-ray free electron laser (XFEL) develop periodic density fluctuations, known as microbunches, which enable the exponential gain of X-ray power in an XFEL. When an electron beam microbunched at a hard X-ray wavelength is kicked, microbunches are often washed out due to the dispersion and R56 of the bend. An achromatic (dispersion-free) bend with small R56, however, can preserve microbunches, which rotate to follow the new trajectory of the electron bunch. Rotated microbunches can subsequently be lased in a repointed undulator to produce a new beam of off-axis X-rays. In this work, we demonstrate hard X-ray multiplexing in the Linac Coherent Light Source (LCLS) Hard X-ray Undulator Line (HXU) using microbunch rotation through a 10 microrad first-order-achromatic bend created by transversely offsetting quadrupole magnets in the FODO lattice. Quadrupole offsets are determined analytically from beam-matrix theory. We also discuss the application of microbunch rotation to out-coupling a cavity-based XFEL (CBXFEL) [1].

Cavity-Based X-ray Free-Electron Lasers (CBXFELs) employ an X-ray cavity formed by crystal mirrors such that X-ray pulses receive periodic FEL-amplification and Bragg-monochromatization. CBXFELs enable improved longitudinal coherence and spectral brightness over single-pass self-amplification of spontaneous radiation (SASE) FELs [1,2] for high-repetition rate FELs. Construction and alignment of a stable low-loss cavity of Bragg-reflecting mirrors has been considered a daunting challenge and has not seen previous experimental implementation of large X-ray cavities in the hard X-ray regime. In this work, we demonstrate stable operation of a low loss 14-m-roundtrip rectangular cavity of four Bragg-reflecting diamond (400) mirrors. 9.831 keV X-rays from the Linac Coherent Light Source (LCLS) were in-coupled into the cavity via a thin diamond transmission grating. X-ray ring-down was characterized using fast photodiodes and a nanosecond-gated camera. Intra-cavity focusing was introduced to further stabilize the cavity, enabling observation of X-ray storage at >50 round trips. This experiment demonstrates feasibility of a stable low-loss hard X-ray cavity that will support future CBXFEL tests and operation [3].