DPB Dissertation Award: Progress Towards Cavity-Based X-Ray Free-Electron Lasers: Demonstration of Cavity Ringdown and Microbunch Rotation Out-Coupling

Current X-ray Free-Electron Lasers (XFELs), including the Linac Coherent Light Source (LCLS) at SLAC, are single-pass SASE (self-amplified spontaneous emission) machines, where electrons produce X-rays as they pass through an undulator line just once. The exponential gain of X-ray power in an XFEL occurs as X-rays act on the electron beam, causing it to form microbunches at the X-ray wavelength. In a SASE XFEL, the X-rays which initiate this process come from noise in the beam, and thus X-rays produced by current XFELs are transversely coherent, but longitudinally noisy. In proposed cavity-based XFELs (CBXFELs), by contrast, Xrays are spectrally filtered and returned by mirrors from the end of the undulator line to the beginning, so a strong X-ray pulse is used to seed the FEL process on subsequent passes. Future X-ray light sources based on CBXFELs can thus improve longitudinal coherence and spectral brightness, and promise to be more stable than current XFELs. In this talk I discuss the requirements to build a CBXFEL cavity and several aspects of CBXFEL development. I cover the characterization of CBXFEL-quality diamond mirrors, two cavity out-coupling methods including active cavity Q-switching and microbunch rotation, and a demonstration of a 14m Bragg reflecting cavity.