Journal Article photon diagnostics FEL Research Support, U.S. Gov't, Non-P.H.S. free-electron lasers hard X-rays CVD diamond EuXFEL HIREX diagnostic spectrometer Huge Aperture MultiPlier Research Support, N.I.H., Extramural Research Support, Non-U.S. Gov't SASE Wigner X-ray free-electron laser X-ray gas monitors XFEL physics diffraction efficiency duration fixed elliptical geometry K–B mirrors free-standing diamond membrane grating structures hard X-ray micro-focusing optics instrumentation optics and detector beamline photoelectron spectrometer polarimeter soft X-rays spectrogram