research papers 708 https://doi.org/10.1107/S1600577519003382 J. Synchrotron Rad. (2019). 26, 708–713 Received 21 December 2018 Accepted 9 March 2019 Edited by D. Cocco, SLAC National Accelerator Laboratory, USA 1 This article will form part of a virtual special issue containing papers presented at the PhotonDiag2018 workshop. Keywords: CVD diamond; free-standing diamond membrane; X-ray free-electron laser; photon diagnostics; diffraction efficiency; grating structures; HIREX diagnostic spectro- meter. Characterizing transmissive diamond gratings as beam splitters for the hard X-ray single-shot spectrometer of the European XFEL 1 Naresh Kujala, a * Mikako Makita, a Jia Liu, a Alexey Zozulya, a Michael Sprung, b Christian David c and Jan Gru ¨nert a a European XFEL GmbH, Holzkoppel 4, 22869 Schenefeld, Germany, b P10 Beamline, PETRA III, Deutsches Elektronen- Synchrotron (DESY), Notkestrasse 85, 22607 Hamburg, Germany, and c Paul Scherrer Institut (PSI), 5232 Villigen, Switzerland. *Correspondence e-mail: naresh.kujala@xfel.eu The European X-ray Free Electron Laser (EuXFEL) offers intense, coherent femtosecond pulses, resulting in characteristic peak brilliance values a billion times higher than that of conventional synchrotron facilities. Such pulses result in extreme peak radiation levels of the order of terawatts cm 2 for any optical component in the beam and can exceed the ablation threshold of many materials. Diamond is considered the optimal material for such applications due to its high thermal conductivity (2052 W mK 1 at 300 K) and low absorption for hard X-rays. Grating structures were fabricated on free-standing CVD diamond of 10 mm thickness with 500 mm silicon substrate support. The grating structures were produced by electron-beam lithography at the Laboratory for Micro- and Nanotechnology, Paul Scherrer Institut, Switzerland. The grating lines were etched to a depth of 1.2 mm, resulting in an aspect ratio of 16. The characterization measurements with X-rays were performed on transmissive diamond gratings of 150 nm pitch at the P10 beamline of PETRA III, DESY. In this paper, the gratings are briefly described, and a measured diffraction efficiency of 0.75% at 6 keV in the first-order diffraction is shown; the variation of the diffraction efficiency across the grating surface is presented. 1. Introduction Over the past decade, several X-ray free-electron laser (XFEL) facilities have been developed and constructed around the world. Two technologies have been pursued to construct electron accelerators for XFEL applications: warm, normal-conducting machines and cold, super-conducting accelerators (Emma et al., 2010; Allaria et al., 2012; Schreiber et al., 2012; Ishikawa et al., 2012; Harmand et al., 2013; Pelle- grini et al., 2015). The European XFEL (EuXFEL) in the Hamburg metropolitan area, Germany, is an X-ray source based on a super-conducting linear accelerator which started user operation in fall 2017 (Saldin et al., 2000; Altarelli et al., 2006; Tschentscher et al., 2017; Decking et al., 2019). EuXFEL operates with a 10 Hz bunch train structure. Each train can contain up to 2700 pulses. Individual X-ray pulses are of the order of 2 fs to 100 fs in duration, contain energy of the order of several mJ, and are separated by 222 ns (at the nominal intra-train repetition rate of 4.5 MHz). This unique time structure provides not only very high peak brilliance but also the highest average brilliance over all of the other XFEL facilities. There are in total three undulators that can produce self-amplified spontaneous emission (SASE) radiation, among which two hard X-ray undulators, SASE1 and SASE2, generate hard XFEL radiation up to 25 keV while the third undulator, SASE3, provides soft X-rays up to 3 keV. ISSN 1600-5775 # 2019 International Union of Crystallography