TEST OF AN X-RAY CAVITY USING DOUBLE-BUNCHES FROM THE LCLS Cu-Linac* K.-J. Kim † , L. Assoufid, R.R. Lindberg, X. Shi, D. Shu, Y. Shvyd’ko, M. White Argonne National Laboratory, Argonne, IL, USA F.-J. Decker, Z. Huang, G. Marcus, T. Raubenheimer, T.-F. Tan, D. Zhu SLAC National Accelerator Laboratory, Menlo Park, CA, USA Abstract We discuss a proposal to test the operation of an X-ray cavity consisting of Bragg reflectors. The test will consti- tute a major step demonstrating the feasibility of either an X-ray regenerative amplifier FEL or an X-ray FEL Oscil- lator. These cavity-based X-ray FELs will provide the full temporal coherence lacking in the SASE FELs. An X-ray cavity of rectangular path will be constructed around the first seven LCLS-II undulator units. The Cu-linac will pro- duce a pair of electron bunches separated by the cavity- round-trip distance during each linac cycle. The X-ray pulse produced by the first bunch is deflected into the cav- ity and returns to the undulator where it is amplified due to the presence of the second bunch. The key challenges are: the precision of the cavity mechanical construction, the quality of the diamond crystals, and the electron beam sta- bility. When the LCLS-II super-conducting linac becomes available, the cavity can then be used for high-repetition rate studies of the X-ray RAFEL and XFELO concepts. INTRODUCTION X-ray free electron lasers (XFELs) such as the LCLS [1], based on Self-Amplified Spontaneous Emission (SASE) [2], are capable of producing extremely bright, transversely coherent, ultra-short X-ray pulses suitable for the investigation of ultra-fast chemical and physical pro- cesses [3-5]. A characteristic feature of single-pass SASE FEL amplifiers, however, is poor longitudinal coherence. Temporally-coherent FEL pulses can be obtained by storing and recirculating the output of an amplifier in an X- ray cavity so that the X-ray pulse can interact with the fol- lowing fresh electron bunches over many passes. The X- ray cavity is formed by a set of narrow bandwidth diamond Bragg crystals, which provide high reflectivity and mono- chromatization. This is the concept behind the cavity-based X-ray FELs (CBXFELs) such as the X-ray free-electron laser oscillator (XFELO) [6] and the X-ray regenerative amplifier free-electron laser (XRAFEL) [7]. The supercon- ducting technology adopted by LCLS-II and LCLS-II-HE [8], will be capable of producing a constant stream of elec- tron bunches (rather than pulsed/burst mode) with repeti- tion rates up to 1 MHz, making the prospect of the CBXFEL realistic. The XFELO relies on a low-loss cavity supporting a low-gain FEL, While RAFEL leverages a high-gain FEL interaction. The defining properties of the XFELO are the extremely narrow and stable spectral bandwidths that can be as small as a few meV [9]. These characteristics conspire to push the average brightness of an XFELO source ~ 4 orders of magnitude higher than that of SASE at LCLS-II/-HE, as shown in Fig. 1[10]. Indeed, the ultrafine spectral capabil- ities along with the high spectral photon density enabled by the XFELO would be complementary to the ultrafast temporal capabilities and high temporal photon density of the hard-X-ray SASE FELs. The scientific case for an XFELO is discussed in Ref. [11]. The XRAFEL concept, on the other hand, excels at bridging the gap between the performance characteristics of an HXR SASE FEL and an XFELO while directly ad- dressing the science case that is targeted by the LCLS-II and LCLS-II-HE upgrades. The RAFEL system aims at the production of fully-longitudinally-coherent but shorter FEL pulses, and as such, has the unique ability to produce FEL X-ray pulses with both high average brightness and high peak brightness. The spectral output of a RAFEL is compared to SASE in Fig. 2. ___________________________________________ * Work supported by U.S. DOE, Office of Science, Office of BES, under Contract No. DE-AC02-06CH11357 (ANL) and DE-AC02-76SF00515 (SLAC) † kwangje@anl.gov Figure 1: Brightness of various X-ray sources. Figure 2: RAFEL and SASE. 10th Int. Particle Accelerator Conf. IPAC2019, Melbourne, Australia JACoW Publishing ISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-TUPRB096 MC2: Photon Sources and Electron Accelerators A06 Free Electron Lasers TUPRB096 1887 Content from this work may be used under the terms of the CC BY 3.0 licence (© 2019). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI