# X-ray Free Electron Laser Accelerator Lattice Design Using Laser-Assisted Bunch Compression
## 1 Introduction
In our accelerator design for MaRIE XFEL using an LABC scheme, the first stage of the electron bunch compression employed the bulk compression method as is the case in the conventional scheme using a chicane bunch compressor. For the second stage of the bunch compression, our scheme used an LABC method, which was accomplished in two steps [17; 18; 19]. In the first step, the beam was modulated by a laser modulator downstream of the first bunch compressor; in the second step, a C-chicane bunch compressor converted the energy modulation formed in the beam by the laser modulator into a longitudinal electron density modulation, creating periodic, high current spikes with intervals equal to the laser wavelength. It was the electrons residing inside the high current spikes that participated in the effective X-ray lasing in the undulator. The two steps of our LABC method were not necessarily occurring at the same beam energy. Because the bunch compressor following the laser modulator entailed only a small \(R_{56}\) value, it could be located downstream of the last linac section with the beam at the final lasing energy [17]. Architecture compactness was also pursued in the design of the MaRIE XFEL while using an LABC scheme. Compact XFEL was demonstrated with the SPring-8 Angstrom Compact free electron LASer (SACLA) [28] and at the SwissFEL XFEL facility [29; 30], assisted with high-gradient C-band normal conducting linac structures. Recent progress in the application of a laser wakefield accelerator in the demonstration of free electron lasing has provided an alternative approach for achieving compact XFEL [31]. In our accelerator lattice design, the linac sections were assumed to be built with cryogenic copper RF high-gradient accelerator cavities [32; 33], which significantly shortened the length of the linac sections, enabling an overall compact XFEL architecture. Joint experimental research by UCLA and SLAC found that at X-band, cryogenic cooled copper accelerator structures could outperform the room temperature structures in terms of achieving a significantly higher accelerating gradient at the same breakdown rate. While the C-band cryogenic copper RF high gradient accelerator technology was still under development [19; 27; 34], we assumed here that each accelerator cell in the C-band linac sections was already capable of achieving an accelerating gradient of up to 120.0 MV/m. The simulations in the work reported in this paper were performed entirely using Elegant code [35], assisted by in-house Python [36] scripts. The content of this paper is arranged as follows. An overview of the accelerator lattice configuration, including the major specifications, is provided in Section 2. The overview addresses the beamline elements in the order of the geometry, from the electron bunch entering the first section of the linac at the starting point to the beam entering the X-ray undulator at the end. Section 3 through Section 5 elaborates on the designs of the beamline elements by their different categories, in the order of the laser modulator, the bunch compressors, and the linac sections, respectively. A final beam analysis at the entrance of the X-ray undulator is presented in Section 6, followed by the conclusions in Section 7.