# X-ray Free Electron Laser Accelerator Lattice Design Using Laser-Assisted Bunch Compression
## 1 Introduction  
An X-ray free electron laser (XFEL) is a light source that generates high brightness coherent X-ray pulses [1]. XFEL facilities, such as the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory, have found multidisciplinary applications in the scientific research fields of material science, biology, chemistry and physics [2; 3]. At Los Alamos National Laboratory (LANL), scientists have been investigating XFEL as a candidate technology for the Matter-Radiation Interactions in Extremes (MaRIE) initiative [4; 5], with the goal of eventually achieving the Dynamic Mesoscale Material Science Capability (DMMSC) [6]. The MaRIE XFEL requires 42 keV X-ray photon energy, \(5\times 10^{10}\) photons per pulse and \(2\times 10^{-4}\) fractional spectral bandwidth [5]. The major challenges facing the endeavor of pursuing an XFEL with lower electron beam emittance as well as lower energy spread in order to meet the MaRIE XFEL requirements include coherent synchrotron radiation (CSR) effects [7; 8; 9; 10] and microbunching instability (MBI) [11; 12; 13]. The undulator resistive wall wakes (URWW) [14; 15; 16] degrade the electron beam lasing performance in the X-ray undulator. In an effort to alleviate the above-mentioned negative effects commonly found in XFEL beamline designs that routinely use bulk compression schemes and to realize the conditions for MaRIE, an XFEL scheme using the approach of a laser-assisted bunch compression (LABC) was developed [17; 18; 19], based on the scheme of enhanced self-amplified spontaneous emission (ESASE). As a general concept, an LABC scheme utilizes the integration of linear logics and dispersive beamline elements to achieve electron bunchcompression [20], shaping the input bulk electron beam into a train of compressed periodic microbunches with a high peak current. One type of well-established LABC architecture is the combination of a laser modulator (LM) and a downstream chicane bunch compressor (BC), which was first proposed to implement enhanced self-amplified spontaneous emission (ESASE) for XFEL [21; 22; 23]. The original motivation of the ESASE method was to maximize the microbunch peak current, so as relieve the demanding requirement on the electron beam emittance for achieving gain length reduction. Inspired by the pioneering ESASE concept, our accelerator lattice design discussed in this paper used an LABC scheme with a laser modulator and a downstream chicane bunch compressor, as well; however, the goal of our design was different, and the parameter regime was more advanced. We prioritized the reduction of the beam instabilities, mentioned above, by investigating an electron beam with ultra-low emittance produced by a well-developed cryogenic normal conducting radiofrequency (RF) photoinjector [24; 25; 26; 27] at the very starting point of the accelerator lattice. The ultra-low emittance was preserved until the end of the beamline, where the beam also presented a small energy spread. The practicality of the novel cryogenic normal conducting RF photoinjectors has been revolutionary in the field of XFEL instrumentation. It immediately eliminated the necessity of maximizing the beam current in order to overcome the difficulty of achieving a short gain length due to the otherwise comparatively large beam emittance, as in the original description of ESASE [21; 22; 23]. Meanwhile, this novel photoinjector technology made it possible to explore the unprecedented LABC parameter space, leading to potentially improved XFEL performance as presented in this paper. Moreover, we used fine manipulation of the microbunch longitudinal phase space to carefully overcompress the microbunch so as to achieve a greater utilization rate of the beam charge than that in the previously reported ESASE design [17; 21] and to realize higher quality X-ray lasing. For narrative convenience and terminology consistency with prior studies [17; 18], we use LABC to refer to our beamline design developed from the ESASE scheme.