Design of an X-ray undulator: optimization considerations for SASE operation

Accelerator-based fourth-generation light sources, namely, free-electron lasers (FELs), offer unique radiation characteristics, such as tunable, coherent, high-power, ultra-short pulses. They rely on novel technology with challenging parameters, from which the practicability is currently being proved by world-class facilities like the European XFEL, LCLS, FLASH, and SACLA. When contriving such superior light characteristics, "state-of-the-art" linear accelerator (linac) and undulator technologies come into prominence. In this respect, design and simulation studies for a planar X-ray undulator are considered to optimize the FEL generation process by self-amplified spontaneous emission (SASE). Three main performance parameters for SASE operation (i.e., 1D gain length, saturation power, and saturation length) are compared and discussed by means of numerical calculations and simulation results. It is shown that hard X-ray FEL pulses (down to sub-angstroms) are generable via in-vacuum hybrid undulators driven by an 8 GeV electron linac. (author)