Feasibility of a low-energy X-ray free-electron laser

X‐ray free‐electron lasers based on self‐amplified spontaneous emission promise users unprecedented X‐radiation that is extremely bright, extremely short and transversely coherent. However, hard X‐ray free‐electron laser facilities under construction are all huge and expensive, consisting of high‐energy linear accelerators and long undulators. The benefit of hard X‐ray free‐electron lasers may be limited to only a few regions in the world, unless it is possible to reduce the size. This paper discusses how small a hard X‐ray free‐electron laser facility can be. It is shown that a 1.5 Å X‐ray free‐electron laser is achievable using electron energy down to 4.5 GeV or lower if we use the third‐harmonic radiation, but at the expense of the transverse coherence.