Realization of ultra-stable hard X-ray Free Electron Laser

The use of electron-beam-based alignment incorporating undulator radiation spectrum analysis has allowed reliable operation of PAL-XFEL with unprecedented stability in terms of orbit, energy, and timing. A timing jitter of smaller than 20 fs for the FEL photon beam, a transverse position jitter of smaller than 10
^%
of the photon beam size, and a variation of FEL intensity of smaller than 5% are consistently achieved due to the use of state-of-the-art design of the electron linear accelerator and the 3-Be lattice less vulnerable to RF jitters. The low timing jitter of the electron beam makes it possible to observe Bi(111) phonon dynamics without the need for timing-jitter correction, indicating that PAL-XFEL will be an extremely useful tool for hard X-ray time-resolved experiments.