Fig. 1: Experimental setting of XPCS using split-and-delay optics and reflection self-seeding at BL3, SACLA18.

The seed pulse was produced by monochromatizing the SASE from the upstream undulator segments. Then the seed was amplified in the downstream undulators. The details are described in ref. 36. The amplified X-ray pulse was split into two sub-pulses using the SDO31. The sub-pulses with a delay time Δt are focused by mirrors40 and hit the water jet. Scattered X-rays were recorded by the MPCCDs42.