Megahertz-compatible angular streaking with few-femtosecond resolution at x-ray free-electron lasers

Highly brilliant, coherent, femtosecond x-ray pulses delivered by free-electron lasers (FELs) constitute oneof the pillars of modern ultrafast science. Next generation FEL facilities provide up to megahertz repetitionrates and pulse durations down to the attosecond regime utilizing self-amplification of spontaneous emission.However, the stochastic nature of this generation mechanism demands single-shot pulse characterization toperform meaningful experiments. Here we demonstrate a fast yet robust online analysis technique capableof megahertz-rate mapping of the temporal intensity structure and arrival time of x-ray FEL pulses withfew-femtosecond resolution. We performed angular streaking measurements of both neon photo- and Augerelectrons and show their applicability for a direct time-domain feedback system during ongoing experiments.The fidelity of the real-time pulse characterization algorithm is corroborated by resolving isolated x-ray pulsesand double pulse trains with few-femtosecond substructure, thus paving the way for x-ray-pump–x-ray-probeFEL science at repetition rates compatible with the demands of LCLS-II and European XFEL