We propose a novel method to characterize the temporal duration and shape of femtosecond x-ray pulses in a free-electron laser (FEL) by measuring the time-resolved electron-beam energy loss and energy spread induced by the FEL process, with a transverse radio-frequency deflector located after the undulator. Its merits are simplicity, high resolution, wide diagnostic range, and noninvasive to user operation. When the system is applied to the Linac Coherent Light Source, the world’s most powerful x-ray FEL, it can provide single-shot measurements of the electron-beam and x-ray pulses with a resolution on the order of 1–2 femtoseconds rms.
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