Abstract
There is growing interest in the generation and characterization of femtosecond and subfemtosecond pulses from linac-based free-electron lasers (FELs). In this report, following the method of Ricci and Smith [Phys. Rev. ST Accel. Beams 3, 032801 (2000)], we investigate the measurement of the longitudinal bunch profile of an ultrashort electron bunch produced by these FELs. We show that this method can be applied in a straightforward manner at x-ray FEL facilities such as the Linac Coherent Light Source by slightly adjusting the second bunch compressor followed by running the bunch on an rf zero-crossing phase of the final linac. We find that the linac wakefield strongly perturbs the measurement, and through analysis show that it can be compensated in a simple way. We demonstrate the effectiveness of this method and wakefield compensation through numerical simulations, including effects of coherent synchrotron radiation and longitudinal space charge. When used in conjunction with a high-resolution electron spectrometer, this method potentially reveals the temporal profile of the electron beam down to the femtosecond and subfemotsecond scale.
Highlights
The advent of x-ray free-electron lasers (FELs) is expected to revolutionize the ultrafast x-ray sciences
Techniques exist for measuring the coherent radiation spectrum of a short bunch in order to reconstruct its temporal profile
Information about the bunch length can be obtained from the statistical fluctuation of the incoherent radiation intensity [6,7]
Summary
The advent of x-ray free-electron lasers (FELs) is expected to revolutionize the ultrafast x-ray sciences. We show that this method can be adapted to linac-based FELs such as the LCLS; one needs only to (1) slightly adjust the final bunch compressor strength and (2) run the beam on the zero-crossing phase in the linac that follows. This method is insensitive to the initial beam energy spread or chirp and is more effective than rf zero phasing for very short bunches. It normally requires no extra hardware (e.g., a high-frequency deflecting cavity) in a linac-based FEL.
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