Abstract
The chirp-taper free-electron laser (FEL) scheme has been proposed to generate subfemtosecond broadband pulses using laser-modulated electron bunches in x-ray FELs. In this paper, we extend the chirp-taper concept to the postsaturation superradiant regime by combining an energy-modulated beam with a sinelike undulator taper. We show that subfemtosecond duration pulses with peak powers approaching a terawatt may be obtained with typical x-ray FEL parameters such as those at the Linac Coherent Light Source. The method shown here can produce a stable source of attosecond-scale, high-power x-ray pulses suitable for probing electronic dynamics within molecules.
Highlights
Subfemtosecond x-ray pulses are highly desirable, as they enable the investigation of electron dynamics within atoms, molecules, and solids by probing samples faster than typical valence electron evolution timescales [1]
High-harmonic generation in gas jets is capable of producing attosecond pulses [2,3], yet the produced pulse energies are typically limited to nanojoules for extreme ultraviolet and picojoules for x rays
Superradiant amplification via resonant energy slicing presented in this paper offers unique benefits compared to existing short-pulse free-electron laser (FEL) techniques
Summary
Subfemtosecond x-ray pulses are highly desirable, as they enable the investigation of electron dynamics within atoms, molecules, and solids by probing samples faster than typical valence electron evolution timescales [1]. Simulation studies have shown that subfemtosecond x-ray pulses can be generated by modulating an electron bunch with a high-power infrared laser and employing the chirp-taper matching condition [17] Whereas these studies have investigated chirped-tapered FELs up to the saturation point, extending the interaction by tapering the undulator to maintain resonance as the radiation slips past the initial lasing region to interact with fresh electrons leads to superradiant amplification of the initial pulse. SASE radiation pulses can form along the resonant part of the beam, spoiling the gain of the superradiant pulse by increasing the electron beam’s energy spread This competing effect can be suppressed in a sinusoidal chirp-taper setup, since the resonance condition is met only at specific locations in the electron bunch, suppressing the FEL instability everywhere else in the bunch. Whereas fresh-bunch or fresh-slice FEL schemes can be used for pulse shortening via superradiance at a cost of skipping electrons during the delay, continuous resonant energy slicing allows all electrons the pulse passes to contribute to the lasing for greater overall power conversion efficiency
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