Abstract
Generation of attosecond x-ray pulses is attracting much attention within the x-ray free-electron laser (FEL) user community. We propose a novel scheme for the generation of coherent stable attosecond x-ray pulse trains in a seeded FEL, via a process of mode-locked amplification. Three modulators and two chicanes are used for generating separated attosecond scale microstructures in the electron beam using the beam echo effect. Such electron beam will produce high harmonic radiation with a comb of longitudinal modes at the very beginning of the radiator. By using a series of spatiotemporal shifts between the copropagating radiation and electron beam in the radiator, all these modes can be preserved and amplified to saturation. Using a representative realistic set of parameters, three-dimensional simulation results show that trains of 200 attosecond soft x-ray pulses with stable peak powers at gigawatt level can be generated directly from ultraviolet seed lasers. The even spacing between the attosecond pulses can be easily altered from subfemtosecond to tens of femtoseconds by slightly changing the wavelength of one seed laser.
Highlights
Ultrashort x-ray sources have the potential to open new regimes in atomic and electronic processes, benefiting widespread fields in physics, chemistry, and biology
We propose a novel scheme for the generation of coherent stable attosecond x-ray pulse trains in a seeded free-electron laser (FEL), via a process of mode-locked amplification
The electron beam is first energy modulated by a seed laser with wave number k1 in the first modulator (M1) and sent through a strong dispersion section (DS1), which makes the energy modulation induced in M1 macroscopically smeared out, but many well-structured beamlets appear in the phase space
Summary
Ultrashort x-ray sources have the potential to open new regimes in atomic and electronic processes, benefiting widespread fields in physics, chemistry, and biology. As with the conventional atomic laser, this mode locking modifies the temporal envelop of the output field from continuous wave to a train of short pulses, periodically spaced by the wavelength of the seed laser. As the radiation originates from the shot noise of the electron beam, these schemes may still have large shot-to-shot output power fluctuations and usually need a large number of undulator-chicane modules to produce a clean series of short pulses [22].
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