Abstract
Free-electron lasers (FELs) seeded with external lasers hold great promise for generating high power radiation with nearly transform-limited bandwidth in the soft x-ray region. However, it has been pointed out that the initial seed laser phase error will be amplified by the frequency up-conversion process, which may degrade the quality of the output radiation produced by a harmonic generation scheme. In this paper, theoretical and simulation studies on frequency chirp amplification in seeded FEL schemes with slippage effect taken into account are presented. It is found that the seed laser imperfection experienced by the electron beam can be significantly smoothed by the slippage effect in the modulator when the slippage length is comparable to the seed laser pulse length. This smoothing effect allows one to preserve the excellent temporal coherence of seeded FELs in the presence of large frequency chirp in the seed laser. Our studies show that the tolerance on frequency chirp in the seed laser for generating nearly transform-limited soft x-ray pulses in seeded FELs is much looser than previously thought and fully coherent radiation at nanometer wavelength may be reached with current technologies.
Highlights
The recent success of self-amplified spontaneous emission (SASE) based x-ray free-electron laser (FEL) facilities [1,2] has paved the way for novel types of experiments in many scientific disciplines
We have shown that nearly transform-limited high harmonic radiation pulses can be generated via seeded FELs, when the slippage length in the modulator is comparable to the pulse length of the seed laser
By adopting a seed laser with pulse length comparable to the slippage length in the modulator, the initial frequency chirp induced by the imperfect seed laser pulse can be significantly smoothed and the production of nearly transform-limited radiation pulses is possible. 3D simulations have been carried out and the results confirm the theoretical predictions
Summary
The recent success of self-amplified spontaneous emission (SASE) based x-ray free-electron laser (FEL) facilities [1,2] has paved the way for novel types of experiments in many scientific disciplines. In seeded harmonic generation FELs, typically an external coherent seed laser pulse is first used to interact with electrons in a short undulator, called modulator, to produce energy modulation in the electron beam. The seed power and frequency variations as a function of time due to the slippage effect in the modulator were neglected This assumption is reasonable in the case of using an ideal seed laser with infinite pulse length and flat spectral phase distribution. Our studies show that the tolerance on laser frequency chirp for generating nearly transform-limited soft x-ray pulses in seeded FELs is much looser than that suggested in [15,16] and fully coherent radiation at nanometer wavelength may be reached with current technologies
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