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Register a new userInterferometry for full temporal reconstruction of laser-plasma accelerator-based seeded free electron lasers
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The spectacular development of Laser-Plasma Accelerators (LPA) appears very promising for a free electron laser application. The handling of the inherent properties of those LPA beams already allowed controlled production of LPA-based spontaneous undulator radiation. Stepping further, we here unveil that the forthcoming LPA-based seeded FELs will present singular spatio-spectral distributions. Relying on numerical simulations and simple analytical models, we show how those interferometric patterns can be exploited to retrieve, in single-shot, the spectro-temporal content and source point properties of the FEL pulses.
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Free-electron lasers (FELs) seeded with external lasers hold great promise for generating high power radiation with nearly transform-limited bandwidth in soft x-ray region. However, it has been pointed out that the initial seed laser noise 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 for laser phase error 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 laser pulse length. This smoothing effect allows one to preserve the excellent temporal coherence of seeded FELs in presence of large laser phase errors. For ultra-short UV seed lasers with FWHM around 16 fs, the slippage length in a modulator with ~30 undulator periods is typically comparable to the laser width; for longer seed laser pulses with FWHM around 80 fs, the slippage length can be made comparable to the laser width using a modulator tuned at the sub-harmonic of the UV seed laser. Three-dimensional simulations have been carried out for a soft x-ray facility using seed laser pulses with large frequency chirp and the results show that the sensitivity of the bandwidth of the seeded FEL to the initial frequency chirp can be significantly reduced by a proper design of the modulator such that the slippage length is comparable to the laser width. Our studies show that the tolerance on laser phase error 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.
In this paper, we report results of simulations, in the framework of both EuPRAXIA cite{Walk2017} and EuPRAXIA@SPARC_LAB cite{Ferr2017} projects, aimed at delivering a high brightness electron bunch for driving a Free Electron Laser (FEL) by employing a plasma post acceleration scheme. The boosting plasma wave is driven by a tens of SI{}{terawatt} class laser and doubles the energy of an externally injected beam up to GeV{1}. The injected bunch is simulated starting from a photoinjector, matched to plasma, boosted and finally matched to an undulator, where its ability to produce FEL radiation is verified to yield $O( um{e11})$ photons per shot at m{2.7}.
The spectroscopic techniques for time-resolved fine analysis of matter require coherent X-ray radiation with femtosecond duration and high average brightness. Seeded free-electron lasers (FELs), which use the frequency up-conversion of an external seed laser to improve temporal coherence, are ideal for providing fully coherent soft X-ray pulses. However, it is difficult to operate seeded FELs at a high repetition rate due to the limitations of present state-of-the-art laser systems. Here, we report the novel self-modulation method for enhancing laser-induced energy modulation, thereby significantly reducing the requirement of an external laser system. Driven by this scheme, we experimentally realize high harmonic generation in a seeded FEL using an unprecedentedly small energy modulation. An electron beam with a laser-induced energy modulation as small as 1.8 times the slice energy spread is used for lasing at the 7th harmonic of a 266-nm seed laser in a single-stage high-gain harmonic generation (HGHG) setup and the 30th harmonic of the seed laser in a two-stage HGHG setup. The results mark a major step towards a high-repetition-rate, fully coherent X-ray FEL.
Laser Plasma Accelerators (LPA) can sustain GeV/m accelerating fields offering outstanding new possibilities for compact applications. Despite the impressive recent developments, the LPA beam quality is still significantly lower than in the conventional radio-frequency accelerators, which is an issue in the cases of demanding applications such as Free Electron Lasers (FELs). If the electron beam duration is below few tens of femtosecond keeping pC charges, the mrad level divergence and few percent energy spread are particularly limiting. Several concepts of transfer line were proposed to mitigate those intrinsic properties targetting undulator radiation applications. We study here the robustness of the chromatic matching strategy for FEL amplification at 200~nm in a dedicated transport line, and analyze its sensitivity to several parameters. We consider not only the possible LPA source jitters, but also various realistic defaults of the equipment such as magnetic elements misalignements or focussing strength errors, unperfect undulator fields, etc...
Plasma driven particle accelerators represent the future of compact accelerating machines and Free Electron Lasers are going to benefit from these new technologies. One of the main issue of this new approach to FEL machines is the design of the transfer line needed to match of the electron-beam with the magnetic undulators. Despite the reduction of the chromaticity of plasma beams is one of the main goals, the target of this line is to be effective even in cases of beams with a considerable value of chromaticity. The method here explained is based on the code GIOTTO [1] that works using a homemade genetic algorithm and that is capable of finding optimal matching line layouts directly using a full 3D tracking code.
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