Generating intense fully coherent soft x-ray radiation based on a laser-plasma accelerator

Features and futures of X-ray free-electron lasers

Linear accelerator-based free-electron lasers (FELs) are the leading source of fully coherent X-rays with ultra-high peak powers and ultra-short pulse lengths. Current X-ray FEL facilities have proved their worth as useful tools for diverse scientific applications. In this paper, we present an overview of the features and future prospects of X-ray FELs, including the working principles and properties of X-ray FELs, the operational status of different FEL facilities worldwide, the applications supported by such facilities, and the current developments and outlook for X-ray FEL-based research.

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X-ray free-electron lasers (XFELs) generate X-ray by electrons flying through a periodic magnetic field.

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XFELs are the leading X-ray sources with ultra-high brightness and ultra-short duration.

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XFELs can be launched from either the shot noise of the electron beam or the seed.

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XFEL-laser collision is proposed to learn the nature of vacuum at SHINE.

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XFELs are being combined with intense lasers and synchrotron radiation light sources.

A coherent harmonic generation method for producing femtosecond coherent radiation in a laser plasma accelerator based light source

The electron beam generated in laser plasma accelerators (LPAs) has two main initial weaknesses - a large beam divergence (up to a few milliradians) and a few percent level energy spread. They reduce the beam brightness and worsen the coherence of the LPA-based light source. To achieve fully coherent radiation, several methods have been proposed for generating strong microbunching on LPA beams. In these methods, a seed laser is used to induce an angular modulation into the electron beam, and the angular modulation is converted into a strong density modulation through a beamline with nonzero longitudinal position and transverse angle coupling. In this paper, an alternative method to generate microbunching into the LPA beam by using a seed laser that induces an energy modulation and transverse-longitudinal coupling beamlines that convert the energy modulation into strong density modulation is proposed. Compared with the angular modulation methods, the proposed method can use more than one order of magnitude lower seed laser power to achieve similar radiation performance. Simulations show that with the proposed method a coherent pulse of a few microjoules pulse energy and femtosecond duration can be generated with a typical LPA beam.

Angular dispersion enhanced prebunch for seeding ultrashort and coherent EUV and soft X-ray free-electron laser in storage rings

Prebunching is an effective technique to reduce the radiation saturation length and to improve the longitudinal coherence and output stability in storage-ring-based free-electron lasers (FELs). A novel technique is proposed which uses angular dispersion to enhance the high-harmonic bunching with very small laser-induced energy spread. This technique can effectively reduce the radiation saturation length without significantly reducing the peak power of the FEL. Numerical simulations demonstrate that this technique can be used for the generation of 100 MW scale level, fully temporal coherent femtosecond extreme-ultraviolet and soft X-ray radiation pulses through a 10 m-long undulator based on a diffraction-limited storage ring.

Generation of ultrashort coherent radiation based on a laser plasma accelerator

A laser plasma accelerator (LPA) has the potential to realize compact free-electron laser (FEL) radiation at the regular laboratory scale. However, large initial angular divergence and energy spread dramatically hinder ways to transport the beam and realize FEL radiation. Although methods have been proposed to solve these problems, the relatively large jitter, including transverse position jitter and energy jitter, still limits the advance of these experiments. In this paper a simple method to realize coherent harmonic generation based on a LPA beam is proposed. The scheme is very compact, adopting a high-power laser split from the driver laser, a short modulator and a short radiator which has a great tolerance to these typical types of jitter. Numerical simulations indicate that coherent third-harmonic radiation with gigawatt-level power and single spike spectra can be obtained, verifying the feasibility of the scheme and indicating the capability to generate ultrashort fully coherent radiation.

Coherent X-ray source generation with off-resonance laser modulation

The generation of ultrashort coherent radiation in the extreme-ultraviolet (EUV) and soft X-ray regimes is of great significance in a broad range of research. In this paper, a promising scheme for generating coherent harmonic radiation using off-resonance seed laser modulation is discussed. The off-resonance seed laser, whose wavelength differs from the resonant wavelength of the undulator, is first used to modulate the angular distribution of the electron beam (e beam) in the undulator (modulator). After passing through a dispersion section, strong coherent micro-bunching is introduced into the e beam, which contains high-order harmonic components of the seed laser. Theoretical analysis and simulations indicate that this method can be used for the generation of coherent EUV and soft X-ray radiation at sub-gigawatt power in a meter-scale undulator (radiator).

High-Brightness High-Energy Electron Beams from a Laser Wakefield Accelerator via Energy Chirp Control

By designing a structured gas density profile between the dual-stage gas jets to manipulate electron seeding and energy chirp reversal for compressing the energy spread, we have experimentally produced high-brightness high-energy electron beams from a cascaded laser wakefield accelerator with peak energies in the range of 200-600 MeV, 0.4%-1.2% rms energy spread, 10-80 pC charge, and ∼0.2  mrad rms divergence. The maximum six-dimensional brightness B_{6D,n} is estimated as ∼6.5×10^{15}  A/m^{2}/0.1%, which is very close to the typical brightness of e beams from state-of-the-art linac drivers. These high-brightness high-energy e beams may lead to the realization of compact monoenergetic gamma-ray and intense coherent x-ray radiation sources.