B. W. J. McNeil, N. R. Thompson, D. J. Dunning, B. Sheehy
A model is presented which demonstrates that the attosecond pulse structure of a High Harmonic Generation (HHG) seed may be retained through to saturation in an FEL amplifier. At wavelengths of ~12nm a train of attosecond pulses of widths ~300 attoseconds with peak powers in excess of 1 GW are predicted from full 3D simulation. Methods for improving these results are discussed.
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The successful operation of the hard x-ray self-seeding experiment at the LCLS opens the era of fully coherent hard x-ray free electron lasers (FELs). However, the shot-to-shot radiation fluctuation is still a serious issue. In this paper, high-gain, single-pass x-ray FEL schemes seeded by the narrow bandwidth radiation signal from an x-ray FEL oscillator were proposed and investigated, which are expected to generate high brightness, fully coherent and stable x-ray pulse. A simple model has been developed to figure out the temporal and the spectral structures of the output pulses in x-ray FEL …
We present for the first time the concept of a seeded γ quantum Free-Electron-Laser (QFEL) at 478 keV, which has very different properties compared to a classical. The basic concept is to produce a highly brilliant γ beam via SASE. To produce highly intense and coherent γ beam, we intend to use a seeded FEL scheme. Important for the production of such a γ beam are novel refractive γ -lenses for focusing and an efficient monochromator, allowing to generate a very intense and coherent seed beam. The energy of the γ beam is 478 keV, corresponding to a wavelength in the sub-Ångstrøm regime (1/38 Å…
Ultra-high harmonic conversion of a seeded free-electron laser via harmonic optical klystron
本文提出一种结合谐波光学速调管与EEHG的新方法，将FEL的谐波转换阶数提升至约90，能产生波长3 nm的强且几乎完全相干的FEL脉冲，且所需种子激光强度更低，利于高重复率运行。
We study high-order harmonic generation (HHG) driven by seeded plasmonic-enhanced fields. On one hand, plasmonic-enhanced fields have shown a great potential to extend the HHG cutoff, an instrumental pre-requisite for the generation of attosecond pulses. On another hand, the use of XUV seeds appears to have a considerable potential to improve the HHG conversion efficiency, which is typically modest when a unique fundamental laser pulse is employed. By mixing these two sources, we show it is possible to, simultaneously, boost the HHG cutoff and to increase the harmonic photon flux. The combinat…
Independently from electron beam based procedures, photon beam based diagnostics is an alternative way for alignment and commissioning of the numerous undulator cells in a high-gain short-wavelength free-electron laser (FEL). In this paper, using the seed laser modulated electron beam and the undulator fine tuning technique, a coherent photon beam based diagnostic was proposed for seeded FEL, and some preliminary experimental results at Shanghai deep ultraviolet FEL test facility were presented. It demonstrates that the spatial distribution of the coherent harmonic radiation from one individua…
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 signific…
We demonstrate the ability to control and shape the spectro-temporal content of extreme-ultraviolet (XUV) pulses produced by a seeded free-electron laser (FEL). The control over the spectro-temporal properties of XUV light was achieved by precisely manipulating the linear frequency chirp of the seed laser. Our results agree with existing theory, which allows retrieving the temporal properties (amplitude and phase) of the FEL pulse from measurements of the spectra as a function of the FEL operating parameters. Furthermore, we show the first direct evidence of the full temporal coherence of FEL …
Self-enhanced coherent harmonic amplification in seeded free-electron lasers
提出基于自调制和超辐射的种子自由电子激光新方案，在MHz重复率下产生水窗范围内约2GW的飞秒软X射线脉冲，实验验证了高达15次谐波的自增强相干信号产生。
Spectral-phase interferometry for direct electric-field reconstruction applied to seeded extreme-ultraviolet free-electron lasers
本研究提出一种基于SPIDER技术的飞秒脉冲表征装置，通过种子激光简单排布实现双延迟频移脉冲副本，可良好复原极紫外种子型FEL脉冲的时间形状与相位，为FEL光谱相位的实验研究与调控提供支撑。
Recently, the self-modulation scheme of a weakly pre-bunched electron beam has been proposed [Yan et al., Physical Review Letters 126, 084801 (2021)], which is of great promise for high-repetition-rate seeded free-electron lasers (FELs), such as high-gain harmonic generation (HGHG). In this paper, the self-modulation scheme is systematically analyzed and optimized, and further experiments in which the self-modulator is resonant at the second harmonic of the seed laser are conducted. The three-dimensional numerical simulations show that the required seed laser intensity in the self-modulation s…
X-ray beams carrying orbital angular momentum (OAM) are an emerging tool for probing matter. Optical elements, such as spiral phase plates and zone plates, have been widely used to generate OAM light. However, these optics are challenging to use at x-ray free-electron lasers (XFELs) due to the high impinging intensities. Here, we propose a self-seeded FEL method to produce intense x-ray vortices. Unlike passive filtering after amplification, an optical element will be used to introduce the helical phase in the linear regime, significantly reducing the thermal load on the optical element. The g…
Besides the designed beam acceleration, the energy of electrons changed by the longitudinal wakefields in a real free-electron laser (FEL) facility, which may degrade FEL performances from the theoretical expectation. In this paper, with the help of simulation codes, the wakefields induced beam energy loss in the sophisticated undulator section is calculated for Shanghai soft X-ray FEL, which is a two-stage seeded FEL test facility. While the 1st stage 44 nm FEL output is almost not affected by the wakefields, it is found that a beam energy loss about 0.8 MeV degrades the peak brightness of th…
As a new-generation light source, free-electron lasers (FELs) provide high-brightness X-ray pulses at the angstrom-femtosecond space and time scales. The fundamental physics behind the FEL is the interaction between an electromagnetic wave and a relativistic electron beam in an undulator, which consists of hundreds or thousands of dipole magnets with an alternating magnetic field. Here, we report the first observation of the laser-beam interaction in a pure dipole magnet, in which the electron beam energy modulation with 40-keV amplitude and 266-nm period is measured. We demonstrate that such …
Intense extreme-ultraviolet (XUV) pulses enable the investigation of XUV-induced nonlinear processes and are a prerequisite for the development of attosecond pump - attosecond probe experiments. While highly nonlinear processes in the XUV range have been studied at free-electron lasers (FELs), high-harmonic generation (HHG) has allowed the investigation of low-order nonlinear processes. Here we suggest a concept to optimize the HHG intensity, which surprisingly requires a scaling of the experimental parameters that differs substantially from optimizing the HHG pulse energy. As a result, we are…
We propose an effective scheme for the generation of intense coherent extreme ultraviolet light beams carrying orbital angular momentum (OAM). The light is produced by a high-gain harmonic-generation free-electron laser (FEL), seeded using a laser pulse with a transverse staircase-like phase pattern. During amplification, diffraction and mode selection drive the radiation profile towards a dominant OAM mode at saturation. With a seed laser at 260 nm, gigawatt power levels are obtained at wavelengths approaching those of soft x-rays. Compared to other proposed schemes to generate OAM with FELs,…
We propose a method for a straightforward characterization of the temporal shape of femtosecond pulses in the extreme-ultraviolet/soft X-ray spectral region. The approach is based on the presence of a significant linear frequency chirp in the pulse. This allows to establish an homothetic relation between the pulse spectrum and its temporal profile. The theoretical approach is reminiscent of the one employed by Fraunhofer for describing far-field diffraction. As an application, we consider the case of a seeded free-electron laser (FEL). Theory is successfully benchmarked with numerical simulati…
Generation and Measurement of Ultrashort Free Electron Laser Pulses in Ultraviolet and Soft-X-ray Spectral Regions
本文报告了外种子自由电子激光器产生和测量紫外至软X射线超短相干脉冲的新进展，重点介绍了FERMI装置上少飞秒极紫外脉冲的产生与自相关表征。
White-light-seeded, CEP-stable, 4.5-W, 4-micron KTA parametric amplifier driven by a 1.4-ps Yb:YAG thin disk laser
本研究利用1.4 ps Yb:YAG薄片激光器驱动基于KTA的光参量放大器，实现长脉冲下超连续谱种子稳定产生，得到CEP稳定的高功率中红外脉冲，并验证了其在高次谐波产生中的应用潜力。
A 2D Holographic Approach for All-Optical Single-Shot Temporal Characterization of SASE FEL Pulses
该研究针对SASE自由电子激光脉冲的时间抖动问题，提出基于双盲全息术的全光单次方法，实现亚10飞秒脉冲的阿秒级精度波形重建与延迟标记，填补了相关技术空白。
Soft X-ray high-harmonic generation in an anti-resonant hollow core fiber driven by a 3 $μ$m ultrafast laser
本文利用3 μm中红外OPCPA激光驱动充气反谐振空芯光纤，在高效导波相位匹配结构中实现了高达碳K边（284 eV）的软X射线高次谐波产生，克服了传统波导高损耗与短相互作用长度的限制。
Extreme-ultraviolet (XUV) sources including high-harmonic generation, free-electron lasers, soft x-ray lasers and laser-driven plasmas are widely used for applications ranging from femtochemistry and attosecond science to coherent diffractive imaging and EUV lithography. The bandwidth of the XUV light emitted by these sources reflects the XUV generation process used. While light from soft-x-ray lasers and XUV FELs typically has a relatively narrow bandwidth, plasma sources and HHG sources driven by few-cycle laser pulses emit broadband XUV pulses. Since these characteristic properties of a giv…
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