The NIGMS structural biology facility at the NSLS

FLASH at DESY in Hamburg, the first and currently only free-electron laser for VUV and soft X-ray radiation, started user operation in the summer of 2005. Currently it covers a wavelength range from 13 nm to 50 nm with GW peak power and pulse durations between 10 fs and 50 fs. Approximately 20 weeks of beamtime are provided per year for scientific experiments that are not possible on any other radiation source.     

Self-seeded FEL wavelength extension with high-gain harmonic generation

We study a self-seeded high-gain harmonic generation(HGHG) free-electron laser(FEL) scheme to extend the wavelength of a soft X-ray FEL. This scheme uses a regular self-seeding monochromator to generate a seed laser at the wavelength of 1.52 nm, followed by a HGHG configuration to produce coherent, narrow-bandwidth harmonic radiations at the GW level. The 2nd and 3rd harmonic radiation is investigated with start-to-end simulations.Detailed studies of the FEL performance and shot-to-shot fluctuations are presented.     

X射线自由电子激光试验装置

X射线自由电子激光试验装置(以下简称"SXFEL试验装置")是中国第一台X射线相干光源,其输出波长小于9 nm。这台基于0.84 GeV直线加速器、以掌握装置相关技术和实验演示种子型自由电子激光(FEL)级联与短波长回声型FEL为主要目标的自由电子激光装置,于2020年11月通过国家验收。本文将介绍SXFEL试验装置的基本情况和主要进展。     

First observation of self-amplified spontaneous emission in a free-electron laser at 109 nm wavelength

We present the first observation of self-amplified spontaneous emission (SASE) in a free-electron laser (FEL) in the vacuum ultraviolet regime at 109 nm wavelength (11 eV). The observed free-electron laser gain (approximately 3000) and the radiation characteristics, such as dependency on bunch charge, angular distribution, spectral width, and intensity fluctuations, are all consistent with the present models for SASE FELs.     

First ultraviolet high-gain harmonic-generation free-electron laser

We report the first experimental results on a high-gain harmonic-generation (HGHG) free-electron laser (FEL) operating in the ultraviolet. An 800 nm seed from a Ti:sapphire laser has been used to produce saturated amplified radiation at the 266 nm third harmonic. The results confirm the predictions for HGHG FEL operation: stable central wavelength, narrow bandwidth, and small pulse-energy fluctuation.     

Small rare gas clusters in XUV laser pulses

Semi-classical molecular dynamics simulations of small rare gas clusters in short laser pulses of 100 nm wavelength were performed. For comparison, the cluster response to 800 nm laser pulses was investigated as well. The inner ionization dynamics of multi-electron atoms inside the cluster was treated explicitly. The simulation results underpin the belief that at XUV (extreme ultraviolet) wavelengths collisions play an important role in the energy absorption. The generation of the surprisingly high charge states of Xe atoms inside clusters, as they were observed in a free-electron laser experiment at DESY, Hamburg, Germany by Wabnitz et al. is due to the reduced ionization potential of atoms inside charged clusters, the ionization ignition mechanism, and collisions. PACS 36.40.Gk; 36.40.Vz; 31.15.Gy     

Generation of Coherent Radiation in the Near X-Ray Band by a Cascade FEL with a Two-Frequency Undulator

Generation of X-ray radiation in a cascade self-amplified spontaneous emission free-electron laser (SASE FEL) using the harmonics of a two-frequency undulator is studied. The advanced phenomenological model of a one-pass FEL that accounts for the main losses in real FELs is presented: the electron energy spread in the beam, the beam divergence, diffraction, and the fact that emission losses are greater at higher harmonics than in the main frequency range. The FEL mathematical model was performed using the Mathematica software and calibrated within the experiment carried out at the operating SPARC facility via complex three-dimensional numerical simulations. The phenomenological model is used to analyze FEL dynamics for generation of a high-energy X-ray emission at a relatively short length. It is proposed to use a two-frequency undulator for the initial electron grouping and subsequent frequency multiplication in a cascade FEL with higher harmonic amplification (HGHG). The advantages of the two-frequency undulator are presented for electron grouping at higher harmonics of the undulator radiation (UR). The operation of several types of FEL is simulated with amplification of the seed laser wave frequency in two and three cascades to generate the soft X-ray radiation. A seed laser with a wavelength of 11.43 nm corresponding to the peak reflectivity of mirror coatings with MoRu/Be is proposed for generating the intensive X-ray laser radiation with λ ~ 1.27–3.37 nm. Here, the intensive radiation power reaches 50 MW at a length of only 35 meters; the radiation shows good temporal coherence corresponding to the performance of a low-power seed laser with a lower frequency.     

First Observation of z-Dependent Electron-Beam Microbunching Using Coherent Transition Radiation

We report the first measurements of the electron-beam microbunching z dependence in a self-amplified spontaneous-emission (SASE) free-electron laser (FEL) experiment by the observation of visible wavelength coherent transition radiation (CTR). In this case the fundamental SASE wavelength was at 537 nm, and the CTR exhibited an exponential intensity growth similar to the SASE radiation. In addition, we observed for the first time structure in the CTR angular distribution patterns that may be useful for optimizing SASE FEL performance.     

Acceleration complex for extreme ultraviolet nanolithography based on a free-electron laser with kilowatt-scale average radiation power

The project of an acceleration complex is described that is based on a 0.7-GeV superconducting linear accelerator for the free-electron laser used for extreme ultraviolet lithography at a 13.5-nm wavelength with a 0.5-kW average power of laser radiation, as well as for examination of materials using X-ray and vacuum-ultraviolet radiations.     

SASE自由电子激光

 SASE自由电子激光可以产生短至0.1nm的高亮度（峰值亮度比当前的第三代同步辐射高10个量级;平均亮度高3~5个量级）、短脉冲（脉冲长度小于2个量级、达到亚皮秒水平）硬X射线相干光。因而被称为是继第三代同步辐射之后的第四代光源。SASE依据的是高增益自由电子激光原理,利用了光阴极微波电子枪技术和电子直线加速器技术。综述了SASE的历史发展、基本原理、基本结构、主要物理特征和对电子束的要求。     

小周期波荡器自由电子激光器：数值模拟与实用化设计

采用三维自洽非线性理论计算方法,针对小周期波荡器自由电子激光器进行了数值模拟计算研究.结果表明,在我所现有脉冲线加速器基础上,采用周期为10mm的波荡器,可获得输出功率为20MW,波长为1.7mm的自由电子激光.最后给出实用化总体实验设计方案.     

Coherent single-cycle pulses with MV/cm field strengths from a relativistic transition radiation light source

Terahertz (THz) pulses with energies up to 100 μJ and corresponding electric fields up to 1 MV/cm were generated by coherent transition radiation from 500 MeV electron bunches at the free-electron laser Freie-Elektronen-Laser in Hamburg (FLASH). The pulses were characterized in the time domain by electro-optical sampling by a synchronized femtosecond laser with jitter of less than 100 fs. High THz field strengths and quality of synchronization with an optical laser will enable observation of nonlinear THz phenomena.     

Generation of GW radiation pulses from a VUV free-electron laser operating in the femtosecond regime

Experimental results are presented from vacuum-ultraviolet free-electron laser (FEL) operating in the self-amplified spontaneous emission (SASE) mode. The generation of ultrashort radiation pulses became possible due to specific tailoring of the bunch charge distribution. A complete characterization of the linear and nonlinear modes of the SASE FEL operation was performed. At saturation the FEL produces ultrashort pulses (30-100 fs FWHM) with a peak radiation power in the GW level and with full transverse coherence. The wavelength was tuned in the range of 95-105 nm.     

Multiple ionization of rare gas atoms irradiated with intense VUV radiation

The interaction of intense vacuum-ultraviolet radiation from a free-electron laser with rare gas atoms is investigated. The ionization products of xenon and argon atomic beams are analyzed with time-of-flight mass spectroscopy. At 98 nm wavelength and approximately 10(13) W/cm(2) multiple charged ions up to Xe6+ (Ar4+) are detected. From the intensity dependence of multiple charged ion yields the mechanisms of multiphoton processes were derived. In the range of approximately 10(12)-10(13) W/cm(2) the ionization is attributed to sequential multiphoton processes. The production of multiple charged ions saturates at 5-30 times lower power densities than at 193 and 564 nm wavelength, respectively.     

Photon Sources at DESY

During the past 10 years, photon science activities at DESY in Hamburg, Germany, have expanded significantly and this development is expected to continue in the coming years. The soft X-ray free-electron laser (FEL) FLASH has been in user operation for over 10 years and the high-brilliance hard X-ray synchrotron radiation source PETRA III started serving the user community five years ago. Access to both light sources has since been highly demanded by scientists not only from Germany and Europe, but from all over the world. The request for beamtime far exceeded the capacity of available experimental infrastructure at both facilities and, for this reason, it was necessary to add further beamlines and also to broaden the portfolio of techniques. Therefore, new facilities have recently been built to almost double the existing capacity for user beamtime at both sources (Figure 1).     

Crossed beam photodissociation imaging of HeH+ with vacuum ultraviolet free-electron laser pulses

Molecular photofragmentation has been studied by event imaging on HeH+ ions at 32 nm (38.7 eV) in a fast ion beam crossed with the free-electron laser in Hamburg (FLASH), analyzing neutral He product directions and energies. Fragmentation into He(1snl,n > or = 2)+H+ was observed to yield significant photodissociation at 32 nm with an absolute cross section of (1.4+/-0.7) x 10(-18) cm2, releasing energies of 10-20 eV. A clear dominance of photodissociation perpendicular to the laser polarization was found in contrast to the excitation paths so far emphasized in theoretical studies.     

Lasing of middle-infrared free-election lasers using the storage ring NIJI-IV

We report for the first time to our knowledge the experimental realization of a storage ring free-electron laser (FEL) in the middle-infrared (MIR) region. A technique to adjust the optical cavity using higher harmonic FELs was developed for a fundamental FEL in the MIR region. The MIR FELs were oscillated in the wavelength region of 2475 to 2673 nm, and the relative linewidth was 5×10??. A quasi-monochromatic x-ray beam with an energy of 700 keV was generated using FEL Compton backscattering. We were able to realize a quasi-monochromatic x-ray beam, whose energy is difficult to generate even in advanced synchrotron radiation facilities.     

Evidence for microbunching "sidebands" in a saturated free-electron laser using coherent optical transition radiation

We report the first measurements of z-dependent coherent optical transition radiation (COTR) due to electron-beam microbunching at high gains ( >10(4)) including saturation of a self-amplified spontaneous emission free-electron laser (FEL). In these experiments the fundamental wavelength was near 530 nm, and the COTR spectra exhibit the transition from simple spectra to complex spectra ( 5% spectral width) after saturation. The COTR intensity growth and angular distribution data are reported as well as the evidence for transverse spectral dependencies and an "effective" core of the beam being involved in microbunching.     

Undulators and generation of X-ray pulses in free-electron lasers with self-amplified spontaneous emission

The results of theoretical examination and comparative analysis of synchrotron radiation sources (specifically, undulators and X-ray free-electron lasers (FELs)) are presented. The problem of generation of shorter radiation pulses is prioritized; undulator systems and their corresponding FELs, which are considered to be the most promising in terms of generation of high-frequency ultrashort pulses of such radiation (in particular, in the X-ray range) are studied. The possibility of generation of higher harmonics is explored. The advantages and disadvantages of single-pass (with no reflecting elements) and multi-pass (with mirrors) FEL lasing schemes are revealed. The potential to reduce the duration of laser pulses produced by undulators and FELs and use them as sources of femtosecond pulses is investigated. The prospects for further development of X-ray free-electron lasers and the ways to improve the quality of their radiation with the given parameters are discussed.