Self-amplified spontaneous emission free-electron laser with an energy-chirped electron beam and undulator tapering

We report the first experimental implementation of a method based on simultaneous use of an energy chirp in the electron beam and a tapered undulator, for the generation of ultrashort pulses in a self-amplified spontaneous emission mode free-electron laser (SASE FEL). The experiment, performed at the SPARC FEL test facility, demonstrates the possibility of compensating the nominally detrimental effect of the chirp by a proper taper of the undulator gaps. An increase of more than 1 order of magnitude in the pulse energy is observed in comparison to the untapered case, accompanied by FEL spectra where the typical SASE spiking is suppressed.     

北京大学SASE FEL的参数优化研究

SASEFEL是获得短波长X射线激光的最佳途径,SASEFEL的理论和实验研究是当前FEL研究的热点.本文利用北京大学超导加速器装置提供的电子束,通过解析理论和3D模型的数值模拟方法得到了实现SASEFEL的扭摆器的优化参数,并讨论了电子束的束流品质参数对SASEFEL的饱和长度和功率的影响,对加速器的设计和调试有重要的参考意义.     

An analysis of undulator optimization in self-seeding free electron lasers

A simple analysis is given for the optimum length of undulator in a self-seeding free electron laser(FEL).The obtained relations show the correlation between the undulator length and the system parameters.The power required for the seeding in the second part of the undulator and the overall efficiency of monochromatizating the seeding determine the length of the first part of the undulator;the magnitude of seeding power dominates the length of the second part of the undulator;the whole length of the undulator in a self-seeding FEL is determined by the overall efficiency for getting coherent seed,and is about half as long again as that of SASE,not including the dispersion section.The requirement of the dispersion section strength is also analyzed.     

磁压缩对高频稳定性的要求

 曲柄式磁压缩系统是北京大学SASE自由电子激光装置中非常重要的部分,通过其对电子束团的压缩为扭摆器提供高流强、短脉冲的电子束,使电子束在扭摆器内较短的距离实现饱和出光。曲柄式磁压缩需要利用偏离高频峰位的加速相位使得电子束产生能量-位置关联,主要讨论高频相位抖动与能量-位置关联的相互关系,高频相位抖动使得束团的能量-位置关联不同,即束团内电子能量随位置分布不同。进而研究其对磁压缩性能的影响,即能量-位置关联不一样会导致磁压缩得到的束团长度出现涨落。     

交叉型波荡器软X射线自由电子激光极化控制的影响因素

 交叉型波荡器是一种实现软X射线自由电子激光极化控制的有效方式。以自放大自发辐射自由光电子激光为例,采用统计的方法系统地分析了交叉型波荡器软X射线自由电子激光极化控制的影响因素。通过对光场相干长度、光脉冲两分量之间相对滑移长度和光场分量平均功率差异等的分析,给出了优化交叉型波荡器极化控制方案遵循的原则,即：辐射场相干时间尽可能长,光场分量相对滑移长度尽可能短,辐射场分量功率差异尽可能小等。该原则为交叉型波荡器软X射线自由电子激光极化控制方案的优化提供了依据。     

Fully coherent x-ray pulses from a regenerative-amplifier free-electron laser

We propose and analyze a regenerative-amplifier free-electron laser (FEL) to produce fully coherent, hard x-ray pulses. The method makes use of narrow-bandwidth Bragg crystals to form an x-ray feedback loop around a relatively short undulator. Self-amplified spontaneous emission (SASE) from the leading electron bunch in a bunch train is spectrally filtered by the Bragg reflectors and is brought back to the beginning of the undulator to interact repeatedly with subsequent bunches in the bunch train. The FEL interaction with these short bunches regeneratively amplifies the radiation intensity and broadens its spectrum, allowing for effective transmission of the x rays outside the crystal bandwidth. The spectral brightness of these x-ray pulses is about 2 to 3 orders of magnitude higher than that from a single-pass SASE FEL.     

Possible application of X-ray optical elements for reducing the spectral bandwidth of an X-ray SASE FEL

A new design for a single pass X-ray Self-Amplified Spontaneous Emission (SASE) FEL is proposed. The scheme consists of two undulators and an X-ray monochromator located between them. The first stage of the FEL amplifier operates in the SASE linear regime. After the exit of the first undulator the electron bunch is guided through a non-isochronous bypass and the X-ray beam enters the monochromator. The main function of the bypass is to suppress the modulation of the electron beam induced in the first undulator. This is possible because of the finite value of the natural energy spread in the beam. At the entrance to the second undulator the radiation power from the monochromator dominates significantly over the shot noise and the residual electron bunching. As a result the second stage of the FEL amplifier operates in the steady-state regime when the input signal bandwidth is small with respect to that of the FEL amplifier. Integral losses of the radiation power in the monochromator are relatively small because grazing incidence optics can be used. The proposed scheme is illustrated for the example of the 6 nm option SASE FEL at the TESLA Test Facility under construction at DESY. As shown in this paper the spectral bandwidth of such a two-stage SASE FEL (Δλ/λ 5 × 10⁻⁵) is close to the limit defined by the finite duration of the radiation pulse. The average brilliance is equal to 7 × 10²⁴ photons/(s × mrad² × mm² × 0.1% bandw.) which is by two orders of magnitude higher than the value which could be reached by the conventional SASE FEL. The monochromatization of the radiation is performed at a low level of radiation power (about 500 times less than the saturation level) which allows one to use conventional X-ray optical elements (grazing incidence grating and mirrors) for the monochromator design.     

Diagnostic technique applied for FEL electron bunches

Diagnostic technique applied for FEL ultrashort electron bunches is developed at JINR-DESY collaboration within the framework of the FLASH and XFEL projects. Photon diagnostics are based on calorimetric measurements and detection of undulator radiation. The infrared undulator constructed at JINR and installed at FLASH is used for longitudinal bunch shape measurements and for two-color lasing provided by the FIR and VUV undulators. The pump probe experiments with VUV and FIR undulators provide the bunch profile measurements with resolution of several femtosecond. The new three microchannel plates (MCP) detectors operated in X-ray range are under development now in JINR for SASE1-SASE 3 European XFEL.     

Specific features of operation of free-electron lasers with a short bunch

The physical specific features of a free-electron laser in the short-wave region in which the bunch length in the proper frame is smaller than the undulator length in the same reference frame are discussed. The regimes of amplification of the external harmonic signal and the induced amplification of proper noise with forming the quasi-coherent radiation, i.e. the self-amplified spontaneous emission (SASE) regime, are considered.     

A high-harmonic generation source for seeding a free-electron laser at 38 nm

Direct seeding with a high-harmonic generation (HHG) source can improve the spectral, temporal, and coherence properties of a free-electron laser (FEL) and shall reduce intensity and arrival-time fluctuations. In the seeding experiment sFLASH at the extreme ultraviolet FEL in Hamburg FLASH, which operates in the self-amplified spontaneous emission mode (SASE), the 21st harmonic of an 800 nm laser is refocused into a dedicated seeding undulator. For seeding, the external light field has to overcome the noise level of SASE; therefore, an efficient coupling between seed pulse and electron bunch is mandatory. Thus, an HHG beam with a proper divergence, width, beam quality, Rayleigh length, pointing stability, single-shot pulse energy, and stability in the 21st harmonic is needed. Here, we present the setup of the HHG source that seeds sFLASH at 38.1 nm, the optimization procedures, and the necessary diagnostics.     

Optimization considerations for a SASE free electron laser based on a superconducting undulator

A self amplified spontaneous emission (SASE) free electron laser (FEL) based on a new generation superconducting planar undulator, is optimized. It is shown that the laser wavelength should be down to soft X-rays range (～2–3 nm) of the spectrum via a dedicated undulator driven by a 1 GeV electron linear accelerator (linac). Numerical calculations and simulation results of the three main performance parameters for SASE operation, namely 1D gain length (L_G,1D), saturation power (P_sat) and saturation length (L_sat), are compared and discussed.     

Observation of anomalously large spectral bandwidth in a high-gain self-amplified spontaneous emission free-electron laser

Observation of ultrawide bandwidth, up to 15% full-width, high-gain operation of a self-amplified spontaneous emission free-election laser (SASE FEL) is reported. This type of lasing is obtained with a strongly chirped beam (deltaE/E approximately 1.7%) emitted from the accelerator. Because of nonlinear pulse compression during transport, a short, high current bunch with strong mismatch errors is injected into the undulator, giving high FEL gain. Start-to-end simulations reproduce key features of the measurements and provide insight into mechanisms, such as angular spread in emitted photon and electron trajectory distributions, which yield novel features in the radiation spectrum.     

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.     

Experimental characterization of nonlinear harmonic radiation from a visible self-amplified spontaneous emission free-electron laser at saturation

Nonlinear harmonic radiation was observed using the VISA self-amplified, spontaneous emission (SASE) free-electron laser (FEL) at saturation. The gain lengths, spectra, and energies of the three lowest SASE FEL modes were experimentally characterized. The measured nonlinear harmonic gain lengths and center spectral wavelengths decrease with harmonic number, n, which is consistent with nonlinear harmonic theory. Both the second and third nonlinear harmonics energies are about 1% of the fundamental energy. These experimental results demonstrate for the first time the feasibility of using nonlinear harmonic SASE FEL radiation to produce coherent, femtosecond x rays.     

A SASE free electron laser optimization based on new generation in-vacuum undulators

Optimization studies for an accelerator based light source, namely self-amplified spontaneous emission (SASE) free electron laser (FEL), based on new generation in-vacuum hybrid and superconducting undulator configurations, are compared and discussed. It is shown that the FEL wavelength should be down to soft X-rays ($～3\mathrm{nm}$) part of the spectrum while keeping the same linear accelerator (linac) energy about 1 GeV. On the other hand, numerical calculations and simulation results of the main performance parameters for SASE operation (1D gain length, saturation power and saturation length), are optimized. Finally, technological advantages and challenges for both cases, are briefly mentioned.     

Observation of an optical vortex beam from a helical undulator in the XUV region

The observation of an optical vortex beam at 60 nm wavelength, produced as the second‐harmonic radiation from a helical undulator, is reported. The helical wavefront of the optical vortex beam was verified by measuring the interference pattern between the vortex beam from a helical undulator and a normal beam from another undulator. Although the interference patterns were slightly blurred owing to the relatively large electron beam emittance, it was possible to observe the interference features thanks to the helical wavefront of the vortex beam. The experimental results were well reproduced by simulation.     

Time-resolved phase measurement of a self-amplified free-electron laser

We report on the first time-resolved phase measurement on self-amplified spontaneous emission (SASE) free-electron laser (FEL) pulses. We observed that the spikes in the output of such free-electron laser pulses have an intrinsic positive chirp. We also observed that the energy chirp in the electron bunch mapped directly into the FEL output. Under certain conditions, the two chirps cancel each other. The experimental result was compared with simulations and interpreted with SASE theory.     

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.     

High energy gain of trapped electrons in a tapered, diffraction-dominated inverse-free-electron laser

Energy gain of trapped electrons in excess of 20 MeV has been demonstrated in an inverse-free-electron-laser (IFEL) accelerator experiment. A 14.5 MeV electron beam is copropagated with a 400 GW CO2 laser beam in a 50 cm long undulator strongly tapered in period and field amplitude. The Rayleigh range of the laser, approximately 1.8 cm, is much shorter than the undulator length yielding a diffraction-dominated interaction. Experimental results on the dependence of the acceleration on injection energy, laser focus position, and laser power are discussed. Simulations, in good agreement with the experimental data, show that most of the energy gain occurs in the first half of the undulator at a gradient of 70 MeV/m and that the structure in the measured energy spectrum arises because of higher harmonic IFEL interaction in the second half of the undulator.     

Self-Seeding XFELs: Operation Principle and Challenges

A high-gain free electron laser gradually became one of the most promising hard X-ray sources after its experimental demonstration in 1997. The baseline mode of operation since then remains the self-amplified spontaneous emission (SASE), which is based on the shot noise amplification. Numerous statistically independent modes emerge in the electron density modulation of the electron beam and, as a result, in the temporal structure of the pulse. In a radiation spectrum, the same number of modes would be present (Figure 1(a)). In this way, SASE radiation has a poor temporal coherence.