纯永磁波荡器基波与谐波磁场的三维计算与测试

 对标淮的纯永磁波荡器进行了三维磁场的有限元计算,给出了纯永磁波荡器的基波与谐波磁场的幅值与相位,并与测试结果进行对比,结果表明,标准的纯永磁波荡器谐波磁场幅值比基波幅值低两个数量级。     

W波段同轴静磁波荡器自由电子激光优化设计

基于同轴静磁波荡器的自由电子激光主要利用环形电子束与同轴TE模式的相互作用产生电磁辐射,是一种重要的毫米波辐射源。分析了同轴结构作用区内外半径、工作模式、电子束电压、波荡器周期等参数对辐射频率的影响规律,研究了电子束平均半径的选取原则和束波互作用腔的设计方法,设计了辐射频率在W波段的基于同轴静磁波荡器的自由电子激光,并进行了粒子模拟,在电子束电压为720 kV,电子束流为1 kA的情况下,获得了频率107 GHz、辐射功率35 MW的TE01模,束波转换效率为4.9%,束波作用腔总长度小于200 mm,同轴静磁波荡器的磁场幅度0.34 T。     

CAEPXFEL波荡器束线初步物理设计

中国工程物理研究院计划开展XFEL光源的研制,主要通过电子束在波荡器中的辐射特性产生硬X射线。根据理论分析和数值模拟,在电子束最高能量可达12GeV的实际条件下,初步设计了一条间隙可调的真空外波荡器束线,波荡器分为25段,每段长3.86m,间隙7~10mm,周期长度25.4mm,磁场0.636~1.03T,束线总长120m左右,辐射光子能量为3~25keV。     

Experimental study of magnet undulators with pulsed-wire method

Experimental study of magnetic characteristics of planar magnet undulator for terahertz FEL has been carried out with use of pulsed-wire method. Special attention was given to the procedure of measurement of the second integral of undulator magnetic field, which shows variation of the beam position at passing the undulator magnetic system.     

混合型波荡器的3维优化

 以波荡器辐射波和共振电子能及混合型波荡器的解析计算为基础，通过3维磁场的有限元计算，在欧洲光源横向优化基础上，给出了同步辐射和自由电子激光用混合型波荡器纵向优化参数，进而给出了用于DUV FEL混合型波荡器的设计计算参数。加侧位和顶部永磁块后，峰值磁场分别提高到0.722T和0.773T，辅助以1μm量级分辨率的磁间隙调节机械系统，磁场分辨率好于10^-4T量级。     

X‐Treme beamline at SLS: X‐ray magnetic circular and linear dichroism at high field and low temperature

X‐Treme is a soft X‐ray beamline recently built in the Swiss Light Source at the Paul Scherrer Institut in collaboration with École Polytechnique Fédérale de Lausanne. The beamline is dedicated to polarization‐dependent X‐ray absorption spectroscopy at high magnetic fields and low temperature. The source is an elliptically polarizing undulator. The end‐station has a superconducting 7 T–2 T vector magnet, with sample temperature down to 2 K and is equipped with an in situ sample preparation system for surface science. The beamline commissioning measurements, which show a resolving power of 8000 and a maximum flux at the sample of 4.7 × 10¹² photons s^?1, are presented. Scientific examples showing X‐ray magnetic circular and X‐ray magnetic linear dichroism measurements are also presented.     

用于THz源的混合型波荡器数值模拟

波荡器是基于自由电子激光的小型THz源关键器件, 其可调节的周期性磁场结构与两端的光腔配合, 使得穿越的电子束产生带增益的相干辐射, 最终达到THz源所需要的功率. 同纯永磁结构相比, 混合型波荡器通过软铁材料调节由永磁块磁化方向性差异导致的磁场分布误差, 同时可提供更高的场强. 本文针对小型THz源需求, 对混合型波荡器进行了相关物理设计. 在解析方法分析的基础上, 采用OPERA3D/TOSCA有限元分析软件, 对波荡器进行了三维磁场数值模拟和积分场优化. 通过对波荡器端部结构的调整, 优化后模型的一次场积分(导向误差)小于0.01Gs﹒m, 电子轨迹偏移小于0.02mm.     

Magnetic design and modelling of a 14 mm‐period prototype superconducting undulator

The magnetic design of a ten‐period (each period 14 mm) prototype superconducting undulator is reported using RADIA. The results of modelling the magnetic flux density are presented in an analytical formula. The dependence of the field integrals and phase error on the current density and undulator gap has been calculated, and temperature curves are determined for the models and are compared with earlier reported Moser–Rossmanith fits.     

Fast continuous energy scan with dynamic coupling of the monochromator and undulator at the DEIMOS beamline

In order to improve the efficiency of X‐ray absorption data recording, a fast scan method, the Turboscan, has been developed on the DEIMOS beamline at Synchrotron SOLEIL, consisting of a software‐synchronized continuous motion of the monochromator and undulator motors. This process suppresses the time loss when waiting for the motors to reach their target positions, as well as software dead‐time, while preserving excellent beam characteristics.     

Diffractive–refractive optics: X‐ray splitter

The possibility of splitting a thin (e.g. undulator) X‐ray beam based on diffraction–refraction effects is discussed. The beam is diffracted from a crystal whose diffracting surface has the shape of a roof with the ridge lying in the plane of diffraction. The crystal is cut asymmetrically. One half of the beam impinges on the left‐hand part of the roof and the other half impinges on the right‐hand side of the roof. Owing to refraction the left part of the beam is deviated to the left whereas the right part is deviated to the right. The device proposed consists of two channel‐cut crystals with roof‐like diffraction surfaces; the crystals are set in a dispersive position. The separation of the beams after splitting is calculated at a distance of 10 m from the crystals for various asymmetry and inclination angles. It is shown that such a splitting may be utilized for long beamlines. Advantages and disadvantages of this method are discussed.     

波荡器谐波场增强效应对电子自发辐射谱的影响

 有效利用电子束在波荡器中运动时产生的谐波辐射,是获得更短波长辐射最直接的方法之一。提高波荡器磁场的谐波分量可以提高电子束的谐波辐射光场强度。分析了一种改进Halbach型波荡器结构,计算了其磁场的构成,分析表明这种结构可以使磁场的三次谐波分量提高至基波分量的25%左右。以合肥光源的电子束参数为例,计算了波荡器磁场谐波分量增强后对电子束自发辐射谱的影响。计算结果表明,波荡器磁场谐波分量增强至25%时,可以使电子束自发辐射的三次谐波辐射增强至基波辐射光子通量的67%左右。     

Fourier treatment of near-field synchrotron radiation theory

In this paper, we couple synchrotron radiation (SR) theory with a branch of physical optics, namely laser beam optics. We show that the theory of laser beams is successful in characterizing radiation fields associated with any SR source. Both radiation beam generated by an ultra-relativistic electron in a magnetic device and laser beam are solutions of the wave equation based on paraxial approximation. It follows that they are similar in all aspects. In the space-frequency domain SR beams appear as laser beams whose transverse extents are large compared with the wavelength. In practical situations (e.g. undulator, bending magnet sources), radiation beams exhibit a virtual “waist” where the wavefront is often plane. Remarkably, the field distribution of a SR beam across the waist turns out to be strictly related with the inverse Fourier transform of the far-field angle distribution. Then, we take advantage of standard Fourier Optics techniques and apply the Fresnel propagation formula to characterize the SR beam. Altogether, we show that it is possible to reconstruct the near-field distribution of the SR beam outside the magnetic setup from the knowledge of the far-field pattern. The general theory of SR in the near-zone developed in this paper is illustrated for the special cases of undulator radiation. Using known analytical formulas for the far-field pattern and its inverse Fourier transform we find analytical expressions for near-field distributions in terms of far-field distributions.     

Optimization of the lattice function in the planar undulator applied for terahertz FEL oscillators

Since the beta function of the electron beam within the undulator has a great influence on the power gain of the free electron laser (FEL), optimization of the undulator lattice becomes important. In this paper, the transfer matrix of the planar undulator is obtained from differential equations of the electron motion. Based on this, the lattice function of the planar undulator in a terahertz FEL oscillator proposed by Huazhong University of Science and Technology (HUST-FEL) is optimized and the expressions of the average beta function are derived. The accuracy of the optimization result was confirmed well by the numerical method. The application range of this analytical method is given as well. At last, the emittance growth in the horizontal direction due to the attenuation of the magnetic field is discussed.     

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.     

Mini‐beam collimator enables microcrystallography experiments on standard beamlines

The high‐brilliance X‐ray beams from undulator sources at third‐generation synchrotron facilities are excellent tools for solving crystal structures of important and challenging biological macromolecules and complexes. However, many of the most important structural targets yield crystals that are too small or too inhomogeneous for a `standard' beam from an undulator source, ～25–50 µm (FWHM) in the vertical and 50–100 µm in the horizontal direction. Although many synchrotron facilities have microfocus beamlines for other applications, this capability for macromolecular crystallography was pioneered at ID‐13 of the ESRF. The National Institute of General Medical Sciences and National Cancer Institute Collaborative Access Team (GM/CA‐CAT) dual canted undulator beamlines at the APS deliver high‐intensity focused beams with a minimum focal size of 20 µm × 65 µm at the sample position. To meet growing user demand for beams to study samples of 10 µm or less, a `mini‐beam' apparatus was developed that conditions the focused beam to either 5 µm or 10 µm (FWHM) diameter with high intensity. The mini‐beam has a symmetric Gaussian shape in both the horizontal and vertical directions, and reduces the vertical divergence of the focused beam by 25%. Significant reduction in background was achieved by implementation of both forward‐ and back‐scatter guards. A unique triple‐collimator apparatus, which has been in routine use on both undulator beamlines since February 2008, allows users to rapidly interchange the focused beam and conditioned mini‐beams of two sizes with a single mouse click. The device and the beam are stable over many hours of routine operation. The rapid‐exchange capability has greatly facilitated sample screening and resulted in several structures that could not have been obtained with the larger focused beam.     

Theoretical and numerical analysis of electron motion in a three-dimensional undulator field

The results of numerical calculations of electron trajectories in the three-dimensional ideal magnetic field of an undulator are given. These trajectories can be regarded as exact results in our case. It is shown that, in a series of cases, results of the calculation of trajectories using approximate analytical formulas significantly differ from those obtained using numerical methods. The obtained numerical results show that the influence of the undulator on the dynamics of the electron beam is very complicated and cannot be reduced only to the focusing properties of the undulator magnetic field or wiggler.     

用模拟退火法进行纯永磁波荡器磁块组合优化

 纯永磁波荡器由多个磁块组成,磁块的剩磁离散性会引起波荡器磁场误差,从而影响储存环工作状态和自发辐射谱质量。在波荡器磁块安装之前,使用模拟退火法对磁块进行组合排序优化,可以使峰值场强误差降低到10 ^-4量级以下,磁场一次积分降低到10^-6 T·m量级,二次积分降低到10 ^-6 T·m ²量级,优化结果不依赖于初始状态的选择。给出优化的详细过程,提出了根据磁块剩磁快速计算波荡器峰值场强误差和积分场的方法。     

Momentum spread in a relativistic electron beam in an undulator

The motion of the relativistic electron beam in the spatially periodic magnetic field of an undulator has been considered taking into account the effect of the incoherent field of the spontaneous undulator radiation on the motion of the electrons. An expression for the rms momentum of the electrons has been obtained. It has been shown that the momentum spread in the ultrarelativistic electron beam increases in the spontaneous incoherent emission mode. Conditions for the self-amplification of the spontaneous undulator radiation in ultrashort-wavelength free-electron lasers have been discussed.     

Particle beams guided by electromagnetic vortices: new solutions of the Lorentz, Schrödinger, Klein-Gordon, and Dirac equations

It is shown that electromagnetic vortices can act as beam guides for charged particles. The confinement in the transverse directions is due to the rotation of the electric and magnetic fields around the vortex line. A large class of exact solutions describing various types of relativistic beams formed by an electromagnetic wave with a simple vortex line is found both in the classical and in the quantum case. In the second case, the motion in the transverse direction is fully quantized. Particle trajectories trapped by a vortex are very similar to those in a helical undulator.     

Phase‐merging enhanced harmonic generation free‐electron laser with a normal modulator

A phase‐merging enhanced harmonic generation free‐electron laser (FEL) was proposed to increase the harmonic conversion efficiency of seeded FELs and promote the radiation wavelength towards the X‐ray spectral region. However, this requires a specially designed transverse gradient undulator (TGU) as the modulator to couple the transverse and longitudinal phase space of the electron beam. In this paper, the generation of the phase‐merging effect is explored using the natural field gradient of a normal planar undulator. In this method, a vertical dispersion on the electron beam is introduced and then the dispersed beam travels through a normal modulator in a vertical off‐axis orbit where the vertical field gradient is selected properly in terms of the vertical dispersion strength and modulation amplitude. The phase‐merging effect will be generated after passing through the dispersive chicane. Theoretical analysis and numerical simulations for a seeded soft X‐ray FEL based on parameters of the Shanghai Soft X‐ray FEL project are presented. Compared with a TGU modulator, using the natural gradient of a normal planar modulator has the distinct advantage that the gradient can be conveniently tuned in quite a large range by adjusting the beam orbit offset.