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.     

Sm_2Co_(17)型永磁合金的辐照效应研究

Sm_2Co_(17)型永磁合金大量使用在上海同步辐射光源储存环的永磁型波荡器上,在受到长期辐照后会发生磁性能损失的现象,进而影响同步辐射光的品质.为了探索其潜在的微观机理,本文首先对Sm_2Co_(17)型永磁合金所处混合辐射场的粒子及相关物理量进行了计算分析,确定引发磁性能损失的主要粒子是中子.然后采用Ar离子模拟中子辐照损伤的方法对其进行辐照,采用透射电镜对其辐照前后的微观形貌及微观结构进行了研究探讨,采用振动样品磁强计对永磁合金辐照前后的饱和磁化强度进行了分析对比,并讨论了微观结构演化与宏观磁性能变化的联系.结果表明,Ar离子辐照后Sm_2Co_(17)型永磁合金饱和磁化强度的不可逆损失与微观结构变化有直接的关系,其2:17相从单晶结构转变为非晶结构是造成其磁性能损失可能的微观机制.     

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.     

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.     

一种用于波荡器磁中心高精度标定的磁靶标

波荡器磁中心轴的标定是保证自由电子激光装置波荡器安装准直精度的重要前提。介绍了一种利用磁靶标实现波荡器磁中心高精度标定的方法。设计制造了由若干块永磁块组合构成的磁靶标,其能产生一正一斜两个高梯度四极场。测出了两四极场垂直分量的零点位置分布并依此给出了磁靶标的具体使用方法。结果表明:标定后的波荡器磁中心在磁靶标坐标系中水平方向测量精度好于±20 μm,垂直方向测量精度好于±2 μm。     

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.     

Three-dimensional numerical investigations of the laser-beam interactions in an undulator

Laser-beam interaction in an undulator is commonly suggested in the development of free electron laser(FEL)schemes. In this paper, a three-dimensional algorithm is developed to assist in laser-beam interaction simulation in an undulator, which is built on the basis of the fundamentals of electrodynamics, i.e.the electron's behavior is determined by the magnetic field and the laser electric field in the time domain. On the basis of the algorithm, the detuning effect in a laser heater, the carrier envelope phase effect of a few-cycle laser in attosecond X-ray FEL schemes and output wavelength tuning in a high gain harmonic generation FEL are numerically discussed.     

Efficiency and sideband observations of a Raman FEL oscillator witha tapered undulator

Power and spectral measurements are reported from the Columbia Raman free-electron laser (FEL) oscillator experiment. High-power radiation pulses (~12 MW, 100 ns) are generated at a wavelength of ~2.5 mm, using a 750-kV electron beam injected into a helical undulator. The undulator is made up of a 40-cm long constant-period (1.45 cm) section followed by an equal length of tapered undulator. The period is decreased by 7.6% in such a way that the on-axis field remains constant. It is reported that the taper allows an increase in total power efficiency from ~4 to ~12%. Most noteworthy is that the tapered undulator reduces the sideband radiation compared with a constant-period undulator FEL which is studied in the same configuration. The power was measured calorimetrically and compared with the results of a 1-D Raman code. The reduction of sideband power observed in the experiment was consistent with computational results obtained with a 2-D sideband code     

Design of an Efficiency-Enhanced Raman FEL Oscillator

The paper reports numerical calculations appropriate for the design of an "efficiency-enhanced" Raman free electron laser (FEL) oscillator. A helical undulator is used which consists of a constant period section followed by a section where the period is decreased roughly 10 percent. Simulation of the FEL is done by following the radiation dynamics in both the small- and large-signal regimes, using the "generalized pendulum equation" with self-consistent radiation field. It is found that the efficiency of the oscillator may be enhanced from ? 8 (no taper) to ? 15 percent (with taper). The configuration of an experimental oscillator is described; this device should produce ? 25 MW at a wavelength ? 1.8 mm using an 800-kV electron beam.     

On Undulator Radiation in Infrared and Millimeter Wave Ranges: Part 1—Classical Description

In this paper the results of theoretical and experimental research of the motion and radiation of low-voltage electron beams in Motz undulator are given. Constant magnetic field of undulator ordinary magnet under some conditions can be substituted for field of magnet dipole. Such substitution allows us to accurately integrate the electron trajectory. We can with the high accuracy consider this field as chaotic and this fact allow us to use well-known Ginsburg theory for estimation of power of braking radiation. There are longitudinal and transversal components of magnetic fields of described structure. The longitudinal component can be gained by covering of external longitudinal constant magnetic field. In this connection certain resonance effects are observed.     

A free electron laser experiment on "angular steering"

A paper by Jerby (1990) has discussed a number of mechanisms whereby FEL radiation may be directed electronically into different radiation patterns. We have devised an experiment (1992) to test this concept using the Columbia microwave FEL, which amplifies radiation at 24 GHz to a level <1 MW. A 4 mm dia, electron beam (580 kV) is propagated in a guiding field of 0.8 T inside an overmoded 24 mm dia. cylindrical waveguide. A TE11 mode is grown in a 33 cm long first undulator section (period 3.36 cm), and upon entering the following undulator section (period 2.26 cm, length 40 cm), the electron bunches convert to TM11 radiation which is further amplified. The far-field pattern of the TM11 emitted power is distinct from the TE11 pattern. Numerical and experimental studies are described in this paper showing the resulting radiation pattern.<>     

Studies of classical radiation emission from plasma wave undulators

The authors examine the characteristics of the classical radiation emitted by a relativistic electron beam that propagates perpendicularly through a large amplitude relativistic plasma wave. Such a study is useful for evaluating the feasibility of using relativistic plasma waves as extremely short wavelength undulators for generating short wavelength radiation. The electron trajectories in a plasma wave undulator are obtained using perturbation techniques and are then compared to numerical simulation results. The frequency spectrum and angular distribution of the spontaneous radiation emitted by a single electron and the stimulated radiation gain are obtained analytically, and are then compared to 3-D numerical simulations. The characteristics of the plasma wave undulator are compared to the AC free-electron laser (FEL) undulator and the conventional FEL     

Undulators and ‘free-electron lasers’

This article outlines the early development of the undulator in which an electron beam travelling through a system of magnetic fields with directions alternating in space emits a species of linearly polarized synchrotron radiation peaked at a wavelength which is short compared to the magnet period when the electron energy is high compared with the rest energy. The conditions of synchronism and the departure from synchronism that leads to gain during a single passage of the beam are examined. In later developments, stemming from this, single pass gain of circularly polarized radiation was experimentally demonstrated by means of a ‘twister’ structure and an optical cavity, obtained by adding mirrors to the magnet assembly, was used to achieve build-up of the radiation in an oscillator device known as the ‘free-electron laser’. Applications of this device and further theoretical developments are discussed.     

Undulator beamline optimization with integrated chicanes for X‐ray free‐electron‐laser facilities

An optimization of the undulator layout of X‐ray free‐electron‐laser (FEL) facilities based on placing small chicanes between the undulator modules is presented. The installation of magnetic chicanes offers the following benefits with respect to state‐of‐the‐art FEL facilities: reduction of the required undulator length to achieve FEL saturation, improvement of the longitudinal coherence of the FEL pulses, and the ability to produce shorter FEL pulses with higher power levels. Numerical simulations performed for the soft X‐ray beamline of the SwissFEL facility show that optimizing the advantages of the layout requires shorter undulator modules than the standard ones. This proposal allows a very compact undulator beamline that produces fully coherent FEL pulses and it makes possible new kinds of experiments that require very short and high‐power FEL pulses.     

Design,construction and characterization of the compact ultrafast terahertz free-electron laser undulator

A compact ultrafast terahertz (CUTE) free-electron laser (FEL) is being developed at the Raja Ramanna Centre for Advanced Technology (RRCAT), Indore. The undulator required for the CUTE-FEL has recently been developed. We have designed, built and characterized a variable gap, 5 cm period, 2.5 m long pure permanent magnet undulator in two identical segments. The tolerable error in the magnetic field was 1% in rms, and we have measured it to be 0.7%. The obtained rms phase shake is around 2°. To ensure that the trajectories do not have an exit error in position or angle, corrector coils have been designed. Shimming coils have been applied for both the undulator segments to reduce the amplitude of the betatron oscillations in the vertical trajectory. Details of novel corrector coils and soft iron shims are given and their performance is discussed.      

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.     

Physical design of a wavelength tunable fully coherent VUV source using a self-seeding free electron laser

In order to meet the requirements of the synchrotron radiation users, a fully coherent VUV free electron laser (FEL) has been preliminarily designed. One important goal of this design is that the radiation wavelength can be easily tuned in a broad range (70—170 nm). In the light of the users' demand and our actual conditions, the self-seeding scheme is adopted for this proposal. Firstly, we attempted to fix the electron energy and only changed the undulator gap to vary the radiation wavelength; however, our analysis implies that this is difficult because of the great difference of the power gain length and FEL efficiency at different wavelengths. Therefore, we have considered dividing the wavelength range into three subareas. In each subarea, a constant electron energy is used and the wavelength tuning is realized only by adjusting the undulator gap. The simulation results show that this scheme has an acceptable performance.     

Gain of harmonic generation in high gain free electron laser

In a planar undulator employed free electron laser(FEL),each harmonic radiation starts from linear amplification and ends with nonlinear harmonic interactions of the lower nonlinear harmonics and the fundamental radiation.In this paper,we investigate the harmonic generation based on the dispersion relation driven from the coupled Maxwell-Vlasov equations,taking into account the effects due to energy spread,emittance,betatron oscillation of electron beam as well as diffraction guiding of the radiation field.A 3D universal scaling function for gain of the linear harmonic generation and a 1D universal scaling function for gain of the nonlinear harmonic generation are presented,which promise rapid computation in FEL design and optimization.The analytical approaches have been validated by 3D simulation results in large range.     

Investigation of parameters of undulators with permanent magnets

Investigations of a hybrid type undulator for the terahertz FEL with ferrite magnets were carried out. The study began with an undulator simulation in FEMLAB environment and measurements on the created model. The work presents the results of study of the dependence of magnetic field on the height of the magnetic elements and study of the magnetic induction depending on the ratio of the width of magnetic elements to the distance between them. It is shown that the hybrid undulator scheme allows selecting the optimum parameters of the undulator. The obtained results can be used for choosing the optimum parameters of undulators of the terahertz-range FEL.     

Calculation and analysis of the magnetic field of a linearly tapered undulator

There is an empirical formula describing the relationship between the peak magnetic field and the undulator structure parameters for a uniform-parameter hybrid undulator.In this paper, we investigate the relationship for a linearly tapered undulator through numerical calculation by using the code RADIA, and check it with the empirical formula.The results imply that this empirical formula is also effective for linearly tapered undulators at a big enough scope for the requirements of normal FEL experiments.Therefore, for a linearly tapered undulator,we can use the empirical formula to design the variation of the undulator gap.For the tapering rate demanded by normal FEL experiments, the gap of a linearly tapered undulator increases almost linearly, and the tapering rate will keep constant while adjusting the undulator gap with the same variation for each undulator period.