简谐近似与晶体摆动场辐射的一般特征

晶体摆动场辐射的能量很高,是获得短波长激光的重要途径之一。关键问题是如何提高摆动场辐射强度,如何将摆动场辐射与沟道辐射分离,将摆动场辐射中的相干辐射与自发辐射分离。首先,在经典力学框架内和线性近似下,讨论了摆动场辐射的瞬时辐射强度和平均辐射强度;引入无量纲偏转角讨论了这两种辐射的强度比、频率比和相干性。结果表明,在无量纲偏转角大于大于1时,只需在束流方向上放置一个张角为Δθ≈γ-1的接受器就可以将相干的摆动场辐射成功收集,而这时两种辐射的频率比和强度比都很大。     

正弦平方势与晶体摆动场辐射的非线性特征

在经典力学框架内和偶极近似下,引入正弦平方势讨论了系统非线性对摆动场辐射的影响。首先,把粒子在弯晶中的运动方程化为具有周期外力作用的摆方程,用Jacobian椭圆函数和椭圆积分解析地描述了粒子运动行为;用多尺度度法和三角函数的广义Bessel展开,找到了受迫摆方程的近似解。而后,在超相对论情况下讨论了系统的瞬时辐射强度和平均辐射强度,并对两种辐射的强度比进行了讨论。结果表明,摆动场辐射强度与摆动场振幅平方成正比,振幅越大摆动场辐射强度越强。     

An Analysis of Nonlinear Harmonic Generation in High Gain Free-Electron Laser

The odd harmonic evolution in the free-electron laser (FEL) employing planar undulator is studied analytically. The evolution equation of the harmonic is deduced, the dependence relation of the harmonics interactions is revealed, and both the linear and the nonlinear harmonic evolutions can be described. The analytical formula of the nonlinear harmonic power in self-amplified spontaneous emission FELs is given. For the saturation power of the harmonic, an estimation formula is given. The harmonic evolution equation for a high-gain harmonic generation (HGHG) FEL is also deduced, and in the fundamental case, it gives the optical field equation of HGHG, which describes the optical field evolution from the coherent enhancement to the exponential gain until the saturation occurrence.     

The Radiation Properties of a Novel Wire Antenna for the Security Fence Radar

The radiation properties of a novel wire antenna are investigated. The main part of this antenna is a sinusoidal wire undulator, which radiates by coupling electromagnetic energy from a Goubau line located near the antenna. Far-field patterns and S-parameters composed of three sets of antenna are measured. The measured patterns are compared with the calculated ones and the phase and attenuation constants of all the antennas are calculated. Frequency behavior and the dependency on the antenna dimensions of those wave parameters are investigated. Measured and calculated field patterns are also compared with the MoM patterns and some properties of the wave propagating along structure are explained by using MoM current distributions. It is shown that a broadside transversal radiation occurs in a narrow frequency band. The radiation intensity strongly depends on the coupling distance between the Goubau line and the sinusoidal undulator. This antenna is used as the basic element of the security fence radar antenna array working at 1.25 GHz in the L-band, which detects intruders approaching the fence. The performance of the antenna in the array is investigated and the near field distribution of the array is measured. The received signals caused by an intruder are given. The effects of rain and wind are also considered. The results suggests that the security fence radar introduced in this study can be used for the perimeter control of closed areas such as airports, malls, etc.      

Theory of microwave amplification in a coaxial ubitron

Results are presented from a theoretical study of the microwave amplification process in a coaxial ubitron with an undulator based on permanent annular magnets. Such an undulator is used for both focusing of a high-current relativistic electron beam (REB) and excitation of microwave radiation. The spatial structure of the magnetic field in a coaxial undulator is obtained. It is shown that optimum transport of the REB is possible under certain conditions for the parameters of the coaxial undulator (the intensity of the magnetic field and the undulator period).     

基于小周期永磁型错列波荡器的紧凑型XFEL装置设计

X射线自由电子激光(XFEL)装置具有规模庞大、造价高昂的缺点,因此XFEL装置小型化成为该领域一个重要的研究方向,减小波荡器周期是小型化XFEL装置的重要手段之一。小周期永磁型错列波荡器周期可做到10 mm,同时产生约0.8 T的峰值磁场,在小型化XFEL装置上具有潜在应用价值。本文将小周期永磁型错列应用到SASE型XFEL装置中,分析了纵向磁场对起振过程和辐射性能可能的影响,设计了一台辐射波长1 nm的SASE型XFEL装置并计算了其辐射性能,峰值功率约2.2 GW,单脉冲能量约2.4μJ。通过本文证明了小周期永磁型错列波荡器在缩减基于加速器光源规模上的作用。     

Spontaneous emission and gain in a waveguide free-electron laser

A free-electron laser (FEL) enclosed in a waveguide of narrowly spaced parallel plates has been proposed as a compact, coherent source of far-infrared radiation. The spontaneous emission and small-signal gain of such a device are analyzed. Maxwell's equations are solved for the fields of a relativistic electron beam passing through a linearly polarized undulator in the presence of a parallel-plane waveguide. The radiation intensity is resolved into its component waveguide modes for the fundamental frequency and for all harmonics. The intensity profile in a given harmonic mode is altered significantly when a parameter involving the undulator period, beam energy, and transverse dimension of the guide is such that the radiation group velocity is close to the electrons' axial velocity. The small-signal gain in the waveguide FEL is calculated and related to the spontaneous emission. Near zero slip, the gain curve is significantly different from that of a free-space FEL with the same parameters     

超相对论粒子的晶体摆动场辐射与同步辐射

注意到晶体摆动场辐射是基于沟道粒子的横向运动,而晶体的周期弯曲则等效为作用在粒子上的力。把晶体的周期弯曲视为相互作用势,并考虑在周期摆动场中粒子的"同步辐射"。在经典力学框架内和偶极近似下,把粒子的纵向运动方程化为摆方程,并在小振幅近似下,把它进一步化为Duffing方程。用Jacobian椭圆函数和第一类椭圆积分严格地给出了粒子轨道和它的运动周期;讨论了沟道辐射、摆动场辐射和"同步辐射"能量(频率)。结果表明,三种辐射之间的频率差别比较大,在实验上很容易将他们分开。选择如下一组参数V0=0.5eV,波长λp=100nm,能量γ=104,计算结果表明,"同步辐射"频率Ω=8.83×1019,已进入X-能区。     

两维摇摆场自由电子激光的谐波特性研究

本文应用Nadey定理对两维摇摆场自由电子激光的谐波特性进行了研究。导出了这种新型摇摆场结构自由电子激光高次谐波的自发辐射功率密度和小信号增益公式。以及电子与辐射场的耦合系数的表达式。计算结果表明,采用两维摇摆场可以增强电子与辐射场之间的耦合,提高谐波的辐射强度和增益。而且在某些条件下,在轴上既可获得奇次谐波又可获得偶次谐波。     

Accelerator Sources for THz Science: A Review

Free Electron Lasers have been around since 1977 providing not only a test bed for the physics of FELs and electron/photon interactions but as a workhorse of scientific research. More than 30 FELs are presently operating around the world spanning a wavelength range from the millimeter region to the hard x-ray using direct current and rf linear accelerators or storage rings as electron sources. The characteristics that have driven the development of these sources are the desire for high peak and average power, high micropulse energies, wavelength tunability, timing flexibility, and wavelengths that are unavailable from more conventional laser sources. Operation of FELs in the far infrared to terahertz regime poses special challenges which have been and are being addressed at a number of facilities around the world. This paper will review a number of former and existing FELs operating in this regime and discuss future efforts. Broadband collective radiation from relativistic electrons also plays a significant role in the production of FIR/THz radiation and several groups are taking advantage of this source for users. Applications for use of the radiation have evolved from simple imaging to complex pump probe tests of insulator/metal transitions and energy flow in organic molecules. We will discuss the technologies for generating the IR/FIR/THz radiation and cover some of the unique applications of such sources.     

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.     

Direct observation of local atomic structure in arsenic implanted silicon

We have investigated the implantation fluence dependence of the local atomic structure around arsenic (As) dopant atoms in low-energy (10 keV) implanted crystalline silicon by extended X-ray absorption fine structure (EXAFS) spectroscopy performed using a high-brilliance synchrotron radiation beam from an in-vacuum undulator in a third-generation light source. To obtain complementary information on the structural properties, high-resolution transmission electron microscopy (HR-XTEM) and nanobeam electron diffraction (NBED) measurements were also performed. We present the first observation of the initial stages of lattice disorder in As-implanted Si.     

采用飞秒装置的高功率宽带太赫兹源

真空电子器件产生的THz辐射通常是基于环形或直线型的加速器装置。飞秒级的电子束团通过周期性的磁铁可产生高功率、宽带可调谐的相干太赫兹辐射。这种高功率的太赫兹源为太赫兹技术的应用研究提供了新的手段。介绍了一种基于飞秒直线加速器装置产生的相干太赫兹波荡器辐射源,它主要由S波段热阴极微波电子枪,磁铁和SLAC型加速管组成,该装置能够提供具有20~30 MeV能量、束团长度为100~300 fs的电子束团。波荡器采用的是Apple?鄄II型波荡器,通过调节波荡器两平行磁块的位置可以产生具有不同极化特性的太赫兹辐射。为了测量波荡器产生的相干THz辐射谱,采用改进型的迈克尔逊干涉仪来进行测量,给出了实验装置的介绍以及实验结果。     

激光同步辐射在不同观测角度上的辐射特性研究

利用高能电子在强激光场中散射发出激光同步辐射,根据Lorentz方程与电子能量方程构建高能电子与强激光场的对撞模型,并通过MATLAB软件模拟高能电子在与强激光场对撞时电子运动的三维轨迹以及不同观测角度上同步辐射的脉宽和最大功率,进而分析不同观测角度对激光同步辐射特性的影响。模拟结果表明,观测角度由0°增大到360°期间,激光同步辐射的最大功率先减小后增大,而其脉宽先减小后增大,两者都于观测角为180°处左右呈现出一定程度的对称。且在0度或360度处获得的最大辐射功率取值最大,脉宽最小,能量最集中。     

柱面非均匀弱磁场对光纤陀螺磁敏感性的影响

以光纤陀螺(FOG)某电路板为例,扫描分析了其磁场分布规律,并利用矩阵光学和光传输理论,建立了柱面非均匀磁场中Faraday非互易相位差理论模型,进而仿真分析了实际电路板的辐射磁场对光纤陀螺的影响.分析结果表明:1)柱面非均匀磁场与光纤环距离越小,FOG磁敏感相位误差越大;2)柱面非均匀磁场对FOG所产生的磁敏感误差与角度之间呈倾斜正弦曲线,且倾斜度随磁场与光纤环间距减小而加剧;3)当R＜5r(磁场源位于光纤环外部,r为光纤环半径,R为磁场源与光纤环左侧边缘的距离)或R＜0.5r(磁场源位于光纤环内部)时,非均匀磁场均大于同量级均匀磁场对FOG的影响;4)非均匀场的存在影响FOG偏离固有磁轴方向;5)对于距离FOG较近的一些辐射强度非常小的普通电路板,也会导致FOG输出的不稳定和方向相关性.上述结果对于理解和分析实际磁场对FOG的影响,具有实际指导意义.     

瞬态阵列电磁脉冲同相合成特性研究

根据电磁理论和光栅方程给出线源天线能量辐射的解析公式,得到波的矢量积分的最大值与阵元数目的平方成正比。并在不同实验条件下测试天线阵列在轴向的电场强度和能量密度,在实验误差范围内可以得出,电场强度与天线单元数成正比,能量密度与天线单元数的平方成正比。这说明瞬态阵列电磁脉冲可以很好的实现同相合成。     

Parallel velocity spread induced in a relativistic electron beam by an undulator

The parallel momentum-spread of an intense relativistic electron beam (1 kA/cm², 700 kV), used in a Raman free-electron laser, has been determined by a Thomson backscattering experiment. Infrared radiation at 9.6 μm, provided by a CO2TEA laser, is backscattered by the electron beam and is observed at a wavelength of 0.5 μm. The spectral width of this scattered light is a quantitative measure of the beam energy spread or temperature. The beam itself has a fractional momentum spread, = 0.6 percent. When a region of periodic, transverse helical field is imposed by an undulator, the momentum spread is found to increase, becoming ≈2 percent when the quiver velocity of the electrons induced by the undulator is roughly 15 percent C. The data also show how the intrinsic spread of beam combines with that induced by the undulator.     

Burst error statistics of simulated Viterbi decoded BPSK on fadingand scintillating channels

The author presents and analyzes burst error statistics of a soft-decision Viterbi decoder when the transmitted signal is encoded with the 313 (3, 1/2) or 31123 (5, 1/2) convolutional codes, modulated via coherent binary phase-shift keying (BPSK) for the additive white Gaussian noise (AWGN) channel, and subjected to slow and nonselective scintillation/fading modeled by the Nakagami-m distribution. These statistics were generated by Monte-Carlo simulations, and presented in terms of burst error length average and quantile (90 and 99%) statistics versus SNR (E_b/N₀) parameterized by the fading intensity parameter m. The results indicate how Viterbi decoder burst error statistics vary with the fading/scintillation intensity m for Nakagami-m channels, and, consequently, provide information important to the design of interleaved or noninterleaved concatenated coding schemes for such channel environments     

太赫兹Smith-Purcell自由电子激光研究进展

太赫兹电磁波在材料表征、医学成像、无线通信、安全检测等领域有广泛的应用前景,是当前科学研究热点之一。基于自由电子与周期性光栅结构相互作用产生的Smith-Purcell辐射的Smith-Purcell自由电子激光凭借其易加工、可调谐、高功率等优点成为发展高功率太赫兹源的有效途径之一。本文对太赫兹Smith-Purcell自由电子激光的近期研究进展进行了综述,对基于特异Smith-Purcell辐射新型Smith-Purcell自由电子激光以及基于预群聚电子注的Smith-Purcell太赫兹源进行了着重介绍,这两类新型太赫兹自由电子激光结构克服了传统自由电子激光的若干缺点,有望发展为具有重要应用前景的紧凑型、大功率太赫兹源。     

Backward Wave Excitation and Generation of Oscillations in Free-Electron Lasers in the Absence of Feedback—Beyond the High Gain Approximation

Quantum and free-electron lasers (FELs) are based on distributed interactions between electromagnetic radiation and gain media. In an amplifier configuration, a forward wave is amplified while propagating in a polarized medium. Formulating a coupled mode theory for excitation of both forward and backward waves, we identify conditions, leading to efficient excitation of backward wave without any mechanism of feedback or resonator assembly. The excitations of incident and reflected waves are described by a set of coupled differential equations expressed in the frequency domain. The induced polarization is given in terms of an electronic susceptibility tensor. In quantum lasers the interaction is described by two first-order differential equations. In FELs, the excitation of the forward and backward modes is described by two coupled third-order differential equations. In our previous investigation analytical and numerical solutions of reflectance and transmittance for both quantum lasers and high-gain FELs were presented. In this work we extend the study to a general FEL without restriction of the high-gain approximation. It is found that when the solutions become infinite, the device operates as an oscillator, producing radiation at the output with no Held at its input, entirely without any localized or distributed feedback.