Optimization of the beam configuration in the Raman-type free-electron laser

A theoretical discussion is given for the two-dimensional Raman-type free-electron laser composed of a hollow relativistic electron beam of arbitrary thickness contained in a parallel plate waveguide and an array of permanent magnets for the pumping source. Under the influence of the magnetostatic field pump, the coupling between the scattered wave of the even TE mode (positive-energy wave) and the electron plasma wave of the odd TM mode (negative-energy wave) is investigated in detail. With the aid of numerical illustrations, the electron plasma wave with odd symmetry is found to be split into two modes: one mode is dominant for a thick beam and the other mode dominant for a thin beam. For the case where the scattered wave interacts with the electron plasma wave of the latter mode, an optimum beam thickness is obtained for which the growth rate becomes maximum. The optimum beam thickness is found to be comparable with the wavelength of the scattered wave and yet considerably greater than the reactive skin depth of the electron beam. In addition, the efficiency for energy conversion is found to be greatly improved by utilizing a hollow beam instead of a solid beam.     

A compact Cherenkov laser with a Bragg cavity composed ofdielectric gratings

The growth and saturation characteristics of a Cherenkov laser with a Bragg cavity composed of dielectric gratings are investigated in detail with the aid of numerical simulation based upon the fluid model of the electron beam, For the analysis of the problem, a two-dimensional (2-D) model of the Cherenkov laser is considered which consists of a planar relativistic electron beam and a parallel plate waveguide, one plate of which is loaded with a dielectric sheet. For confinement and extraction of the electromagnetic wave, a Bragg cavity is formed by dielectric gratings fabricated at both ends of a dielectric-loaded parallel plate waveguide. The result of numerical simulation shows that a compact Cherenkov laser, the longitudinal dimension of which is about one order of magnitude smaller than that of the corresponding single-pass Cherenkov laser, can be realized by using a Bragg cavity composed of dielectric gratings     

Noise in free-electron laser amplifier

The equations of the free-electron laser amplifier are generalized to include higher order modes. The density and velocity fluctuations in the entering electron beam cause noise excitation in the amplifier. The electron beam fluctuations have been studied extensively, both theoretically and experimentally, in traveling wave tubes, and hence the well-tested formalism developed for this purpose is conveniently applied to the present problem. It is found that the fluctuations put a severe constraint on the achievable exponential gain in a proposed Raman-type free-electron laser operating at optical frequencies. A1/lambda^{3}scaling law is derived.     

等离子体圆柱波导慢电磁波的激励

本文在考虑等离子体厚度效应的情况上,详尽地推导了充填圆环状等离子体的圓柱波导中TM模慢电磁波的色散方程;利用相对论空间电荷波理论,导出了相对论电子注在轴向磁场引导下沿等离子体内表面传输时的空间电荷波方程;求得了电子注和慢空间电荷波相互作用的线性增益和频率漂移;讨论了等离子体厚度、密度对色散特性和互作用增益的影响。     

Efficiency enhancement in a Cherenkov laser loaded with a Kerr-likemedium

Efficiency enhancement in energy conversion for a Cherenkov free elctron laser loaded with a Kerr-like medium is discussed with the aid of particle simulation. For the analysis of the problem, a two-dimensional model for the Cherenkov free electron laser is considered which is composed of a planar relativistic electron beam and a parallel plate waveguide one plate of which is loaded with a nonlinear dielectric sheet exhibiting a Kerr-like effect. To follow the growth of an electromagnetic wave and the decrease in the kinetic energy of the electron beam in the specified model of the Cherenkov free electron laser, a particular segment of the electron beam with the longitudinal length of one guide wavelength is picked out. In that segment, as it travels down the waveguide, the interaction between the electromagnetic wave and a group of electrons is analyzed with the use of the finite-difference time-domain (FDTD) method. The result of numerical simulation shows that the efficiency of energy transfer from the electron beam to the electromagnetic wave is greatly enhanced by the proper choice of the nonlinear parameter for a Kerr-like medium. The enhanced efficiency is due to the improved velocity matching between the electron beam and the electromagnetic wave, and to the self-focusing effect of a Kerr-like medium     

Nonlinear saturation and efficiency enhancement in a Cherenkovlaser using a dielectric grating

With the aid of particle simulation, this paper discusses nonlinear saturation and efficiency enhancement in a single-pass Cherenkov laser using a dielectric grating, taking into account the nonlinear properties of a relativistic electron beam. For the analysis of the problem, a two-dimensional model for the Cherenkov laser is considered which is composed of a planar relativistic electron beam and a parallel plate waveguide, one plate of which is loaded with a dielectric grating. First, the nonlinear characteristics for the growth of the electromagnetic wave are considered for a dielectric grating with constant parameters. The variation of the groove depth or the slot width in a dielectric grating is found to have the same effect on the growth characteristics as the variation of the permittivity of a dielectric sheet with constant thickness. Then, in order to enhance the efficiency in energy transfer from the electron beam to the electromagnetic wave in the Cherenkov laser, one of the grating parameters or the groove depth is adiabatically decreased in accordance with the decrease in the average velocity of the electron beam. The result of numerical simulation shows that sufficient efficiency enhancement in energy transfer is achieved with the use of a dielectric grating with a variable parameter     

Linear Theory of Free Electron Laser with a Plasma-Loaded Cylindrical Waveguide

The dispersion characteristics of plasma–loaded free-electron laser has been analyzed using linear fluid model. The device under consideration consists of the cylindrical metallic waveguide, completely filled with background plasma and a relativistic electron beam which passes through a helical wiggler magnetic field. The result predicts that reasonable plasma density tends to improve the growth rate of the low-frequency optical wave of FEL and causes an shiftup in the operating frequency, However it has little effect on the growth rate of the high-frequency wave. In the plasma–loaded FEL, for the FEL oscillator, it may be tuned by varying the plasma density; and for the FEL amplifier, the wider frequency bandwidth is gained. A critical density n ^c _p for the background plasma density is found.     

Mode analysis of an open-boundary cerenkov laser in the collective regime

The mode analysis of an open-boundary Cerenkov laser is developed in the collective regime. The Cerenkov laser under consideration consists of a relativistic slab electron beam and a dielectric-loaded conducting plane. The beam and the dielectric are assumed to be arbitrary in thickness, with an arbitrary gap allowed between them. For the Cerenkov laser specified above, the dependence of the growth rate upon the electron density of the beam, the beam-dielectric gap, the beam thickness, and the drift velocity of the beam is clarified. In particular, the following results deserve special attention. First, with other parameters kept invariant, the growth rate approaches a constant value for the beam thickness greater than the reactive skin depth of the beam. Second, the growth rate becomes maximum at the drift velocity of the beam characteristic of a particular electromagnetic wave mode with which the space charge wave interacts. As the mode number increases, the characteristic drift velocity shifts to higher values whereas the value of the maximum growth rate decreases.     

Effects of electron prebunching on the radiation growth rate of acollective (Raman) free-electron laser amplifier

Experiments are reported on the effects of electron prebunching in a mildly relativistic, low-current (200-KV, 1-A) free-electron laser amplifier operating in the collective (Raman) regime at a frequency of ~10 GHz. Prebunching is established by injecting an electromagnetic wave into a bifilar helical wiggler and then transporting the bunched beam into a second magnetic wiggler region. The wave growth rate is deduced from measurements of the radiation intensity as a function of interaction length. Observations show that prebunching can increase the radiation growth rate manyfold as compared with a system without prebunching. Studies are presented both in the small-signal (linear) regime, and in the nonlinear (saturated) regime     

电磁波泵自由电子激光边带不稳定性单粒子理论＊

本文用单粒子理论分析了主频电磁波为常振幅常相移情况下电磁波泵自由电子激光(EM-FEL)的边带不稳定性,推出了色散方程,求出了边带的振荡频率与线性增长率,进行了详细的讨论,得出了一些重要的结论。     

Mode competition in free-electron masers with oversized planar Bragg resonators

The nonlinear dynamics of free-electron masers with planar Bragg resonators is studied. Here, in contrast to previous studies devoted to this class of self-oscillators, the transverse field structure is not fixed and consists of a set of modes of a regular planar waveguide. A criterion for the admissible oversize (the ratio of the gap between the plates forming the resonator to the wavelength) such that the stationary oscillation regime is stable under the variation of the electron beam parameters is obtained. It is demonstrated that, in the case of a large oversize, the synchronism mismatch variation due to the instability of the electron beam parameters should result in jumping of the oscillation frequency. It is found that different oscillation frequencies correspond to excitation of mode bunches with different transverse indices of partial waves. The region of synchronism mismatch corresponding to the excitation of the surface mode whose field localizes close to the corrugated surface is found. It is shown that, unlike regions of excitation of volume modes, this region is weakly sensitive to the gap between the plates.     

Phase variation in free-electron laser amplifiers

The evolution of the phase of the output radiation of free-electron laser amplifiers is investigated by means of a three-dimensional simulation code. The configuration employed consists of the propagation of a relativistic electron beam through a loss-free cylindrical waveguide in the presence of a helically symmetric wiggler and a uniform axial guide magnetic field. The analysis is fully three dimensional, and a set of model equations is discussed which describes the coupling between an ensemble of electrons and the radiation field of either the TE or TM modes. The model equations are solved numerically, and the output phase is studied with respect to variations in either frequency or electron beam energy. The output phase is found to depend sensitively on the wave frequency within the unstable bandwidth. In addition, the phase stability of the output radiation is discussed as a function of beam energy. Finally, the evolution of the phase for tapered wiggler free-electron lasers is studied.     

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     

电磁波泵浦的自由电子激光器

本文研究了电磁波泵浦的自由电子激光器。从跃变磁场结构出来的大半径迴旋环形电子束通过圆柱形波导与圆柱形波导的TE11模入射电磁波相作用。利用电子束的弗拉索夫分布函数理论和三维波导模的波动方程求得在康普顿区域中的散射波色散关系。通过数值分析,讨论了轴向引导磁场,电子束能量,电子迴旋比和电子束环的径向位置等与散射波频率和增长率的关系。     

Linear analysis of cyclotron-cherenkov and cherenkov instabilities in dielectric-loaded coaxial waveguides

We present, based on the cold fluid theory, linear analysis of the Cherenkov and cyclotron-Cherenkov instabilities which are driven when a linear electron beam is injected into a dielectric-loaded waveguide immersed in an axial magnetic field. In the analysis we consider azimuthally symmetric TM_0n modes. We derive dispersion relations for three types of waveguide, and compare computationally obtained linear growth rates of both instabilities. For the type A, which consists of a metallic cylinder with dielectric liner on its inner surface, the growth rate of the Cherenkov instability is larger than that of the cyclotron-Cherenkov instability. For the type B, which consists of a dielectric core and an outer metallic cylinder, both growth rates are comparable. And for the type C, which consists of a metallic core with dielectric liner on its surface and an outer metallic cylinder, the growth rate of the latter instability is higher than that of the former instability. Finally, for the type C, obtained are dependences of the oscillation frequency and the growth rates of both instabilities on the following parameters: the beam energy, the beam current, the axial magnetic field, the dielectric constant, and the thickness of the dielectric.     

Field theory of dielectric Cherenkov maser

The effect of three-dimensional perturbed velocity and three-dimensional perturbed current density on the beam-wave interaction of dielectric Cherenkov maser is analysed by use of the self-consistent linear field theory. Three distinct cases are considered. First, the propagation of the electron beam in an annular dielectric liner enclosed by a loss-free conducting wall is investigated. The dispersion equation and the simultaneous condition of the beam-wave interaction are derived. It's clearly shown that the instability of the interaction results from the coupling of the TM mode in the dielectric lined slow-wave waveguide to the beam mode via the electron beam. And the coupling is proportional to the density of the beam. The growth rate of the wave produced by the electron beam are obtained. Then, the case of a relativistic electron beam guided by a longitudinal magnetic field in the same slow-wave structure is examined. The motion of electrons could be approximated to be one-dimensional when the simultaneous condition of the beam-wave interaction of dielectric Cherenkov maser is satisfied. Finally, the effect of the background plasma on the instability of the beam-wave interaction is studied.     

实心注在等离子体介质筒产生的切伦可夫辐射

利用自洽线性场理论：考虑电子的三维扰动，分析了轴向实心相对论电子注通过填充背景等离子体介质筒慢波波导产生的切伦可夫辐射，导出了描述其不稳定性的色散方程和注液互作用的同步条件，求得出电磁波辐射的时间增长率，分析了背景等离子体密度和实心电子注半径对波增长率的影响。     

考虑纵向动量离散的离子波纹激光动力学理论

利用动力学理论对一种新型自由电子激光－离子波纹激光（ＩＲＬ）进行了研究。考虑电子束的纵向动量离散和空间电荷场效应，导出了ＩＲＬ不稳定性色散关系，求得了ＩＲＬ辐射波的增长率及电子束空间电荷波的线性阻尼率，并对其进行了讨论，得到了一些结论。     

有矩形波导和线性摆动器的自由电子激光器的特性研究

本文研究圆柱电子束通过矩形波导和线性摆动器时产生相干辐射的特性。用线性化弗拉索夫-麦克斯韦方程分析了因对自洽的电子束平衡态引起扰动产生的不稳定性。导出了TMmn模的色散方程并数值计算了辐射频率和增长率与电子束能量、轴向磁场、摆动器强度和波长以及电子束半径的关系。     

CHARACTERISTIC STUDY OF A FREE ELECTRON LASER WITH LINEARLY POLARIZED WIGGLER AND RECTANGULAR WAVEGUIDE

The characteristics of coherent radiation produced by a cylindrical electron beam passing through a rectangular waveguide and linearly polarized wiggler are studied. The instability analysis is based on the linearized Vlasov-Maxwell equations for the perturbations about a self-consistent beam equilibrium. The dispersion equation of TMmn mode is deduced and by making use of numerical calculation the radiation frequency and growth rate as a function of electron beam energy and radius, axial magnetic field, wiggler field and wave length are presented and discussed.