自由电子激光器的横向几何效应

以具有轴向磁场和线偏振摆动器场的自由电子激光器为例研究了波导壁和电子束厚度对自由电子激光不稳定性的影响。用流体力学方法推得了辐射波(TE模)和TM模的耦合方程组,将解得的电磁场代入电子束表面的边界条件便得到色散关系和不稳定性的增长率。数值结果表明体耦合的贡献远大于面电流密度耦合的贡献。存在一个最佳厚度和波导壁距离,此时增长率为极大。     

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.     

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。     

用磁化电子束提高自由电子激光器的增益

正 自由电子激光器有可能产生从毫米波到可见光范围内的相干辐射,实现简单的电压连续调谐并能获得极高的输出功率。因此越来越引起人们的重视。它的工作机理可概述如下:当相对论电子束与入射的泵浦电磁波相互作用时,在电子束中激起纵向密度波并产生正向及反向的散射电磁波。我们仅考虑反向散射波,因为它经过两次多普勒频     

Magneticresonant three dimensional kinetic theory of the free electron laser with waveguide cavity

Vlasov kinetic theory of the free electron laser employing waveguide frifting tube and helical pump is presented. Distinct from the former theories this theory has (1) taken into account the 3-d structure of the scattering field; (2) applied the plasma kinetic theory, which is more accurate than the other methods, to analysing the effect of the longitudinal. field; (3) taken account of the influence of the annular structure of the beam. The magnetic resonant effect appearing when the cyclotron frequency of electrons approaches the effective frequency of the magnetic pump is analysed and the growth rate formula in the linear regime derived. Results show that on taking into account the 3-d structure of the field and beam, one cannot arbitrarily choose the radius of the hollow beam, or else within certain range the instability may disappear altogether. Lastly, the equation for determination of the optimum beam radius is given and the possibility of raising growth rate by operating in high order modes shown.     

Theory of a nonwiggler collective free electron laser in uniform magnetic field

A nonwiggler free electron laser, operating in uniform guide magnetic field, is analyzed. The amplifier problem is solved self-consistently on the basis of the kinetic theory. It is shown that the asymmetry in the azimuthal distribution of the electrons' momentum leads to a coupling between the transverse and the space-charge modes. This, in turn, enhances the gain in the amplifier. In the case of a cold beam, with the electrons gyrating coherently, the spatial growth in the collective nonwiggler free electron laser (FEL) is comparable to that found in conventional free electron lasers operating in similar regimes.     

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。     

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.     

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.     

自由电子激光中自发辐射光脉冲的自干涉现象

本文用格林函数法,计算了螺旋磁场自由电子激光中单脉冲电子注的自发辐射;结果表明,电子注的有限长度会引起自发辐射光脉冲的自干涉现象。     

超强激光与等离子体相互作用中的自生磁场及三维模拟

用3维粒子模拟程序对超热电子在等离子体靶表面中向前传输时所激发的电流密度,电场和磁场的发展过程进行模拟研究。数值模拟表明,在线性强激光作用下,由于电子初始时刻的无规则热运动,在等离子体临界表面上激发不稳定性,而不稳定性随时间发展和激光功率的进一步深入到等离子体内部,最终使等离子体表面处激发饱和自生磁场,饱和自生磁场对激光有质动力推开电子时所形成的高能电子运动产生抑制作用.通过该研究寻找相对论效应条件下的自生磁场,并为进一步开拓其在高能离子束的应用提供理论和技术上的指导。     

集体区自由电子激光器增益特性的研究

工作在超辐射模式下的集体区自由电子激光器的增益特性研究可以通过改变互作用区长度及泵浦强度来进行。我们利用偏转磁场可以获得任意的相互作用区长度。实验结果证明,工作在集体区的自由电子激光的辐射增益随泵浦磁场近似呈1.7次方关系,实验测得的增益系数范围为0.4dB/cm到1.38dB/cm。     

一种GW级3.2cm相对论返波管振荡器的数值模拟

提出了一种3.2cm的相对论返波管振荡器,并利用2.5维粒子模拟软件KARAT研究了引导磁场强度、电子能量、电子束环平均半径、电子束环厚度对输出微波的影响。最后,在电子束为环形电子束(电子束束环平均半径为0.95cm,束环厚度为1mm)、电子束束压为900kV、电子束束流为6.7kA、引导磁场为3.2T时,得到了1.2GW的微波输出。     

A novel Smith-Purcell free electron laser

An experiment on a novel Smith–Purcell free electron laser (FEL) is described in this paper. The FEL is driven by a relativistic sheet electron beam of middle energy. The high frequency system of the device is a quasi-optical resonator composed of a diffraction grating and a three-mirror reflector. Coherent radiation with a peak power of tens of kW at the 3 mm waveband is sucessfully detected from an experimental facility. The main experimental parameters are: sheet beam energy from 400 kV to 500 kV; pulse length of voltage 70 ns; pulse beam current 0.2 kA; synchronous guide magnetic field up to 1.2 T with 10 ms pulse length; and grating period 2.2 mm.     

High-gain plasma Cerenkov maser

The linearized fluid and Maxwell's equations are used to calculate the dispersion relation for the transverse magnetic modes of a partially filled, plasma-lined, cylindrical waveguide interacting with a relativistic electron beam. Both the plasma liner and the electron beam are assumed to be cold, and the system is immersed in an infinite axial magnetic guide field. The dispersion relation is then used to calculate the growth rate for the instability between the slow-wave modes (Trivelpiece-Gould modes) of the plasma guide and the slow-space charge mode of the electron beam. The calculation is done in the high-gain, strong coupling limit     

有强相对论性非中和电子束环的环形自由电子激光器理论

本文用弗拉索夫-麦克斯韦方程研究了有强相对论性非中和电子束环的环形自由电子激光器的工作特性。结果表明,强电子束的空间电荷效应有提高辐射频率和强度的作用。     

喇曼自由电子激光器辐射谱的测量

本文报道了利用微波栅摄谱仪测量在超辐射工作条件下的喇曼自由电子激光器辐射谱的初步结果。观察到了毫米波自由电子激光器的调谐特性。实验结果表明,辐射谱的中心频率随波荡场强度的增加而减小,这与考虑波导模影响的集体制式自由电子激光的有关理论预言相符。     

Comparison of dc electric field and tapered wiggler free electron laser efficiency enhancement schemes

A constant parameter wiggler free electron laser using a dc electric field for efficiency enhancement is compared to several tapered wiggler efficiency enhancement schemes. Analytical expressions for the efficiency in the plane wave, infinitesimally small radius electron beam limit are derived and compared. Numerical simulations for a Gaussian radiation field and finite radius electron beam with an output radiation wavelength of 2 μm are presented. For finite radius electron beams, the extraction efficiency using a dc electric field is somewhat greater than or equal to that using proposed tapering schemes. While the dc field offers flexibility in efficient, tunable laser design, the required field strengths for visible radiation are in the megavolt/meter range.     

Experimental study of Compton type free electron laser

In this paper, we report the initial results of the experimental study for the first Compton free electron laser in China. The relativistic electron beam of 22 MeV was injected into SmCo₅ permanent magnetic undulator with 3 cm period and total period number 98 by an RF LINAC. We obtain free electron radiation of which the average power over 1 μs macroscopic electron beam pulse train is 1·4 W at 10 μm     

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