飞秒激光与固体靶相互作用中背表面的渡越辐射

在飞秒激光与固体靶相互作用中,利用光学CCD相机和光学多道分析仪(OMA),分别在固体薄膜靶背表面法线方向测量了渡越辐射(TR)积分成像图案和渡越辐射光谱.测量结果显示,渡越辐射光斑呈现圆盘状结构,在圆盘中明亮而强的光信号呈局部化分布,并且有分离的光斑出现.该现象表明,超热电子在输运过程中存在成丝效应,引起严重的不稳定性;渡越辐射光谱在800 nm附近出现了尖峰,是基频波(ω0),并且光谱向红光方向移动;基频波产生的原因归结于超热电子束在传输过程中产生的微束团而引起的相干渡越辐射(CTR);光谱红移的原因是由于等离子体临界面的迅速膨胀所致;渡越辐射光强随靶厚度的增加而减小.     

超强脉冲激光辐照固体靶背表面的光辐射

在超强脉冲激光与固体靶相互作用中,利用光学CCD相机和光学多道分析仪(OMA),分别在固体薄膜靶背表面法线方向测量了光辐射积分成像图案和光谱。测量结果显示,光辐射空间分布图案呈圆环状,而辐射区域有发散角、有光强分布,包含多种辐射成分;光辐射光谱在800nm附近出现尖峰,是激光的基频(ω0)波,这一现象归因于超热电子束在输运的过程中产生的微束团而引起的相干渡越辐射(CTR);如果考虑超热电子的产生和加热机制,共振吸收和真空加热对超热电子都有贡献,其中占主导地位的加热机制则是共振吸收对电子的加热。     

基于单模光纤的超短光脉冲产生及其进展

综述了基于单模光纤的超短光脉冲产生方法、原理及其进展,介绍了我们最近的研究成果,讨论了实现高效率脉冲压缩的途径。     

Rayleigh商-降采样滤波器在基于调频广播信号的单站无源定位系统中的应用

根据基于调频广播信号的无源定位系统中相干脉冲压缩处理的特点,指出在相关脉冲压缩中进行降采样处理的可行性,并提出了以信噪比增益最大为评价准则设计降采样滤波器。推导了信噪比增益与降采样滤波器权系数的关系,给出了采用Rayleigh商方法求解给出准则下最优FIR滤波器的方法和步骤。仿真实验验证了在相干脉冲压缩中采用Rayleigh商-降采样滤波器既降低了计算量又保证了脉冲压缩后的信噪比增益。     

Powerful millimeter-wave generators based on the stimulated Cerenkov radiation of relativistic electron beams

Effective relativistic Cerenkoy generators of various types were studied experimentally: orotrons, surface wave oscillators, and Smith-Purcell free-electron masers that were first realized. Single-mode generation was obtained in multimode systems using electrodynamic and electron methods of mode selection. The power obtained in this regime was 100 MW at 8.6 mm with the efficiency 10%, 50 MW at 5.5 mm with the efficiency 5%, and 8 MW at 2.4 mm with the efficiency 3%. In a multimode regime the power of 0.5 MW was obtained at 0.9 mm. The study of the dependence of the generation power on the value of the focusing magnetic field revealed the presence of zones of cyclotron power absorption, that were earlier observed in centimeter-wave generators.     

Electrodynamics of rotating relativistic electron beams

On the basis of the Minkowski formulation, this paper discusses the basic laws governing the small-signal fields propagated along an electron beam which is rotating around its axis with constant angular frequency of rotation and drifting in the axial direction at constant relativistic velocity. In the first preliminary section are described the dc conditions for getting a stable relativistic electron beam in the presence of neutralizing ions with arbitrary number density and externally applied static magnetic field. Then, after a brief discussion of the ac-field equations, the constitutive relations for small-signal fields are obtained in both the laboratory frame, which is assumed to be an inertial frame, and the rest frame of the electron beam, which is not an inertial but a rotating frame. The rotating relativistic electron beam is found to be a nonuniformly moving dispersive medium or, more specifically, an inhomogeneous bianisotropic medium with space and time dispersion. With the use of the constitutive relations derived above, the following sections consider the energy and momentum for the small-signal fields, and their conservation relations, together with their transformation laws between the laboratory frame and the rest frame of the electron beam. Our discussion includes, as the special cases, all the important cases of an ion-neutralized or axially confined beam and the Brillouin beam.     

A theory of narrow relativistic electron beams

Various models describing narrow electron beams are analyzed. The models are based on the theory proposed by Danilov, the theory developed from Ovcharov’s ideas, and the recent results obtained with the use of the geometrized theory of dense beams. The possibilities provided by different approaches are assessed and compared.     

Interaction of relativistic electrons with ultrashort laser pulses:generation of femtosecond X-rays and microprobing of electron beams

The interaction of relativistic electrons with terawatt femtosecond laser pulses is reviewed with emphasis on how the scattering process can be used as a source of ultrashort X-ray pulses and as a time-resolved diagnostic for measuring transverse and longitudinal phase space distributions of an electron beam with microns spatial and subpicosecond temporal resolution. Recent experimental results are presented in which 90° Thomson scattering was utilized to generate 300 fs duration pulses of X-rays with peak energy of 30 keV. It is shown that, through analysis of the scattered X-ray beam properties, transverse and longitudinal distributions of the electron beam can be obtained     

Equilibrium solutions for partially immersed relativistic electron beams

The equations relating the beam radius and axial electron velocity to easily measurable external beam parameters are developed for solid, relativistic beams. The beam parameters are tunnel voltage, beam current, axial magnetic field, cathode magnetic field, tunnel radius, and cathode radius. The equations are sufficiently complex to warrant the use of a digital computer if many cases are to be evaluated. Error indicators are formulated to prevent use of the relations beyond their range of validity. The equilibrium solutions for relativistic electron beams from unshielded cathodes show that the required magnetic focusing field is lower than that computed from nonrelativistic formulas. The angular magnetic field produced by the beam itself aids in focusing the electrons. The potential depression due to space charge is analyzed. Correction curves are given which allow the use of nonrelativistic equations in predicting equilibrium behavior. The maximum possible microperveance of relativistic beams is shown to be lower than the classical value of 25.4.     

Conservation principles for plasmas and relativistic electron beams

Many useful conservation theorems are derived for relativistic electron beams and anisotropic plasmas. All these theorems are valid for confined-flow and irrotational-flow devices, and cold, collisionless plasmas. All the theorems are derived in a similar way, and a generalization of this method, using linear operators, is given. Among the power theorems discussed are: Tonks' theorem, the instantaneous and sinusoidal small-signal theorems, the energy theorem, a large-disturbance theorem, the Manley-Rowe formulas, an ac power theorem, and a cross-correlation theorem. Contributions to power and energy from surface waves are included. Allowing for relativistic flow and writing all equations in the laboratory frame does not significantly complicate the theorems, and in fact clarifies them somewhat. Many simple applications are discussed, although not in detail. Among these are: small-signal energy, power, and passivity; energy velocity in electron beam and plasma wave-guides; and a conservation theorem for multiple electron beams and electron beam-plasma interactions.     

基于直线感应加速器的非相干太赫兹源设计

太赫兹波的产生途径有很多,本文通过理论设计和数值计算模拟了利用强流直线感应加速器神龙一号来产生 THz波。神龙一号直线感应加速器能够产生最大能量~20 MeV、束流强度~2 kA、脉冲宽度~60 ns 的脉冲电子束,脉冲电子束以不同能量通过偏转半径不相同的偏转磁铁后可以辐射出具有连续频率的太赫兹波。模拟计算了不同能量下的电子束通过偏转半径分别为0.2 m、0.5 m和1 m的偏转磁铁后得到的太赫兹波频率与电子束能量、磁铁偏转半径等的关系,太赫兹波的频率范围可达0.1 THz~9 THz,最大瞬时辐射功率~0.5 W。最后根据神龙一号直线感应加速器漂移段布局,设计得到偏转半径为0.5 m的偏转磁铁结构以及模拟结果。     

A geometrized theory of relativistic electron beams with the potential as the longitudinal coordinate

A new version of the geometrized theory of electron beams is presented in which the originally unknown coordinate system is connected with trajectories (or current tubes) and equipotential surfaces and, in the case of emission in the ρ-mode, has a singular metric. Equations of 3D and 2D nonmonoenergetic flows in the presence of a magnetic field and in the electrostatic version, which admits full geometrization of the problem, are derived. Solution for the cases of emission in the ρ- and T-modes in the vicinity of an arbitrary smooth emitting surface are constructed.