同轴波导环形相对论电子束空间极限电流

得到了1维条件下电子均匀分布时,环形电子束在同轴波导内轴向传输的空间极限电流解析表达式。通过讨论解析式在薄电子束和波导内导体半径为0等极限条件下的近似情况,分析了其合理性。在电子束非均匀分布的条件下,对空间极限电流进行了数值求解,并与解析计算结果进行了比较。理论研究表明:环形电子束在同轴波导内具有较圆柱波导内更高的空间极限电流,而且当电子束靠近波导壁时,空间极限电流会显著提高。     

Free electron laser based on the Smith–Purcell radiation

A Smith–Purcell (S–P) free electron laser (FEL) composed of a metallic diffraction flat grating, an open cylindrical mirror cavity and a relativistic sheet electron beam with moderate energy, is presented. The characteristics of this device are studied by theoretical analysis, experimental measurements and particle-in-cell (PIC) simulation method. Results indicate that coherent radiation with output peak power up to 50 MW at millimeter wavelengths can be generated by using relativistic electron beam of moderate energy.     

Simulation of the formation of an electron ring by picosecond electron beams in a cusp-type magnetic system

The possibility of using picosecond high-current relativistic electron beams for the formation of tubular rotating relativistic electron rings is investigated. Longitudinal compression of a tubular beam in a cusp is simulated numerically disregarding the space charge effects. The behavior of the characteristics of a compressed beam, which determines the tolerances in the initial parameters and magnetic system parameters, is investigated.     

Modeling of terahertz radiation emission from a free‐electron laser

In this article, we report the generation of terahertz (THz) radiation using the interaction of a laser‐modulated relativistic electron beam (REB) with a surface plasma wave. Two laser beams propagating through the modulator interact with the REB, leading to velocity modulation of the beam. This results in pre‐bunching of the REB. The pre‐bunched beam travels through the drift space, where the velocity modulation translates into density modulation. The density‐modulated beam, on interacting with the surface plasma pump wave, acquires an oscillatory velocity that couples with the modulated beam density to give rise to a nonlinear current density which acts as an antenna to give THz radiation. By optimizing the parameters of the beam and the wiggler, we obtain power of the order of 10⁻⁴ using the current scheme.     

有限引导磁场下相对论环形电子注色散特性的研究

采用等效媒质处理方法来研究有限引导磁场下沿纵向运动的相对论环形电子注.首先建立运动坐标系以电子注纵向速度匀速运动,在运动坐标系中电子注可以被考虑成静止的磁化等离子体,再通过四维空间的洛伦兹变换得到电子注在静止的实验室坐标系下可以被等效为双各向异性媒质,其不仅具有张量形式的电导率和磁导率,还具有手征特性.在此基础上同时考虑了由于电子注表面波动所引起的表面电流密度.采用该方法研究了有限引导磁场下圆柱波导中沿纵向运动的相对论环形电子注,推导出该模型的色散方程,并进行了数值计算.计算结果表明该研究方法能够得到更准 关键词： 相对论环形电子注 磁化等离子体 色散特性     

新型高功率径向强流速调管振荡器

 提出了一种新结构的高功率径向强流速调管振荡器,该器件利用折叠式同轴谐振腔的微波场与接近空间电荷限制电流的径向电子束强烈相互作用产生高功率微波。首先对这种器件的实现机理进行了初步的分析，提出了有间隙电压情况时的径向同轴间隙的空间电荷限制电流1维近似估计模型。分析表明：对于电子束直流接近但小于直流空间限制电流的径向速调管，当有调制间隙电压时，空间限制电流要小于无调制间隙电压情况下的直流空间限制电流，径向强流电子束电流接近和超过空间电荷限制电流时会出现强烈的调制。然后用PIC程序对其特性进行了粒子模拟，在二极管输入电压500 kV、电子束电流为30 kA条件下，最终得到了峰值功率6 GW、频率1.3 GHz的微波输出。     

Effects of relativistic and ponderomotive nonlinearities on the interaction of high-power laser beam with an inhomogeneous warm plasma

The influence of relativistic-ponderomotive nonlinearities and the plasma inhomogeneity on the nonlinear interaction between a high-power laser beam and a warm underdense plasma are studied. It is clear that the relativistic ponderomotive force and the electron temperature modify the electron density distribution and consequently change the dielectric permittivity of the plasma. Therefore, by presenting the modified electron density and the nonlinear dielectric permittivity of the warm plasma, the electromagnetic wave equation for the propagation of intense laser beam through the plasma is derived. This nonlinear equation is numerically solved and the distributions of electromagnetic fields in the plasma, the variations of electron density, and plasma refractive index are investigated for two different background electron density profiles. The results show that the amplitude of the electric field and electron density oscillations gradually increase and decrease, during propagation in the inhomogeneous warm plasma with linear and exponential density profiles, respectively, and the distribution of electron density becomes extremely sharp in the presence of intense laser beam. It is also indicated that the electron temperature and initial electron density have an impact on the propagation of the laser beam in the plasma and change the plasma refractive index and the oscillations' amplitude and frequency. The obtained results indicate the importance of a proper choice of laser and plasma parameters on the electromagnetic field distributions, density steepening, and plasma refractive index variations in the interaction of an intense laser beam with an inhomogeneous warm plasma.     

Effect of external magnetic field on critical current for the onset of virtual cathode oscillations in relativistic electron beams

In this Letter we research the space charge limiting current value at which the oscillating virtual cathode is formed in the relativistic electron beam as a function of the external magnetic field guiding the beam electrons. It is shown that the space charge limiting (critical) current decreases with growth of the external magnetic field, and that there is an optimal induction value of the magnetic field at which the critical current for the onset of virtual cathode oscillations in the electron beam is minimum. For the strong external magnetic field the space charge limiting current corresponds to the analytical relation derived under the assumption that the motion of the electron beam is one-dimensional [D.J. Sullivan, J.E. Walsh, E. Coutsias, in: V.L. Granatstein, I. Alexeff (Eds.), Virtual Cathode Oscillator (Vircator) Theory, in: High Power Microwave Sources, vol. 13, Artech House Microwave Library, 1987, Chapter 13]. Such behavior is explained by the characteristic features of the dynamics of electron space charge in the longitudinal and radial directions in the drift space at the different external magnetic fields.     

激光功率密度对Al膜靶后表面快电子发射的影响

 报道了在20 TW皮秒激光器上完成的p偏振激光与等离子体相互作用过程中产生的快电子的角分布和能谱测量结果。实验得到：当激光功率密度小于10¹⁷ W/cm²时,电子发射没有明显定向性,在激光入射面内多峰发射;当激光功率密度大于10¹⁷ W/cm2,小于10¹⁸ W/cm²时,电子主要沿靶面法线方向发射;当激光功率密度达到相对论强度时,电子主要沿激光传播方向发射;激光功率密度未达到相对论强度时,靶后表面法线方向快电子能谱拟合平均温度符合共振吸收温度定标率;激光功率密度达相对论强度以上时,靶后表面法线方向快电子能谱拟合平均温度高于已有的温度定标率。     

导引磁场控制相对论速调管放大器电子束收集的方法

 通过改变磁场位形,利用粒子模拟方法,研究了相对论速调管放大器（RKA）中电子束收集位置对器件效率和工作稳定性的影响。合适的电子束收集位置对增加输出腔区束波作用强度、减小输出腔区强流电子束的空间电荷势能以及减少RKA中反射电子数量非常有利。对一个工作频率2.85 GHz的RKA的电子束收集方式进行了改进,在电子束参数为510 keV和8.1 kA,注入微波功率500 kW和导引磁场1.5 T时,模拟得到了1.4 GW的微波输出,效率33.7%,增益33.8 dB,改进电子束收集方式之前的模拟结果为输出功率1.1 GW,效率为26.3%。利用Surperfish设计了改进收集方式后所需的磁场位形,并导入粒子模拟程序进行了模拟,实现了对电子束收集位置的有效控制,输出效率为32%。     

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     

Collective processes in a microsecond relativistic electron beam: Basic rules and mechanisms

Collective processes taking place in the space charge of a microsecond relativistic electron beam with magnetic insulation are considered. The space-time characteristics of the low-frequency and high-frequency oscillations are found, and the effect of the magnetic compression of the beam near the cathode on these oscillations is studied. It is shown that the basic source of the low-frequency oscillations is the collective motion of the space charge, which takes place in crossed electric and magnetic fields near the cathode, while the primary reason for the high-frequency oscillations is two-stream instability in the beam. The possibility of suppressing both types of oscillations by compressing relativistic electron beams near the cathode is demonstrated. The effect of nonuniform magnetic fields, including their effect on the cathode plasma motion, is elucidated.     

Weakly relativistic and space charge effects in interaction of high-power microwave with plasma

In the present work, numerical studies on the effects of weakly relativistic ponderomotive force and space charge in the nonlinear interaction of a high-power microwave beam with a plasma are carried out. It is shown that, the profiles of the electron density and dielectric permittivity contain high peaks, and modulation of wavelength occurs in electron density distribution by increasing the microwave energy flux. In addition, it is indicated that the profiles of the electric and magnetic fields in relativistic regime are lengthened more than non-relativistic regime by increasing the initial electron density and the relativistic effects cause the increase in oscillation wavelength of electron density, dielectric permittivity and space charge field, in comparison with the non-relativistic regime. Finally, the results of the research show that the steepening in electron density distributions and their oscillation wavelength are enhanced, when the relativistic effects appear.     

Gain enhancement in two-stream free electron laser with a planar wiggler and an axial guide magnetic field

The theory for a two-stream free electron laser (FEL) consisting of a relativistic electron beam transported along the axis of a planar wiggler in the presence of an axial guiding magnetic field is proposed and investigated. The electron trajectories and the small signal gain are derived. The characteristic of the linear gain and the normalized maximum gain are studied numerically. The result shows that the normalized maximum gain is considerably enhanced in comparison with that of the single stream. The effect of the difference between the energies of the two beams in this configuration of FEL is also considered, and we find that the gain is affected by the energy differences between groups 1 and 2.     

A New Scheme for High‐Intensity Laser‐Driven Electron Acceleration in a Plasma

We propose a new approach to high‐intensity relativistic laser‐driven electron acceleration in a plasma. Here, we demonstrate that a plasma wave generated by a stimulated forward‐scattering of an incident laser pulse can be in the longest acceleration phase with injected relativistic beam electrons. This is why the plasma wave has the maximum amplification coefficient which is determined by the acceleration time and the breakdown (overturn) electric field in which the acceleration of the injected beam electrons occurs. We must note that for the longest acceleration phase the relativity of the injected beam electrons plays a crucial role in our scheme. We estimate qualitatively the acceleration parameters of relativistic electrons in the field of a plasma wave generated at the stimulated forward‐scattering of a high‐intensity laser pulse in a plasma. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

拉曼型自由电子激光器中相对论电子运动稳定性的比较研究

拉曼型自由电子激光器作为一种兆瓦级高功率毫米波、太赫兹波辐射源, 其电子的运动稳定性对整体器件的性能至关重要.本文采用科尔莫戈罗夫熵方法, 以典型的麻省理工学院公布的实验数据为例, 比较研究拉曼型正向导引磁场和反向导引磁场两类自由电子激光器中相对论电子的运动稳定性. 结果表明:摇摆器绝热压缩磁场对电子运动的稳定性无实质性影响, 但对电子运动影响大; 电子束自身场在拉曼型正向导引磁场自由电子激光器中使电子运动稳定性变差, 而在拉曼型反向导引磁场自由电子激光器中则可改善电子运动稳定性. 关键词： 拉曼型自由电子激光器 相对论电子运动稳定性 科尔莫戈罗夫熵 电子束自身场     

Relativistic microwave devices with postacceleration of an electron beam in the interaction space

We analyzed the possibility of improving the efficiency of microwave devices operating with relativistic electron beams in systems with particle postacceleration in the interaction space. The fundamental feature of this approach is the formation of the accelerating-potential profile with the inherent electric field of a high-current electron beam. It is shown that the use of the space-variant beam-potential sag helps raise the estimated efficiencies of relativistic Cherenkov TWT and BWO up to values of about 50%.     

Gain enhancement in two-stream free electron laser with planar wiggler and axial guide magnetic field

The theory for the two-stream free electron laser (FEL) consisting of a relativistic electron beam transported along the axis of a planar wiggler in the presence of an axial guiding magnetic field is proposed and investigated. The electron trajectories and the small signal gain are derived. The characteristic of the linear gain and the normalized maximum gain are studied numerically. The result shows that the normalized maximum gain is considerably enhanced in comparison with that of the single stream. The effect of the difference between the energies of the two beams in this configuration of FEL is also considered, and we find that the gain is affected by the energy differences between groups 1 and 2.     

Physical properties of relativistic electron beam during long-range propagation in space plasma environment

It is known that ion channel can effectively limit the radial expansion of an artificial electron beam during its longrange propagation in the space plasma environment. Most prior studies discussed the focusing characteristics of the beam in the ion channel, but the establishment process and transient properties of the ion channel itself, which also plays a crucial role during the propagation of the relativistic electron beam in the plasma environment, were commonly neglected. In this study, a series of two-dimensional(2 D) particle-in-cell simulations is performed and an analytical model of ion channel oscillation is constructed according to the single-particle motion. The results showed that when the beam density is higher than the density of plasma environment, ion channel can be established and always continues to oscillate periodically over the entire propagation. Multiple factors, including the beam electron density, initial beam radius, and the plasma density can affect the oscillation properties of ion channel. Axial velocity of the beam oscillates synchronously with the ion channel and this phenomenon will finally develop into a two-stream instability which can seriously affect the effective transport for relativistic electron beam. Choosing appropriate beam parameters based on various plasma environments may contribute to the improvement of the stability of ion channel. Additionally, radial expansion of the beam can be limited by ion channel and a stable long-range propagation in terrestrial atmosphere may be achieved.