Interaction of a modulated electron beam with a magnetoactive plasma

Experimental results concerning the interaction of a modulated electron beam with a magnetoactive plasma in the whistler frequency range are reported. It was shown experimentally that when a beam is injected into the plasma, waves can be generated by two possible mechanisms: Cherenkov emission of whistlers by the modulated beam, and transition radiation from the beam injection point. In the case of weak beam currents (N _b/N ₀)≪⁻⁴) the Cherenkov resonance radiation is more than an order of magnitude stronger than the transition radiation; the Cherenkov emission efficiency decreases at high beam currents. The transformation of the distribution function of the beam is investigated for the case of weak beam currents. It is shown that in the case of the Cherenkov interaction with whistlers the beam is retarded and the beam distribution function becomes wider and acquires a plateau region. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 6, 378–382 (25 March 1998)     

Parametric generation of whistler waves due to the interaction of high-frequency wave beams with a magnetoplasma

The parametric generation of low-frequency whistler waves by a pump wave beam formed by high-frequency whistler waves with close frequencies is studied experimentally. The electromagnetic fields excited by the beats of two co- or counterpropagating high-frequency waves, or by an amplitude-modulated pump are studied. It is shown that the nonlinear currents at the beat (modulation) frequency are generated by a transverse ponderomotive force arising due to the finite width of the high-frequency beam. In this case, the nonlinear azimuthal drift currents enclose the pump beam and can radiate low-frequency whistler waves to the surrounding plasma.     

Upconversion of whistler waves by gyrating ion beams in a plasma

A gyrating ion beam, with a ring-shaped distribution in velocity, supports negative energy beam modes near the harmonics of beam gyro-frequency. An investigation of the non-linear interaction of high-frequency whistler waves with the negative energy beam cyclotron mode is made. A non-linear dispersion relation is derived for the coupled modes. It is shown that a gyrating ion-beam frequency upconverts the whistler waves separated by harmonics of beam gyro-frequency. The expression for the growth rate of whistler mode waves has been derived. In Case 1, a high-amplitude whistler wave decays into two lower frequency waves, called a low-frequency mode and a side band of frequency lower than that of pump wave. In Case 2 a high-amplitude whistler wave decays into two lower frequency daughter waves, called the low-frequency mode and whistler waves. Generation mechanism of these waves has application in space and laboratory plasmas.     

Whistler Wave Emission from a Modulated Electron Beam in a Collisional Magnetoplasma in the Presence of a Density Duct

We study the radiation from a modulated electron beam injected along the axis of a cylindrical density duct in a magnetoplasma. An expression for the average power lost by the beam at the modulation frequency is obtained and analyzed. It is shown that in the case of Cerenkov resonance of the beam with a weakly damped whistler mode of an enhanced-density duct, a noticeable increase in this power is possible compared with the case where the beam is injected in a homogeneous background magnetoplasma. Based on the results of numerical calculations performed for conditions of the Earth's ionosphere, we give estimates of an increase in the power radiated from the beam in the whistler frequency range in the presence of a cylindrical duct with enhanced density. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 48, No. 9, pp. 730–742, September 2005.     

Dynamically stable electron bunches in beam interaction with an electromagnetic wave packet

Nonlinear interaction of electron beam with a whistler wave packet that effectively dissipates through collisions or wave leakage is studied. Independently of the dissipation type and nature of waves, self-organization of the beam structure leads to the formation of bunches continuously decelerated by waves. Strong dissipation prevents phase mixing required for the quasilinear theory and keeps wave phases in the packet correlated. Thus, dynamically stable bunches are present together with a plateau in the velocity distribution; asymptotically, wave emission by bunches is the main process.     

Physical mechanisms for transition radiation of electromagnetic pulses

The excitation of electromagnetic pulses is examined in various generation schemes. The physical mechanism for excitation of the radiation is studied and the field from transition radiation by high current relativistic electron beams is analyzed. An analysis of the efficiency of the various generation schemes shows that the electron efficiency of beam radiators based on transition radiation may be quite substantial for beam parameters that are easily realized experimentally. Zh. Tekh. Fiz. 69, 90–95 (October 1999)     

基于试验粒子模拟的电离层人工调制激发的极低频和甚低频波对磁层高能电子的散射效应

电离层调制加热能够有效激发极低频和甚低频(ELF/VLF)波,其中向上传播进入磁层的ELF/VLF波能够与高能电子发生共振相互作用,具有人工沉降高能电子、消除辐射带等潜在实际用途.本文综合运用射线追踪和试验粒子方法模拟电离层人工激发的单频ELF/VLF波在电离层和磁层的传播,以及在外辐射带层与高能电子的共振相互作用过程,通过投掷角和能量散射系数评估人工ELF/VLF波对磁层高能电子的共振散射效应.研究表明,电离层人工ELF/VLF波传播到磁层后呈现高倾斜性,传播所能跨域的空间范围主要取决于加热的纬度位置和调制频率.在内辐射带,与～100 keV到几个MeV高能电子发生一阶共振相互作用的为10 kHz的VLF波段;在外辐射带,为几百Hz到1 kHz的ELF波段.对于L=4.5的外辐射带,试验粒子模拟结果显示,单个粒子在人工ELF波作用下投掷角和能量(α,E)的改变具有随机性,而所有试验粒子平均化的?α2和?E2随时间呈现出近似线性的增大,说明波粒共振散射过程体现出整体性.基于试验粒子模拟得到的共振散射系数表明,幅度为10 pT的人工ELF波可在外辐射带的磁赤道局地对1 MeV电子产生较强的投掷角散射效应,进而影响高能电子的损失、沉降等动力学过程.当人工ELF/VLF波在传播过程中变得高度倾斜,不仅最基本的一阶共振十分重要,高阶共振散射也具有较大效应.这些定量分析结果表明,通过电离层加热激发人工ELF/VLF哨声波来沉降、消除辐射带高能电子具有可行性.     

Plasma Maser Theory of Whistler Waves in the Presence of Ion Cyclotron Turbulence

In this paper we consider the plasma maser theory of whistler waves in the presence of ion cyclotron waves in a magnetized plasma. In a plasma with low frequency ion cyclotron turbulence and a high frequency test whistler wave, growth of the whistler wave takes place because of the turbulent bremsstrahlung interaction between the resonant electrons and the modulated electric fields. The growth rate of the whistler wave is calculated and the results discussed.     

Theory of Jovian Kilometric Radiation

This report presents a theoretical model of extraordinary mode radiation in presence of low frequency whistler mode. We show that the generation mechanism of extraordinary mode emission in the presence of whistler mode is based on the plasma maser theory. Application of theoretical results to Jovian Kilometric radiation is examined.     

12MeV强流脉冲电子束对钽靶的破坏研究

强流高能电子束由于具有很强穿透能力,在材料内的能量沉积具有体分布特点,因此其对材料的辐照破坏体现出和低能电子以及X光等不同的特点.本文分析了金相显微镜和扫描电镜下的被12MeV的强流电子束轰击后的1.2mm厚度钽靶的破坏点形貌,其破坏断面体现为力学韧性撕裂,且关于靶中心基本对称.为了解释这种现象,文中用蒙特卡罗的数值模拟方法给出了电子束在钽靶内的沉积能量分布:能量沉积呈现靶中心吸能高,两侧低,且关于中心基本对称的特点.针对破坏断面特点和靶的吸热情况,我们给出高能电子束对靶材破坏初步的定性解释:认为高能电子束的强穿透能力使得靶材各部分几乎同时加热.靶材在极短时间里吸收大量能量,发生剧烈膨胀.由于能量沉积特点,中心部分材料膨胀最厉害,受到两侧边界的强烈约束,将产生两大小相近的热激波相对传播.激波在两侧自由界面反射,产生向内传播稀疏波.当两个稀疏波在靶中心区域相遇,就造成了靶对称撕裂的破坏形态.     

Nonlinear generation of sheared flows and zonal magnetic fields by electron whistlers in plasmas

The nonlinear generation of shear field and flow in whistler waves is considered. It is shown that a coherent parametric process leads to modulational instability of four waves whistler interaction. Growth rates for the flow/field are compared with published simulation results.     

Excitation of chorus-like waves by temperature anisotropy in dipole research experiment (DREX): A numerical study

Due to their significant roles in the radiation belts dynamics, chorus waves are widely investigated in observations,experiments, and simulations. In this paper, numerical studies for the generation of chorus-like waves in a launching device,dipole research experiment(DREX), are carried out by a hybrid code. The DREX plasma is generated by electron cyclotron resonance(ECR), which leads to an intrinsic temperature anisotropy of energetic electrons. Thus the whistler instability can be excited in the device. We then investigate the effects of three parameters, i.e., the cold plasma density nc, the hot plasma density nh, and the parallel thermal velocity of energetic electrons, on the generation of chorus-like waves under the DREX design parameters. It is obtained that a larger temperature anisotropy is needed to excite chorus-like waves with a high nc with other parameters fixed. Then we fix the plasma density and parallel thermal velocity, while varying the hot plasma density. It is found that with the increase of nh, the spectrum of the generated waves changes from no chorus elements, to that with several chorus elements, and then further to broad-band hiss-like waves. Besides, different structures of choruslike waves, such as rising-tone and/or falling-tone structures, can be generated at different parallel thermal velocities in the DREX parameter range.     

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.     

虚阴极振荡器中微波产生的解析理论

 用解析方法研究了虚阴极振荡器中微波产生的机理, 给出了小信号增益公式和饱和功率的标度。由此得到的结论是: 虚阴极振荡器的辐射一般情况下是由相位受到RF场调制的从虚阴极反射的电子束流产生的, 当注入电流稍大于第一临界电流时，有最高辐射增益区，但反射电流较小；当注入电流稍大于第二临界电流时，有较高辐射增益区，反射电流也较大。     

Quantum modulation of an electron beam in the field of opposite electromagnetic waves

The quantum modulation of an electron beam in the field of opposite electromagnetic waves, at a frequency equal to the difference of the wave frequencies, and its harmonics, is obtained. The depth of the modulation becomes of order one at relatively small intensities of the laser fields (including a real spreading of the beam). In the case of equal frequencies of the waves (when the Kapitza-Dirac effect occurs particles form a beam. An experiment for obtaining a modulated beam of particles at the frequencies of the laser radiation, and its harmonics, as well as for bunching of particles in the field of a standing wave is suggested.     

Interaction of spatially modulated pump and FIR waves in Raman laser

The propagation of spatially modulated IR pump and FIR generated waves interacting in the active gas medium of Raman laser is simulated numerically. Stationary nonlinear diffraction of monochromatic beams is studied for the isolated homogeneously broadened three-level system without any limitations on the values of field strength and frequency offset. It is shown that FIR radiation is trapped in the active wave guide induced owing to the nonlinear refractive index mechanism and gain inhomogeneity across the pump wave beam. Nonlinear focus positions and on-axis focal intensities for both fields are calculated.     

Coherent and tunable radiation with power enhancement from surface plasmon polaritons

Surface plasmon polaritons excited by an electron beam can be transformed into coherent and tunable light radiation waves with power enhancement in the simple structure of a metal film with a dielectric medium loading. In this paper, the process of the radiation transformation of this radiation, and the dependencies of the radiation characteristics on the parameters of the structure and the electron beam are studied in detail. The radiation power enhancement is greatly influenced by the beam energy and the film thickness in the infrared to ultraviolet frequency region. Up to 122 times radiation power enhancement and 6.5% radiation frequency tuning band can be obtained by optimizing the beam energy and the parameters of the film.     

Consideration of Femtosecond Radiation Generation on NewSUBARU Storage Ring

We studied on realization of short pulse gamma ray and X-ray simultaneously induced by a femtosecond laser on NewSUBARU storage ring. Based on the fact that the transverse dimensions of electron beam are much shorter than the longitudinal one, the laser light is arranged to collide the electron beam at a right angle to generate femtosecond pulse gamma ray, furthermore, the modulated part of the electron bunch gives rise to short pulse X-ray by synchrotron radiation from a downstream bending magnet. The temporal characteristic of the radiation is analyzed in this paper, as well as the performances are estimated.     

Whistler mode based explanation for the fast reconnection rate measured in the mit versatile toroidal facility

Despite the widely discussed role of whistler waves in mediating magnetic reconnection (MR), the direct connection between such waves and the MR has not been demonstrated by comparing the characteristic temporal and spatial features of the waves and the MR process. Using the whistler wave dispersion relation, we theoretically predict the experimentally measured rise time (τ(rise)) of a few microseconds for the fast rising MR rate in the Versatile Toroidal Facility at MIT. The rise time is closely given by the inverse of the frequency bandwidth of the whistler waves generated in the evolving current sheet. The wave frequencies lie much above the ion cyclotron frequency, but they are limited to less than 0.1% of the electron cyclotron frequency in the argon plasma. The maximum normalized MR rate R=0.35 measured experimentally is precisely predicted by the angular dispersion of the whistler waves.     

Parametric generation of low-frequency waves by plasma electrons accelerated under electron cyclotron resonance conditions

It has been shown experimentally that the diamagnetic effect appearing when electrons of a magnetized plasma in the antenna near field are accelerated under electron cyclotron resonance conditions can be used to generate low-frequency waves. The amplitude modulation of a signal supplied to the antenna is accompanied by the modulation of the diamagnetic effect and leads to the emission of waves at the modulation frequency to the surrounding plasma. In this process, the extended plasma region containing accelerated electrons serves as a parametric bodiless antenna. The results of the model laboratory experiments make it possible to propose a method for the parametric generation of low-frequency whistler waves in the Earth’s ionosphere by a powerful amplitude-modulated signal supplied to the satellite-borne antenna.