激光驱动强流电子束产生和控制

在惯性约束聚变(ICF)电子束快点火物理方案中,需要超强拍瓦激光脉冲驱动MeV能量的强流电子束,并沉积数十kJ能量到压缩氘氚芯区。强流电子束的束流品质是影响点火成功的关键因素之一,为深入了解强流电子束产生物理过程,研制成了三维高性能、适应上万CPU核规模的并行粒子模拟程序,并开展了大规模数值模拟研究,探索了强流电子束的产生机制和输运规律。回顾了近几年来快点火研究团队围绕强流电子束产生和控制开展的研究,介绍了导致束流品质差的两大物理原因:预等离子体效应和束流不稳定性磁场的随机散射。针对这两个物理原因,提出了四种提高强流电子束品质的方法:(1)双层金锥靶减弱预等离子体的负面效应;(2)输运丝产生环向磁场准直强流电子束;(3)外加磁场导引强流电子束提高耦合效率;(4)抑制束流不稳定性以降低随机磁场对电子束流的散射。     

Influence of the beam-forming conditions on the development of space-charge oscillations in a long-pulse relativistic electron beam

The characteristics of the space-charge oscillations of a long-pulse relativistic electron beam in magnetically insulated diodes are determined for different geometries of the electron acceleration section and for explosive-emission cathodes of different materials. The important role of the stream of electrons having high transverse velocities in the evolution of the oscillations is demonstrated, and the laws governing the generation of this stream are determined. Possible mechanisms of the space-charge oscillations are described, taking into account the interaction of the electron stream in the beam halo with the main electron stream, the development of diocotron instability in the stream of electrons emitted by the outer lateral surface of the plasma emitter, and the instability of the space charge of “long-lived” electrons in the beam transport channel. Zh. Tekh. Fiz. 68, 102–106 (April 1998)     

Beam-plasma discharge in the propagation of a long-pulse relativistic electron beam in a medium-pressure rarefied gas

A theoretical model is given, along with a numerical analysis of the evolution of beam-plasma discharge in the propagation of a long-pulse relativistic electron beam in a rarefied gas at medium pressure. It is shown that the self-stabilization of beam-plasma discharge as a result of longitudinal inhomogeneity of the density of the discharge plasma makes it possible for the beam to traverse the beam chamber with relatively low total energy losses, including ionization losses and energy losses in the generation of oscillations. During the dissociative recombination of electrons and ions of the discharge-driven plasma, heat is released and spent in raising the temperature of the gas. The investigated collective-discharge mechanism underlying heating of the gas for a relativistic beam can be more efficient than the classical heating mechanism due to ionization losses of the beam in pair collisions of its electrons with gas particles. Zh. Tekh. Fiz. 67, 94–98 (May 1997)     

电子迴旋激射不稳定性

本文分析了下述情况下的电子迴旋波的激射不稳定性:当相对性的单能高能电子斜向注入具有背景等离子体的磁场区域内,并且在电子等离子体频率与电子迴旋频率可以比拟时,考虑了背景等离子体密度远大于单能的高能电子的密度,以及与前者相反的两种情况。当单能的高能电子具有弱相对论性效应时,在电子迴旋频率的基频和二次谐波附近,分别有ο模和χ模的不稳定性存在。文中计算了这两种模的增长率,并作了讨论。 关键词：     

A study of the effect of self-generated pressure gradient sources on the resistive filamentation instability

In the field of fast ignition scheme, self-generated magnetic fields via beam resistive filamentation have a significant role in the angular divergence of the relativistic electron beam, which can be affected by the intensity of other self-generated magnetic fields. In this context, the effects of pressure gradient sources arising from temperature and density gradient of the pellet along the beam flow direction are investigated. The results showed that the resistive filamentation instability can be strongly amplified compared to the fully homogeneous plasma. In this respect, for the distance away from the critical surface, the instability is protected for a longer wave number. Also, the beam and plasma properties such as the beam relativistic factor, the beam number density, and the degree of the plasma temperature anisotropy might be effective.     

Effect of relativistic elliptical beam modulation on excitation of surface plasma waves in a magnetized dusty plasma column with elliptical cross section

Abstract

The excitation of surface plasma waves due to the interaction of an elliptical relativistic density modulated electron beam with the magnetized dusty plasma column with elliptical cross-section has been studied. The dispersion relation of surface plasma waves has been retrieved from the derived dispersion relation by considering that the beam is absent and there is no dust in the plasma elliptical cylinder. It is shown that the Cherenkov and fast cyclotron interactions appear between the beam and eigen-modes of plasma column. The growth rate of the instability increases with the beam density and modulation index as one-third power of the beam density in Cherenkov interaction and is proportional to the square root of beam density in fast cyclotron interaction. The numerical results and graphs are presented, too.     

相对论电子束在等离子体中的能量沉积

 用3维粒子模拟程序LARED-P研究了束-等离子体不稳定性, 不稳定性激发的强电磁场使电子束在非常短的距离内沉积能量。对于10 MeV的单能电子束,束电子数目占总电子数目5％的情况下,最终约损失14％的束能量。推导了等离子体的色散关系,得出了增长率。     

Nonlinear stage of development of resistive hose instability of a high-current relativistic electron beam in a plasma

The nonlinear stage in the development of a resistive hose instability of a highcurrent relativistic electron beam in a finite-conductivity plasma has been studied in the rigid-beam model. The attenuation of the force of the interaction of the beam with the magnetic field of the total current for large beam displacements is shown to result in the stabilization of the instability. The stabilization time and the amplitudes of the oscillations in the saturation regime are determined as functions of the parameters of the beam in the plasma.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fiz., No. 10, pp. 78–81, October, 1987.     

“神龙—Ⅰ”直线感应加速器束流输运系统设计

系统分析了强流相对论电子束在输运过程中影响束流品质的几种因素,针对束流不同能区凸现的主要矛盾,采取对应性措施以抑制空间电荷效应,束包络振荡和束心横向运动,所有措施归结到束流输运系统设计上表现为合适的束输运半径和匹配磁场的选取,提出了束流输运磁场配置的一般策略,初步设计了“神龙－Ⅰ”直线感应加速器束流输运系统的总体布局。     

“神龙-I”直线感应加速器束流输运系统设计

 系统分析了强流相对论电子束在输运过程中影响束流品质的几种因素,针对束流不同能区凸现的主要矛盾,采取对应性措施以抑制空间电荷效应、束包络振荡和束心横向运动。所有措施归结到束流输运系统设计上表现为合适的束输运半径和匹配磁场的选取,提出了束流输运磁场配置的一般策略,初步设计了“神龙 I”直线感应加速器束流输运系统的总体布局。     

Determination of the final equilibrium radius and the increase in the emittance of a nonequilibrium relativistic electron beam during transport along an external magnetic field in a gas-plasma scattering medium

An equation describing the evolution of the transverse energy of a segment of a paraxial axisymmetric relativistic electron beam (REB) propagating in a gas-plasma scattering medium along an external magnetic field is used to find the equation relating the final equilibrium radius of the beam to its initial nonequilibrium value. An analytical expression for the increase in the mean-square emittance of an REB during transport up to achievement of the equilibrium state is found for the case considered. The dependence of the final equilibrium radius and the corresponding increase in the mean-square emittance on the density of the scattering medium and the induction of the external magnetic field is investigated. Pis’ma Zh. Tekh. Fiz. 67, 108–111 (July 1997)     

Relativistic ponderomotive forces

The interaction of a relativistic classical electron with an inhomogeneous electromagnetic field is investigated. In second-order perturbation theory the motion is separated into fast and slow motions, and the relativistic Newtonian equation is averaged over the fast oscillations. The rate of change obtained for the slow component of the electron momentum is interpreted as a relativistic ponderomotive force. The result is generalized to the relativistic case of the wellknown expression for the Gaponov-Miller force acting on an electron at rest. The expressions obtained for the relativistic ponderomotive forces are very complicated in the general case. They simplify in the limit of a stationary field (pulses of long duration) and a small gradient. The most typical and simplest special case of an inhomogeneous field—a stationary plane-focused beam—is investigated. The main difference between relativistic ponderomotive forces and their nonrelativistic limit is they have multiple components. In addition to the usual force directed along the gradient of the field, the relativistic case is also characterized by force components that do not have the form of the gradient of a potential and are parallel to the wave vector and the direction of the field polarization. It is shown that when a relativistic electron travels in a direction close to the direction of the wave vector of a focused laser beam, these components can greatly exceed the gradient force. A force directed along the field polarization vector arises even when the gradient of the field in this direction is zero. Zh. éksp. Teor. Fiz. 116, 1198–1209 (October 1999)     

Neutrino beam plasma instability

We derive relativistic fluid set of equations for neutrinos and electrons from relativistic Vlasov equations with Fermi weak interaction force. Using these fluid equations, we obtain a dispersion relation describing neutrino beam plasma instability, which is little different from normal dispersion relation of streaming instability. It contains new, nonelectromagnetic, neutrino-plasma (or electroweak) stable and unstable modes also. The growth of the instability is weak for the highly relativistic neutrino flux, but becomes stronger for weakly relativistic neutrino flux in the case of parameters appropriate to the early universe and supernova explosions. However, this mode is dominant only for the beam velocity greater than 0.25c and in the other limit electroweak unstable mode takes over.     

The effect of the plasma density gradient on current filamentation instability

The filamentation instability is one of the basic beam-plasma instabilities that play a significant role in the energy deposition mechanism of the relativistic electrons generated by the laser-plasma interaction in the fast ignition scenario. In this paper, the effect of the density gradient into plasma on the filamentation instability was investigated in the Weibel unstable plasma, where the plasma temperature anisotropy can play an important role. Results indicated that the density gradient enhances the instability growth rate so that decreasing the density gradient from the critical surface to the core of fuel leads to instability for longer regions in k space. Also, investigations in the region close to the critical surface showed that for decreasing the beam number density n_b ≤ 0.01n₀, the instability occurs for while this can be different for higher values. Increasing the beam relativistic factor causes a decreasing peak of instability growth rate because of a reduction in beam current, whereas the initial thermal spread of plasma amplifies the filamentation instability.     

Effect of Multiple Scattering of a Relativistic Electron Beam in a Gas–Plasma Medium on the Dynamics of the Resistive Hose Instability

We consider the effect of the multiple Coulomb scattering on the spatial dynamics of the relativistic hose instability of a relativistic electron beam propagating along an Ohmic plasma channel. It is shown that the enhancement of scattering noticeably suppresses this instability.

     

Effect of the ionization process on the focusing of a relativistic electron beam by a gas-plasma lens

A numerical simulation is made of the processes occurring in a plasma lens under conditions when the focusing of a relativistic electron beam is strongly affected by the ionization of the residual gas in the lens region by the beam itself. The paraxial, azimuthally symmetric, 1.5-dimensional, electrostatic kinetic model, taking account of plasma production, expansion of the plasma electrons away from the beam region, and contraction of the ions toward the axis of the beam, was used for the calculation. The dynamics of the formation of a focal spot is studied, and the size and position of the spot are determined as functions of time for different values of the gas pressure, initial plasma density, and energy of the beam electrons. Zh. Tekh. Fiz. 67, 90–94 (October 1997)     

激光等离子体不稳定性的入射光强度效应

用3维粒子模拟程序研究了相对论强激光和高密度等离子体相互作用引起的电磁不稳定。数值模拟表明,在线偏振强激光作用下,等离子体表面出现了电磁不稳定性。形成的不稳定结构随时间发展和激光功率密度的增加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。这种由电子速度各向异性而产生的自生磁场对激光有质动力推开电子时所形成的电子热流产生抑制作用,并将直接影响电子加速效率。     

Explosive instability in a backward wave oscillator

A plasma filled backward wave oscillator supports Trivelpiece-Gould (TG) and TM modes. The former can be driven unstable by a relativistic electron beam via Cerenkov interaction or by the plasma return current as two stream instability. This unstable TG mode can parametrically couple to a TM mode via a negative energy beam mode giving rise to an explosive instability     

Calculation of the characteristics of the radiation from a generator with a virtual cathode

A three-dimensional computer model, described by a system of Maxwell-Vlasov equations, for the interaction of a plasma with the electromagnetic field was used to calculate the excitation of a field by a relativistic electron beam with a virtual cathode oscillating in a resonance chamber. The characteristics of the generator radiation are investigated. Zh. Tekh. Fiz. 69, 87–92 (February 1999)     

Amplification of monochromatic short-wavelength radiation during the stochastic deceleration of a relativistic electron stream in an incoherent pump field

The use of incoherent multiwave pump radiation or randomly varying magnetostatic fields (stochastic undulators) for improving the energy conversion efficiency in free-electron lasers based on stimulated wave scattering and the stimulated undulator emission of relativistic electron beams is proposed. It is shown within the quasilinear approximation that the electronic efficiency increases in proportion to the width of the pump spectrum due to enrichment of the spectrum of combination waves which are synchronous with the electron beam and realization of a mechanism of stochastic particle deceleration when the signal wave is monochromatic. At the same time, the efficiency scarcely depends on the spread of the beam parameters, making the use of the method promising for improving the efficiency of free-electron lasers powered by intense relativistic electron beams. Zh. Tekh. Fiz. 67, 77–81 (July 1997)