电子束长程传输中离子通道的振荡特性研究

离子通道可以有效抑制电子束在等离子体环境内传输过程中的径向扩散,已有工作研究了离子通道对电子束的影响,但离子通道建立过程和暂态特性研究则更有助于理解和利用离子通道在电子束长程传输中的作用。本文利用PIC方法对离子通道的时空分布进行二维模拟,并基于单粒子理论推导出描述离子通道振荡的解析模型,对上述两种模型的结果相互校验。上述模型的计算结果表明,在长程传输过程中,相对论电子束在等离子体内部建立的离子通道是持续周期振荡的,电子束密度、电子束初始半径以及环境等离子体密度都会对离子通道的振荡规律产生影响,针对不同的等离子体环境选择合适的电子束参数可以有效提高离子通道的稳定性,进而提升传输过程中电子束的束流质量。     

相对论行波管慢波结构几何参数研究

 推导了无引导磁场下具有离子通道的波纹波导中的色散方程,并通过数值计算分析了该慢波结构的几何参数如波纹周期、波纹深度以及波导平均半径对相对论行波管的带宽及增益的影响,为相对论行波管的设计提供了一定的依据。     

Effect of a conducting waveguide on the dynamics of ion hose instability of a relativistic electron beam propagating in the ion focusing regime

We analyze the effect of a conducting waveguide on the dynamics of the ion hose instability of a relativistic electron beam propagating in a rarefied gas-plasma medium in the ion focusing regime. It is shown using the linear theory of the instability under investigation that a decrease in the waveguide radius noticeably stabilizes the ion hose instability. In addition, it is noted that radial narrowing of the ion channel relative to the beam considerably decreases the amplitude of hose oscillations.     

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.     

Beam loading in the nonlinear regime of plasma-based acceleration

A theory that describes how to load negative charge into a nonlinear, three-dimensional plasma wakefield is presented. In this regime, a laser or an electron beam blows out the plasma electrons and creates a nearly spherical ion channel, which is modified by the presence of the beam load. Analytical solutions for the fields and the shape of the ion channel are derived. It is shown that very high beam-loading efficiency can be achieved, while the energy spread of the bunch is conserved. The theoretical results are verified with the particle-in-cell code OSIRIS.     

考虑到束-波相互作用的速调管离子噪声二维模拟

采用粒子模拟的方法并考虑电子束与电磁波的相互作用，首次直接得到了速调管输出信号的离子噪声图像，阐述了束电子、二次电子、离子、电磁场之间的相互作用的动力学过程. 指出离子噪声所表现出来的相位波动是由电子束速度的波动引起的，电子束速度的变化来源于管内离子数量的变化，离子的数量的变化又与电子束状态变化相互影响，这是离子噪声产生的根本原因. 二次电子对离子噪声产生过程的影响甚微，但是其行为却反映了离子噪声的形成机理. 离子噪声引发的输出信号幅度波动取决于电子束速度和半径的改变，与离子行为密切相关. 关键词： 离子噪声 速调管 粒子模拟 电子束     

Effects of ion motion in intense beam-driven plasma wakefield accelerators

Recent proposals for using plasma wakefield accelerators (PWFA) as a component of a linear collider have included intense electron beams with densities many times in excess of the plasma density. The beam's electric fields expel the plasma electrons from the beam path to many beam radii in this regime. We analyze here the motion of plasma ions under the beam fields, and find for a proposed PWFA collider scenario that the ions completely collapse inside of the beam. Simulations of ion collapse are presented. Implications of ion motion on the feasibility of the PWFA-based colliders are discussed.     

Parametric amplification of laser-driven electron acceleration in underdense plasma

A new mechanism is reported that increases electron energy gain from a laser beam of ultrarelativistic intensity in underdense plasma. The increase occurs when the laser produces an ion channel that confines accelerated electrons. The frequency of electron oscillations across the channel is strongly modulated by the laser beam, which causes parametric amplification of the oscillations and enhances the electron energy gain. This mechanism has a threshold determined by a product of beam intensity and ion density.     

2 cm电子回旋共振离子推力器离子源中磁场对等离子体特性与壁面电流影响的数值模拟

电子回旋共振离子推力器(electron cyclotron resonance ion thruster,ECRIT)离子源内等离子体分布会影响束流引出,而磁场结构决定的ECR区与天线的相对位置共同影响了等离子体分布.在鞘层作用下,等离子体中的离子或电子被加速对壁面产生溅射,形成壁面离子或电子电流,造成壁面磨损和等离子体损失,因此研究壁面电流与等离子体特征十分重要.为此本文建立2 cm ECRIT的粒子PIC/MCC(particle-in-cell with Monte Carlo collision)仿真模型,数值模拟研究磁场结构对离子源内等离子体与壁面电流特性的影响.计算表明,当ECR区位于天线上游时,等离子体集中在天线上游和内外磁环间,栅极前离子密度最低,故离子源引出束流、磁环端面电流和天线壁面电流较低.ECR区位于天线下游时,天线和栅极上游附近的等离子体密度较高,故离子源引出束流、天线壁面电流和磁环端面电流较高.腔体壁面等离子体分布与电流受磁场影响最小.     

Determination of the final equilibrium radius of a relativistic electron beam mismatched to the equilibrium conditions in transportation in the ion focusing regime

We derive the equation connecting the final equilibrium radius of a paraxial azimuth-symmetrical relativistic electron beam propagating in the ion focusing regime with its initial nonequilibrium value. The dependence of the final equilibrium radius and the increment of the rms emittance on the density of the scattering background medium is analyzed.     

Generalized equations for the dynamics of the resistive firehose instability of an REB with time-dependent radius and current

Generalized equations are derived that describe the linear stage of the resistive firehose instability of a relativistic electron beam whose radius and current change along the pulse. Such factors as reverse current, the perturbations of the plasma channel, and the evolution of the plasma conductivity due to impact ionization, avalanche ionization, and recombination are taken into account.     

The ion Larmor radius effects on the interchange instability in a sheared magnetic field

The localized interchange instability in a low β collisionless plasma is studied as an eigenvalue problem in the presence of magnetic shear. It is found that while shear is always stabilizing, the ion Larmor radius effects become destabilizing if the shear exceeds a certain value.     

Large-amplitude double layers in a dusty plasma with an arbitrary streaming ion beam

Formation of large-amplitude double layers in a dusty plasma whose constituents are electrons, ions, warm dust grains and positive ion beam are studied using Sagdeev’s pseudopotential technique. Existence of double layers is investigated. It is found that both the temperature of dust particles and ion beam temperature play significant roles in determining the region of the existence of double layers.     

Drag of ballistic electrons by an ion beam

Drag of electrons of a one-dimensional ballistic nanowire by a nearby one-dimensional beam of ions is considered. We assume that the ion beam is represented by an ensemble of heavy ions of the same velocity V. The ratio of the drag current to the primary current carried by the ion beam is calculated. The drag current turns out to be a nonmonotonic function of velocity V. It has a sharp maximum for V near v _nF/2, where n is the number of the uppermost electron miniband (channel) taking part in conduction and v _nF is the corresponding Fermi velocity. This means that the phenomenon of ion beam drag can be used for investigation of the electron spectra of ballistic nanostructures. We note that whereas observation of the Coulomb drag between two parallel quantum wires may in general be complicated by phenomena such as tunneling and phonon drag, the Coulomb drag of electrons of a one-dimensional ballistic nanowire by an ion beam is free of such spurious effects.     

Effect of the periodic ion density channel on the behavior of the ion hose instability of a relativistic electron beam

The dynamics of a relativistic electron beam propagating in an ion channel with a periodically varying density is considered. The behavior of the ion hose instability at different parameters of the beam-ion channel system is studied using the spread mass model. Conditions are determined under which the ion hose instability does not hinder the beam propagation over distances on the order of 100 betatron lengths of the beam.     

Self-consistent current motion of electrons and ions inhigh-current plasma channel

This paper analyses a self-consistent current motion of charged particles in high-current plasma channel. Application of the results obtained to real current channels is possible provided that pair collisions do not considerably affect the current motion of plasma charged particles and the depth of the current layer is small as compared to the channel radius. The approximation adopted in this paper can be considered to be true, for instance, in the case of hydrogen channels with millimeter radius and electron energy of the order of 10 keV provided that the plasma concentration in them is in the range of 10 ¹⁷ cm^-3e<10²⁰ cm^-3. In the present paper, advantage is taken of a kinetic plasma model with electrons and ions in the form of particle beams whose motion is governed by the resulting self-consistent electromagnetic field. It is shown that in a plasma with sufficiently high particle concentration, when the collisionless skin depth is small as compared to the channel radius, the ion motion results in the negative electron contribution to the total channel current. Moreover, the ion component of the current exceeds the total current. This is accompanied by high-speed plasma motion in the form of the electroneutral axial flux, whose direction coincides with that of the total channel current     

Intensive ion beams in the transverse magnetic field of anelectromagnetic separator

Based on state-of-the-art knowledge, the processes of the formation of intense ion beams in the transverse magnetic field of a commercial electromagnetic separator are analyzed. It is shown that, given the conditions of closed electron drift, the potentials will surely equalize along the fast ion trajectories all the way through the beam length; hence, no improvement in dynamic decompensation is caused by the low-frequency noise of the source. It is established that increasing the full beam current forces the maximum amplitude of the current density oscillations out to the beam periphery crosswise. For the first time, it is shown that suppressing noise oscillations in the arc discharge of an ion source for the formation of a highly stable ion beam with a minimum inner potential drop is possible. At low pressures, with collective interaction out of play, the drop is determined by Coulomb's collisions between the beam particles and compensating electrons. The analysis of collective phenomena likely to occur in the ion-beam plasma of a separator gives evidence of an experimental space potential buildup with increasing residual gas pressure in the calutron, leading to a high-frequency ion-electron beam instability. It is pointed out that this instability combined with an additional decompensation results in a noticeable broadening of the beam energy spectrum, which in turn makes for a lower quality of separation     

On the Dispersion Properties of a Plasma Filled Rippled Boundary Resonator

Under the condition of considering the plasma electrons redistribution and ion channel and the transverse oscillating of the beam after the REB incident, the dispersion equation of corrugated waveguide filled with plasma was obtained in this paper, its expressions under several conditions were also given. The results showed that whether consider these factors or not, the results would be quite different.     

On the transverse dispersion of a relativistic electron beam in the evolution of ion hose instability in the ion focusing regime

We investigate the transverse expansion of a relativistic electron beam propagating in a rarefied gas-plasma medium in the ion focusing regime during the development of ion hose instability. The expression is constructed for the pinch potential of the beam as a function of the amplitude of transverse deviation of the beam and on the parameter characterizing the ratio of the characteristic radial scales of the ion channel and of the electron beam. It is shown that this potential becomes substantially lower upon an increase in the hose oscillation amplitude and when the ion channel expands relative to the transverse size of the beam cross section.     

Energetic electron and ion beam generation in plasma openingswitches

In this paper, we present measurements of ion and electron flows in a nanosecond plasma opening switch (NPOS) and a microsecond plasma opening switch (MPOS), performed using charge collectors. In both experiments, an electron flow toward the anode, followed by an ion flow, were observed to propagate downstream toward the load side of the plasma during the plasma opening switch (POS) conduction. In the MPOS, ion acceleration was observed to propagate axially through the entire plasma. These results are in satisfactory agreement with the predictions of the electron magnetohydrodynamics (EHMD) theory and the results of fluid and particle-in-cell (PIC) code simulations. At the beginning of the POS opening, a high-current density (≈2 kA/cm²) short-duration (10-30 ns) axial ion flow downstream toward the load was observed in both experiments, with an electron beam in front of it. These ions are accelerated at the load side of the plasma and are accompanied by comoving electrons. In the NPOS, the ion energy reaches 1.35 MeV, whereas in the MPOS, the ion energy does not exceed 100 keV. We suggest that in the NPOS the dominant mechanism for the axial ion acceleration is collective acceleration by the space charge of the electron beam, while in the MPOS, axial ion acceleration is probably governed by the Hall field in the current carrying plasma