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.     

Transverse oscillations of a long-pulse electron beam on alaser-formed channel

An intense relativistic electron beam may be transported in low-pressure gas using an ion channel which focuses and guides the beam. The beam can be unstable to the growth of transverse oscillations caused by the electric force between the beam and channel-the ion hose instability. Beam propagation on channels created by photoionization of gas with an excimer laser is discussed. Ion hose oscillations have been recorded which have a betatron wavelength of approximately 1.5 m. The growth rate of the ion hose instability in the linear regime was measured as 1.67±0.45. At this level of growth, the amplitude of beam oscillations equals the channel radius after a period of one-third of an ion oscillation time     

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.     

Effect of the current rise rate in a relativistic electron beam pulse propagating in the ion focusing regime on the dynamics of ion hose instability

The effect of the current rise rate in a relativistic electron beam pulse propagating in the ion focusing regime on the spatial dynamics of the ion hose instability is considered. Numerical analysis of the formulated equations shows that the lower rate of current rise in the beam pulse at the linear stage of evolution of the instability noticeably reduces the amplitude of hose oscillations.     

Dependence of the transverse dynamics of the plasma electrons in the ion focus regime on the ratio of the radius of a relativistic electron beam to the radius of the ion channel

A study is made of the effect of the magnetic self-field of a relativistic electron beam propagating in the ion focus regime on the transverse dynamics of plasma electrons. For Gaussian radial profiles of the beam and the ion density in the channel, the maximum deviation of the plasma electrons from the axis of the beam-plasma system is determined as a function of the space-charge neutralization fraction, the ratio of the characteristic beam radius to the channel radius, and the net beam 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     

Computation of the tracking force acting on a relativistic electron beam propagating in a conducting waveguide under the ohmic and ion-focused regimes

The force of interaction between a relativistic electron beam deflected by resistive hose instability and the eddy current induced in a tubular plasma channel of finite conductivity is computed. Dependences of the force on channel ohmic conductivity and current rise time in a beam pulse are studied. For a beam propagating through a perfectly conducting waveguide under the ion-focused regime, the interaction of the beam with the ion-channel electrostatic image on the waveguide wall is studied for the case when the beam and the channel are deflected from the waveguide axis as a result of ion hose instability. The dependence of the force on both deflection amplitudes is ascertained for the nonlinear phase of instability. It is demonstrated that the force under study may become comparable to the beam-channel interaction force if the deflections are large.     

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。     

Coherent interaction of ion and electron beams in systems with electron cooling

A hydrodynamic approximation is used to study the behavior of dipole modes of the transverse oscillations of an ion beam in a storage ring with an electron cooling section. It is shown that in addition to the finite interaction time of the beams, instability may be caused by a specific interaction effect between the ion and electron beams in the magnetic field which leads to redistribution of energy between the various modes of the ion beam oscillations. In this case, the condition that the determinant of the transfer matrix for the cooling section does not exceed unity no longer guarantees the stability of the transverse coherent oscillations of the ion beam and all the eigenvalues of the complete matrix of the ion motion including the storage ring must be analyzed. Calculations of the stability of ion beam dipole oscillations are presented for the parameters of CELSIUS.     

相对论电子束在离子通道中的聚焦与输运

 基于单粒子理论,描述了相对论电子束在离子通道中的聚焦输运过程,讨论了离子-电子密度比、相对论因子、束加速电压和入射电流等系统参数对电子束的聚焦半径、纵向聚焦位置的影响。研究表明,离子通道对电子束具有强烈的聚焦效应,束流在离子通道内的传输是类周期波动传输,随传输距离增加,聚焦点处的半径逐渐增加,束流的波动幅度逐渐减小。选择适当的系统参数,可调节束聚焦点位置和聚焦点半径的大小,实现电子束的长距传输并且减少电子束的耗散。     

Hose instability and wake generation by an intense electron beam in a self-ionized gas

The propagation of an intense relativistic electron beam through a gas that is self-ionized by the beam's space charge and wakefields is examined analytically and with 3D particle-in-cell simulations. Instability arises from the coupling between a beam and the offset plasma channel it creates when it is perturbed. The traditional electron hose instability in a preformed plasma is replaced with this slower growth instability depending on the radius of the ionization channel compared to the electron blowout radius. A new regime for hose stable plasma wakefield acceleration is suggested.     

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     

相对论电子束在动态加载等离子体中的自聚焦传输

为了进一步研究相对论电子束-离子通道辐射实验和理论的需要, 研究了相对论电子束入射中性气体以及通过碰撞电离动态加载等离子体实现对高能束流的自聚焦传输过程PIC(particle in cell) 模拟发现, 电子束电离出的离子背景能够实现对电子束的聚焦传输. 但是离子背景横向和纵向的不均匀性对束流的传输特性有显著影响. 在此基础之上, 提出了电子束在横向不均匀离子背景中传输的理论模型, 给出了束流的自聚焦条件.数值计算结果表明, 横向不均匀性会导致电子束的混合相位传输, 使得焦点附近内层电子可能跑到电子束外而被散焦损失, 这与PIC模拟的结果相符. 此外, PIC模拟还发现, 由于电子束的自聚焦, 在焦点处将电离出更多的离子而引起纵向不均匀性, 纵向不均匀性使得碰撞后的低能电子被俘获, 俘获电子效应会大幅降低电子束的传输效率. 但是俘获电子在纵向呈准周期分布, 对传输电子起到静电Wiggler场的作用, 可能实现静电Wiggler场的动态加载. 研究结果对于进一步研究电子束-等离子体系统的实验以及理论模型提出有一定的参考价值.     

Parametric channeling and collapse of beams of charged particles with internal structure in crystals: 2. Parametric processes during channeling of atomic ions, nuclei, and relativistic electrons in crystals

Features of parametric effects during channeling of atomic ions, nuclei, and relativistic electrons (positrons) in crystals were considered. It was shown that parametric coupling between ion channeling states in the field of crystal axes and planes and electronic states in the ion volume leads to the possibility of “parametric collapse” of the beam, i.e., a decrease in the oscillation amplitude of the atomic ion in the channel due to periodic transfer of the ion oscillation energy to the inner electron of the atom. The same effect can be used to cool beams due to energy transfer to intrinsic nuclear states with low energy levels. It was shown that parametric cooling of beams with a decrease in the transverse energy can also occur during axial channeling of relativistic electron beams. This process results from the parametric coupling between channeling states, which are caused by the particle charge and electron spin states in an effective magnetic field induced in the moving coordinate system.     

周期会切磁场聚焦强流相对论带状电子束的理论分析和粒子模拟

对低幅值、短周期、有偏置磁极的周期会切磁场中300 keV,3 kA带状电子束的传输进行了理论分析和粒子模拟。给出了将波导宽度考虑在内的轴向均匀带状电子束的空间电荷场和作用在有限厚度的带状电子束的短边和宽边上的聚焦力的表达式,并利用束匹配的方法得到了磁场的幅值和周期,以及电子束通道的宽度和高度等参数。最后根据理论计算的结果进行了3维粒子模拟验证,结果表明:束宽边上聚焦良好,而在短边上带状电子束的轴向有限长效应使得聚焦力与散焦力沿轴向不能完全匹配,在束包络上产生了Betatron振荡,但在300 mm的距离上传输效率仍能达到98%以上,说明有偏置磁极的周期会切磁场聚焦强流带状电子束在理论上是可行的。     

The Calculation of the Tracking Force in the Evolution of the Resistive Hose Instability of a Relativistic Electron Beam

We have analyzed the tacking force exerted by a low-conductivity Ohmic plasma channel on a relativistic electron beam. It has been shown that, for the given type of evolution of the resistive hose instability of the beam along its pulse, this force substantially depends on the frequency and the increment of the increase in the given instability.     

利用偏置磁极周期会切磁铁产生边聚焦场

 利用理论分析和数值计算的方法,研究了偏置磁极周期会切磁铁产生的、可用于带状电子束宽边聚焦的边聚焦场。结果表明:增加偏置长度,减小电子束通道宽度,增大磁极轴向长度可以在基本不改变边聚焦场在x方向上分布特性的前提下提高幅值;增大电子束通道的高度,增大磁极厚度均可以减小边聚焦场在x方向上分布曲线的曲率,但同时其幅值也会降低,该降低可利用前述方法予以补偿。在对带状电子束宽边聚焦进行束匹配时,可以先进行曲率匹配,再进行幅值匹配。在进行参数选择时,应合理选择束通道高度和磁极厚度的取值,以避免束通道内的边聚焦场在x方向上的分布出现曲率反向。     

Stability of an ion beam in synchrotrons with digital filters in the feedback loop of a transverse damper

The stability of an ion beam in synchrotrons with digital filters in the feedback loop of a transverse damper is treated. A transverse feedback system (TFS) is required in synchrotrons to stabilize the high intensity ion beams against transverse instabilities and to damp the beam injection errors. The TFS damper kicker (DK) corrects the transverse momentum of a bunch in proportion to its displacement from the closed orbit at the location of the beam position monitor (BPM). The digital signal processing unit in the feedback loop between BPM and DK ensures a condition to achieve optimal damping. Damping rates of the feedback systems with digital filters are analysed in comparison with those in an ideal feedback system.     

Effect of a transverse magnetic field on the generation of electron beams in the gas-filled diode

The effect of a transverse magnetic field (0.080 and 0.016 T) on generation of an electron beam in the gas-filled diode is experimentally investigated. It is shown that, at voltage U = 25 kV across the diode and a low helium pressure (45 Torr), the transverse magnetic field influences the beam current amplitude behind a foil and its distribution over the foil cross section. At elevated pressures and under the conditions of ultrashort avalanche electron beam formation in helium, nitrogen, and air, the transverse magnetic field (0.080 and 0.016 T) has a minor effect on the amplitude and duration of the beam behind the foil. It is established that, when the voltage of the pulse generator reaches several hundreds of kilovolts, some runaway electrons (including the electrons from the discharge plasma near the cathode) are incident on the side walls of the diode.     

Effect of the electron beam and ion flux parameters on the rate of plasma nitriding of an austenitic stainless steel

The method of nitriding of metals in an electron beam plasma is used to change the current density and energy of nitrogen ions by varying the electron beam parameters (5–20 A, 60–500 eV). An electron beam is generated by an electron source based on a self-heated hollow cathode discharge. Stainless steel 12Kh18N10T is saturated by nitrogen at 500°C for 1 h. The microhardness is measured on transverse polished sections to obtain the dependences of the nitrided layer thickness on the ion current density (1.6–6.2 mA/cm²), the ion energy (100–300 eV), and the nitrogen-argon mixture pressure (1–10 Pa). The layer thickness decreases by 4–5 μm when the ion energy increases by 100 V and increases from 19 to 33 μm when the ion current density increases. The pressure dependence of the layer thickness has a maximum. These results are in conflict with the conclusions of the theory of the limitation of the layer thickness by ion sputtering, and the effective diffusion coefficient significantly exceeds the well-known reported data.