强流相对论电子束二极管的模拟计算

文章用一个2 1/2维二极管模拟程序,研究了不同形状的无箔二极管在不同电压、不同导引磁场下的电子束输出特性。程序考虑了空间电荷效应,束流自磁场的各个分量及电子的三维运动,并使用了Child-Langmuir发射定律,计算上采用非均匀差分网格。计算结果表明,二极管形状及所加电压和磁场对输出电子束都有很大影响。导引磁场越强,输出束流越大,发散性越小。磁场大到一定程度后,束流不再增加,反而稍有降低。     

电子直线加速器的束流横向包络线方程

本文给出了包含有外加高频场、聚焦磁场、静电场以及束流本身的空间电荷场和初始发散度等效应在内的能量可变的相对论束流的横向包络线方程;由束流的平衡条件,得到了聚焦磁场公式;最后,还推导了能量可变的相对论束流的均方根包络线方程。     

电子环的绝热压缩与加速

本文在不考虑器壁影响下讨论了电子环加速器中电子环的绝热压缩和加速过程动力学,给出了平衡轨道上外磁场、绝热不变量以及对外磁场梯度和外加速电场的限制公式。当电子数较多时,在线模型近似下考虑了空间电荷的影响。最后把有关公式退化到空间电荷的作用可略去情况下的单粒子近似。     

四极磁场中粒子束输运的非线性分析及模拟

为了提高粒子在四极磁场中输运轨迹计算的精度,基于李代数方法,分析和计算粒子束在四极磁场中空间电荷的非线性效应,得到二级近似下粒子坐标的映射公式,编写了粒子束在四极磁场中传输的模拟程序QML。用模拟程序对四极磁场中的传输进行了模拟计算,并对模拟结果进行了分析。模拟结果表明,在空间电荷的非线性效应的影响下相图产生了明显的扭曲,对粒子束包络曲线横向发散作用明显。     

在电子回旋共振等离子体中离子温度的测量

用一台定向分析器，在发散磁场位形或磁镜磁场位形下，测量了在电子回旋共振（ＥＣＲ）等离子体兰平行于和垂直于外加磁场的离子温度。人们发现，作为气压的函数得到的温度是低的（０．２－０．５ｅＶ），并与由Ｄｏｐｐｌｅｒ分布的光发射谱测定法测量的温度差不多。用两种不同的方法测得的垂直于磁场的温度对于两种磁位形是很一致的。本文讨论了用定向分析器所作测量的可靠性，讨论中考虑了在分析器前面的鞘中和在磁场中的离子运动     

水基磁性流体竖直加热棒下的池沸腾传热实验研究

研究了磁性微粒浓度、外加磁场对水基磁性流体在竖直加热棒加热情况下的沸腾传热影响;实验结果显示,磁性微粒浓度和外加磁场对磁性流体的沸腾换热有很大影响：对于中低浓度的磁性流体,存在一个最优的磁粒浓度,在该浓度下沸腾传热的强化效果最显著,施加磁场时,该结论仍然成立。施加磁场能强化磁性流体的沸腾传热。     

低真空中高能电子束传输特性的MC模拟

基于Geant4蒙特卡罗程序包建立了低真空条件下高能电子束的输运模型,并用该模型详细研究了电子束在低真空条件的传输特性。结果表明,低真空条件下电子束传输特性与高真空下的有较大差异,环境气体对电子束有散射作用,而磁场可有效约束高能电子的传输轨迹,最终轨迹是磁场约束作用与环境气体散射作用竞争的结果。对于确定的磁场分布,电子束能量存在一个最佳范围,从而保证电子束的传输特性。     

轴向磁场中电弧螺旋不稳定性的简化数值分析

在静电近似下，通过数量级的分析，简化了能量方程。用数值方法研究了外加轴向磁场，电流分布、弧电流等对电弧曙旋不稳定性的影响。数值结果表明，在具有外加磁场情况下，当弧柱内均匀电流分布时，正向磁场能够使短波长的扰动稳定，反向磁场能够使长波长的扰动稳定。当弧柱内电流为抛物线形分布时，外加正向磁场对于短波情况下的稳定性影响很小，但对于长波情况，稳定性增强，外加反向磁场，同样能稳定中等和长波情况下的电弧。     

波导管毫米波自由电子激光的一维和三维数值模拟

用单粒子理论三维毫米波方形波导自由电子激光放大器程序FEL-2-A,对由可实现的摇摆器-线极化平面Wiggler外加四极子聚焦或平面抛物极面Wiggler-组成的自由电子激光放大器的非线性演变进行了计算、分析并同实验结果作了比较。FEL-2-A程序是由一组耦合非线性微分方程组构成的。它包括用来描写在无损、矩形波导中TEmn和TMmn模自洽演变的光场模拟方程和用来描述电子束的轨迹及其与光场相互作用的电子模拟方程。文中采用了傍轴近似以及绝热慢变近似。不考虑电子束群聚的纵向空间电荷力。电子束在相空间中(r,φ,x_j,v_(xj),j=1,2)的初始分布由蒙特卡罗随机抽样给出。FEL-2-A的数值模拟结果已经与LLNL的实验结果和FRED程序的模拟结果进行了比较(TE01模),其间的符合程度是十分令人满意的。此外还用此程序研究了Wiggler的不同极化形式,不同注入功率及各种其他相关参量对FEL输出功率的影响,所有这些对4MeV FEL放大器的设计和实验都具有一定的参考意义。在该程序中,考虑空间电荷力对输出功率的影响和使用共振粒子近似方程设计Wiggler的振幅和周期,使得Wiggler参数与电子束的能量相匹配以提高输出功率的计算,分析以及同实验结果的比较将在另外的文章中给出。     

波导管毫米波自由电子激光的一维和三维数值模拟

用单粒子理论三维毫米波方形波导自由电子激光放大器程序FEL-2-A,对由可实现的摇摆器-线极化平面Wiggler外加四极子聚焦或平面抛物极面Wiggler-组成的自由电子激光放大器的非线性演变进行了计算、分析并同实验结果作了比较。FEL-2-A程序是由一组耦合非线性微分方程组构成的。它包括用来描写在无损、矩形波导中TEmn和TMmn 模自洽演变的光场模拟方程和用来描述电子束的轨迹及其与光场相互作用的电子模拟方程。文中采用了傍轴近似以及绝热慢变近似。不考虑电子束群聚的纵向空间电荷力。电子束在相空间中(γ,ψ,χ_j,υ_(xj),j=1,2)的初始分布由蒙特卡罗随机抽样给出。FEL-2-A的数值模拟结果已经与LLNL的实验结果和FRED程序的模拟结果进行了比较(TE01模),其间的符合程度是十分令人满意的。此外还用此程序研究了Wiggler的不同极化形式,不同注入功率及各种其他相关参量对FEL输出功率的影响,所有这些对4MeV FEL放大器的设计和实验都具有一定的参考意义。在该程序中,考虑空间电荷力对输出功率的影响和使用共振粒子近似方程设计Wiggler的振幅和周期,使得Wiggler参数与电子束的能量相匹配以提高输出功率的计算,分析以及同实验结果的比较将在另外的文章中给出。     

The influence of magnetic field on the beam quality of relativistic electron beam long-range propagation in near-Earth environment

In recent years,it has been proposed to use satellite-mounted radio-frequency(RF)accelerators to produce high-current relativistic electron beams to complete debris removal tasks.However,when simulating the long-range propagation(km-range)process of the electron beam,it is difficult to directly use the particle-in-cell method to simultaneously consider the space charge effect of beam and the influence of the geomagnetic field.Owing to these limitations,in this paper,we proposed a simplified method.The ps-range electronic micropulses emitted by the RF accelerator were transmitted and fused to form a ns-range electron beam;then,combined with the improved moving window technology,the model was constructed to simulate the long-range propagation process of the relativistic electron beam in near-Earth environment.Finally,by setting the direction of movement of the beam to be parallel,perpendicular and at an inclination of 3° to the magnetic field,we analyzed and compared the effects of the applied magnetic fields in different directions on the quality of the beam during long-range propagation.The simulation results showed that the parallel state of the beam motion and magnetic fields should be achieved as much as possible to ensure the feasibility of the space debris removal.     

Simulation of a low energy beam transport line

A 2.45 GHz electron cyclotron resonance intense proton source and a low energy beam transport line with dual-Glaser lens were designed and fabricated by Institute of Modern Physics for a compact pulsed hadron source at Tsinghua. The intense proton beams extracted from the ion source are transported through the transport line to match the downstream radio frequency quadrupole accelerator. Particle-in-cell code BEAMPATH was used to carry out the beam transport simulations and optimize the magnetic field structures of the transport line. Emittance growth due to space charge and spherical aberrations of the Glaser lens were studied in both theory and simulation. The results show that narrow beam has smaller aberrations and better beam quality through the transport line. To better match the radio frequency quadrupole accelerator, a shorter transport line is desired with sufficient space charge neutralization.     

空间电荷透镜电子云状态研究

本文分析了新型空间电荷透镜磁镜的轴对称磁场(z,r)平面的磁感应强度的变化曲线,由电子的磁流体运动方程组,推导了处于动态平衡状态下电子云中电子密度的波尔兹曼分布。结合磁镜场的磁感应强度分布和电子的波尔兹曼分布,采用四阶龙格 库塔公式对泊松方程进行二阶微分方程数值求解,得到了理想空间透镜中电子云的电势分布。在磁镜场的磁场基础上分析了空间电荷透镜中心轴线的磁约束因子沿z轴方向变化的趋势和数值模拟中心电子密度、磁约束因子、电子温度对电子云状态的影响。磁约束因子越大,对电子云的约束能力越强,电子集中在空间电荷透镜中心轴附近区域的现象越明显;电子温度越高,电子运动越活跃,弥散空间越大,电子集中在空间电荷透镜中心轴附近区域的现象越不明显。     

Effect of Ion Engine Exhaust on the Radiation Field of a Dipole Antenna Part I: Infinite Slab Model for the Exhaust Ion Beam

A theory of the effect of an ion rocket engine exhaust on the radiation pattern of a dipole antenna is presented. The electromagnetic equations are combined with those describing the exhaust plasma beam to calculate its equivalent effective dielectric constant. In part I of this paper, the beam is represented by an infinite slab of a homogenous plasma medium, as is usually considered in space charge neutralization studies of this type of engine. The equations of propagation of electromagnetic waves through the beam medium, are used to calculate the total dipole radiation field. The method of steepest descent is applied for the evaluation of the integrals. It is found that the dipole radiation pattern depends greatly on the beam characteristic parameters.     

电子直线加速器中计入纵向与横向耦合效应的束流包络线方程组

一、引言从束流动力学的观点出发,对于粒子束在直线加速器中的运动,有两种主要的分析方法,也即粒子轨迹跟踪法及考察束流包络运动的方法。前者是通过解单粒子运动方程,跟踪大量的单粒子运动轨迹而后进行分析;后者则是通过解束流的包络线方程,根据束流包络的变化来进     

Numerical simulation of the plasma acceleration process in a magnetically enhanced micro-cathode vacuum arc thruster

A particle-in-cell simulation is conducted to investigate the plasma acceleration process in a micro-cathode vacuum arc thruster. A coaxial electrode structure thruster with an applied magnetic field configuration is used to investigate the effects of the distribution of the magnetic field on the acceleration process and the mechanism of electrons and ions. The modeling results show that due to the small Larmor radius of electrons, they are magnetized and bound by the magnetic field lines to form a narrow electron channel. Heavy ions with a large Larmor radius take a long time to keep up with the electron movement. The presence of a magnetic field strengthens the charge separation phenomenon. The electric field caused by the charge separation is mainly responsible for the ion acceleration downstream of the computation. The impact of variations in the distribution of the magnetic field on the acceleration of the plasma is also investigated in this study, and it is found that the position of the magnetic coil relative to the thruster exit has an important impact on the acceleration of ions. In order to increase the axial velocity of heavy ions, the design should be considered to reduce the confinement of the magnetic field on the electrons in the downstream divergent part of the applied magnetic field.     

The surface charge effect by using boron ions implantation in p-channel MOSFETs fabricated

This article reports on the effects of surface charge on bare wafers and p-channel MOSFETs by a positive ion beam accompanied by an electron beam current for surface charge neutralization. Without the negative electron beam the films show a higher sheet resistance and the pMOSFETs exhibit a lower threshold voltage, a lower breakdown voltage and a lower gain factor. If the electron beam current is equal to or higher than the ion beam current of 6 mA, the uniformity of sheet resistance, fluctuations of breakdown voltage and gain factor are significantly improved by controlling the charge neutralization in this experiment. It prevents the positive charges from penetrating the poly-gate and causing catastrophic damage in the gate oxide layer. The sheet resistance deviation for the samples with a capped photoresist is higher than that for the bare wafer because the insulating property of the photoresist enhances the wafer surface charge accumulation and thus repels the subsequently implanted ions. The thinner gate oxide layer leads to larger deviations of threshold voltage, breakdown voltage and gain factor due to the imbalance of surface charge.     

FEL中的束流发射度分析

根据自由电子激光器(FEL)对电子束流品质的要求,对束流发射度以及引起束流发射度变化的因素,诸如加速过程中微波场的突变、能量增加、波导结构的尾场效应和传输过程中的空间电荷效应、粒子密度分布的影响及偏转系统中的尾场效应等等,进行了物理上的分析,对其中某些情况,给出了束流发射度变化量的具体估算。