飞秒激光辐照铝靶产生快电子发射的实验和模拟研究

采用实验和数值模拟研究了飞秒激光辐照铝靶产生的快电子发射.实验中,在主脉冲前加上一个预脉冲产生预等离子体,然后主脉冲与预等离子体作用产生快电子.在激光反射方向附近,实验测量的快电子束发射与数值模拟的结果高度地一致;在靶背面,发射的快电子的数目小于数值模拟的结果,原因在于快电子在靶内输运受到电荷分离场和碰撞的影响;在数值模拟中未出现的,沿靶表面发射的快电子束,是由表面准静态电磁场的禁闭效应产生.     

飞秒激光辐照铝靶产生快电子发射的实验和模拟研究

采用实验和数值模拟研究了飞秒激光辐照铝靶产生的快电子发射。实验中,在主脉冲前加上一个预脉冲产生预等离子体,然后主脉冲与预等离子体作用产生快电子。在激光反射方向附近,实验测量的快电子束发射与数值模拟的结果高度地一致;在靶背面,发射的快电子的数目小于数值模拟的结果,原因在于快电子在靶内输运受到电荷分离场和碰撞的影响;在数值模拟中未出现的,沿靶表面发射的快电子束,是由表面准静态电磁场的禁闭效应产生。     

空间电荷效应对阴极微凸起热不稳定性的影响

 由于金属微凸起爆炸电子发射的预发射电流密度一般都超过108 A/cm2,因此必须考虑其空间电荷效应的影响。基于金属微凸起爆炸电子发射起始过程模型,通过理论分析和数值模拟,给出了考虑预发射电流空间电荷效应的微凸起爆炸发射延迟时间随二极管平均电场的变化关系。与不考虑预发射电流空间电荷效应的结果进行对比表明,预发射电流的空间电荷效应可以显著增加金属微凸起的爆炸发射延迟时间。     

腔体系统电磁脉冲模拟中的电子发射面

对电子发射面设置问题进行了研究,给出了底入射腔体系统电磁脉冲模拟中电子发射面的实现方法,分析了侧入射腔体系统电磁脉冲模拟中电子发射面的特点,并利用虚拟发射面的方法实现了侧入射腔体系统电磁脉冲模拟中电子发射面的设置。结果显示,利用该方法可以较简洁地实现腔体系统电磁脉冲模拟中平面和曲面电子发射面的设置,发射面电子发射参数和时序符合物理规律。     

三维PIC数值模拟中二次发射的实现

探讨了电子撞击金属表面产生二次电子的理论,介绍了二次电子发射系数、非弹性背散射电子能量分布等重要物理参数的计算式。根据相关的理论及公式,编写了三维PIC数值模拟程序的二次发射模块, 建立相应的数值模型进行模拟。模拟所得的二次电子发射系数及非弹性背散射电子的能量分布等主要参数与理论值一致,验证了模拟过程的正确性。     

Monte Carlo simulation of absolute secondary electron yield of Cu

A Monte Carlo simulation model of electron interaction with solids that includes cascade secondary electron production has been used to study secondary electron emission from Cu. An optical dielectric function was used to describe electron energy loss and the associated secondary electron excitation. From the simulation, the absolute primary energy dependence of the secondary yield and the energy distribution of secondary electrons has been obtained. We have compared the calculated true secondary yield and total secondary yield curves with experiment. Good agreement has been found only for those experiment data obtained in ultrahigh vacuum. PACS 79.20.Hx; 79.20.Ap; 02.70.Uu     

The advantage of blue InGaN multiple quantum wells light-emitting diodes with p-AlInN electron blocking layer

InGaN based light-emitting diodes (LEDs) with different electron blocking layers have been numerically investigated using the APSYS simulation software. It is found that the structure with a p-AlInN electron blocking layer showes improved light output power, lower current leakage, and smaller efficiency droop. Based on numerical simulation and analysis, these improvements of the electrical and optical characteristics are mainly attributed to the efficient electron blocking in the InGaN/GaN multiple quantum wells (MQWs).     

Modeling of argon discharge characteristics of planar-type surface wave plasmas in an electron fluid model

The planar-type surface wave plasma (SWP) device permits the generation of high-density and uniform processing plasmas via 2.45-GHz microwave power without the application of an external magnetic field. In the present study, the discharge characteristics in the SWP device were analyzed using a two-dimensional numerical simulation code, and the results were compared with experimental observations. The simulation code is based on the finite-difference time-domain (FDTD) method for the microwave field and on the electron fluid model for the argon discharge plasma. Experimental measurements were performed, and they showed that the surface-wave discharge at a filling pressure of 10-100 mtorr has characteristic electron-density distributions that have a peak at approximately 2 cm from the surface. This characteristic of the electron density profiles, as well as the electron temperature profiles in the plasma, is reproduced by the simulation code, albeit with some discrepancies. In order to reduce the effects of these discrepancies, intentional changes in the electron heat conductivity were introduced, and the adiabatic assumption was found to result in a reasonable electron temperature profile. The effects of the alumina window thickness were also investigated in the simulation.     

高增益自由电子激光饱和态分析的三维统计理论

考虑电子束横向发射度和电子β振荡,将2005年国际上提出的单通过高增益自由电子激光饱和状态分析的统计物理方法发展到三维情形。首先建立一种描述电子三维运动的归一化简化模型,推导了一维光场下包含电子横向运动的Vlasov方程。在螺旋型波荡器情形下通过引入横向运动守恒量发展了三维统计物理分析方法,并编写了相应计算程序,计算自由电子激光达到饱和时系统的光强增益、聚束因子。作为对比验证,编写包含N个电子自由电子激光系统的三维直接数值模拟程序,结果表明数值模拟和统计计算结果相一致。对比文献中一维模拟和一维统计理论计算结果,所得结果反映了电子束横向发射度以及电子在波荡器中的横向β振荡对饱和点参数的影响。     

强流电子束阻抗对相对论速调管放大器注入及群聚特性的影响分析

通过数值计算及粒子模拟程序, 分析了强流电子束阻抗、电压及电流特性对相对论速调管放大器(relativistic klystron amplifier, RKA)中束流调制、群聚特性的影响, 其中粒子模拟程序中采用 束发射方式以精确控制电子束的阻抗.结果表明, 低阻抗电子束有利于减小群聚距离, 缩短RKA器件的整体长度, 不利于注入微波对电子束的调制, 而高阻抗电子束情况正好相反.在电子束阻抗不变时, 增加电子束加速电压类似于增大电子束阻抗的情况.另外, 用粒子模拟方法确定了不同阻抗电子束对特定输入腔的电子负载电导, 从而可以得到不同阻抗的强流电子束对种子源 功率水平的需求以及对输入腔外观品质因数的要求. 关键词： 相对论速调管放大器 电子束阻抗 群聚距离 输入调制     

高功率微波介质窗表面电子倍增二维粒子模拟

基于第一性原理的粒子模拟方法,对高功率微波器件中介质窗表面电子实际形成和发展的变化情况进行了研究。使用VORPAL粒子模拟软件,建立一个简单的TEM波垂直入射介质窗表面的二维模型,采用Vaughan二次电子发射模型,利用蒙特卡罗碰撞方法处理电子与背景气体之间的弹性碰撞、激发碰撞和电离碰撞,获得了介质窗表面电子倍增的图像。模拟结果表明,介质窗表面电子数量在一定的时间内达到饱和状态,其振荡频率是入射射频电场频率的两倍。改变初始发射种子电子的数量、入射射频电场的幅值以及背景气体的压强等关键性参数,可得到不同条件下介质窗表面电子数量的变化规律。     

Effects of secondary electron emission on plasma characteristics in dual-frequency atmospheric pressure helium discharge by fluid modeling

A one-dimensional(1D) fluid simulation of dual frequency discharge in helium gas at atmospheric pressure is carried out to investigate the role of the secondary electron emission on the surfaces of the electrodes. In the simulation, electrons,ions of He~+ and He_2~+, metastable atoms of He*and metastable molecules of He*_2 are included. It is found that the secondary electron emission coefficient significantly influences plasma density and electric field as well as electron heating mechanisms and ionization rate. The particle densities increase with increasing SEE coefficient from 0 to 0.3 as well as the sheath's electric field and electron source. Moreover, the SEE coefficient also influences the electron heating mechanism and electron power dissipation in the plasma and both of them increase with increasing SEE coefficient within the range from 0 to 0.3 as a result of increasing of electron density.     

电子照相技术成像模拟研究

针对高能电子照相的关键物理过程开展蒙特卡罗照相模拟研究,采用Geant4围绕2.5 GeV电子在四极透镜组内的输运、电子与物质相互作用衰减等照相基本过程展开研究。通过设计不同材料、不同厚度的含缺陷平板作为模拟照相客体,开展放大型电子照相系统缺陷分辨能力模拟。此外,采用不同材料、不同厚度的台阶样品,模拟获得了电子束流穿过相应面密度材料后的线扩展函数,进一步评估电子照相对实心客体的探测分辨能力。     

S波段6 MeV边耦合电子直线加速器的设计研究（英文）北大核心CSCD

近年来,对紧凑、稳定及可靠型电子直线加速器的需求越来越多,其能量主要分布在几百keV到十几个MeV的范围内,其中需求最多的则是能量在MeV量级的微波电子直线加速器。在这种形势下,中国科学院高能物理研究所正在研制一台S波段6 MeV的边耦合电子直线加速器,本文对基于该加速器的模拟计算研究进行了介绍。EGUN和HFSS分别用来设计电子枪和边耦合加速结构。通过将EGUN计算得到的电子束流参数和HFSS计算得到的三维电磁场分布数据引入到PARMELA中,完成了对该加速器的多粒子动力学研究。模拟结果显示,所设计的加速器完全能够满足设计指标的要求。最终,在考虑束流负载效应的因素后,完成了边耦合加速结构的微波结构设计。     

Electron beam modeling and analyses of the electric field distribution and space charge effect

In electron beam technology, one of the critical focuses of research and development efforts is on improving the measurement of electron beam parameters. The parameters are closely related to the generation, emission, operation environment, and role of the electron beam and the corresponding medium. In this study, a field calculation method is proposed, and the electric field intensity distribution on the electron beam's cross-section is analyzed. The characteristics of beam diffusion caused by the space charge effect are investigated in simulation, and the obtained data are compared with the experiment. The simulation demonstrated that the cross-sectional electric field distribution is primarily affected by the electron beam current, current density distribution, and electron beam propagation speed.     

电子双缝衍射的计算机模拟

根据电子双缝衍射强度分布函数,利用Visual Basic编程,实现了电子双缝衍射的计算机模拟.该模拟不仅生动再现了电子衍射图像,而且通过控制电子产生的速度,能让电子逐个打到屏上,能加深电子是概率波的认识.程序语言简单易学,利于进一步开发.     

微波管电子枪三维粒子模拟研究

 分析了以往电子光学模型进行电子枪模拟工作过程及存在的缺陷，指出粒子模拟方法能够全面考虑空间电荷限制和温度限制、阴极热初速、电子注非层流性等因素，研究电子注的瞬态和噪声特性，是一种更准确的电子枪模拟方法。以某电子枪为例，运用Mafia PIC模块进行了模拟计算，与实验结果对比表明Mafia粒子模拟具有相当精度，可用于指导微波管电子枪工程设计。     

PIC/MCC Simulation of Electron and Ion Currents to Spherical Langmuir Probe

The Particle In Cell/Monte Carlo Collisions (PIC/MCC) simulation was used for the calculation of electron and ion currents to a spherical Langmuir (electrostatic) probe. This simulation took into account the collisions of collected charged particles with neutral gas particles around the probe and it can calculate the probe currents at higher neutral gas pressures. The improvements of usual simulation techniques enabled to speed up the simulation and to calculate the probe current even for neutral gas pressures above 1 kPa. The simulations were carried out for two cases: i) probe with radius of 0.5 mm in non‐thermal plasma with high electron temperature, ii) probe with radius of 10 µm in afterglow plasma with low electron temperature. The influence of probe radius on electron probe current was also studied. The simulations showed that thick sheath limit of OML theory provides incorrect values of probe current for probes with radii larger than 200 µm at plasma parameters considered even at very low neutral gas pressures. The probe characteristics were calculated for probe with 0.5 mm radius for pressures up to 500 Pa and for probe with 10 μm radius for pressures up to 3 kPa. The influence of collisions on electron and ion probe current was demonstrated and the procedure for determination of electron and ion densities from the probe measurement at higher pressures was developed. The results from PIC/MCC simulations were compared with results from continuum theory. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The effects of a small transverse magnetic field upon acapacitively coupled RF discharge

A capacitively-coupled RF argon discharge at a pressure of 10 mTorr with a plate separation of 7.5 cm has been studied both experimentally and using a one-dimensional particle in cell simulation with Monte Carlo collisions. A magnetic field of 0 to 60 G is applied in the direction parallel to the capacitor plates. In the simulation it was found that as the magnetic field was increased such that the electron cyclotron orbit radius of the hot electrons became smaller than of the order of the discharge length, the electron heating in the bulk of the discharge increased. The dominant electron heating mechanism was observed to change from a stochastic sheath to a bulk ohmic electron heating mode, with a variation of field from a to 10 G. This was accompanied by a drop in the plasma density at small magnetic fields, which was also observed experimentally. At higher magnetic fields the plasma density was found to increase, A detailed discussion of the simulation results is presented drawing comparisons with the experimental results, with which there is good agreement, and a simple magnetohydrodynamic model for the bulk heating     

Exact solution for temerature field due to non-equilibrium heating of solid substrate

Non-equilibrium energy transfer between electron and lattice sub-systems due to short-pulse heating is formulated and the closed form solution for electron and lattice site temperatures is presented. The electron kinetic theory approach is incorporated to formulate non-equilibrium energy transfer in the electron and lattice sub-systems. The method of Lie point symmetries is used in the exact solution of governing energy equation. In the analysis, the volumetric heat source, representing the laser heating pulse, and surface heat source, corresponding to short thermal contact of the surface, are incorporated and the analytical solutions for each heating source are presented. Electron temperature distribution obtained from the closed form solution is compared with its counterpart predicted from the numerical simulation. It is found that the results obtained from the closed form agree well with electron temperature predictions obtained from numerical simulation.