沿面闪络流体模型电离参数粒子模拟确定方法

介绍了粒子模拟确定高功率微波介质沿面闪络击穿流体模型相关电离参数的方法.对粒子模拟方法 (包括带电粒子动力学方程、次级电子发射以及蒙特卡罗碰撞模型)和流体整体模型方法 (包括连续性方程和能量守恒方程)做了简介.基于自编的1D3V粒子模拟-蒙特卡罗碰撞程序给出了在高(低)气压、不同气体种类以及不同微波场强和微波频率下流体模型电离参数的粒子模拟结果,包括电离频率、击穿时间、平均电子能量、电子能量分布函数类型.研究结果表明:平均电子能量与电子能量分布函数类型关系不大;中低气压下,电子能量接近Maxwell分布,电子能量分布函数类型对电离参数几乎没有影响;中高气压下,电子能量分布函数类型对电离参数有重要影响,其依赖系数X趋于高阶形式.不同气体的电子能量分布函数类型不同,需要利用粒子模拟对电子能量分布函数类型进行标定.同时,电子能量分布函数依赖系数与微波场强和频率也有关系,其随微波场强增加而增大,随微波频率增加而减小.在给定考察范围(微波场强在7 MV/m以下,微波频率在40 GHz以内),中低气压下,平均电子能量随微波场强增加而迅速增大,电离频率随微波场强增加先增大后降低,平均电子能量随微波频率增加而降低,电离频率随微波频率增加先增加后降低;高气压下,平均电子能量随微波场强增加而缓慢增大,电离频率随微波场强增加而增大,微波频率对平均电子能量和电离频率影响不大.     

高功率微波及材料特性参数对沿面击穿的影响

为研究高功率微波及材料特性参数对介质沿面闪络击穿过程的影响,采用自编的1D3V PIC-MCC程序,通过粒子模拟手段,得到了电子与离子数目、电子及离子密度分布、空间电荷场时空分布、电子平均能量、放电功率、表面沉积功率、激发电离损耗功率、电离频率等重要物理量。结果表明:电离频率随场强增加而增加,达到饱和后缓慢下降,强场诱发的二次电子数目更多导致本底沉积功率增高;电离频率随频率减小而增加,达到饱和后缓慢下降,频率太高会抑制次级电子倍增;因此,低频强场下击穿压力较大;反射引发表面电场下降及磁场增加效应,降低表面场强虽使表面击穿压力下降,但磁场的增加会导致二次电子倍增起振时间缩短,且会增加器件内部击穿风险;圆极化相对线极化诱导二次电子数目更多、本底沉积功率更高,击穿风险增加;短脉冲产生电子、离子总数少,平均能量低,沉积功率低,击穿风险低于长脉冲;脉冲上升时间的缩短和延长,只会提前或推后击穿时间,并不会改善击穿压力;材料二次电子发射率的增加会给击穿造成巨大压力,表面光滑度对击穿过程影响不大;电离频率和电子平均能量随释气压强增加均先增加后减小,低气压二次电子倍增占优,高气压碰撞电离占优。     

入射电子能量对低密度聚乙烯深层充电特性的影响

高能带电粒子与航天器介质材料相互作用引起的深层带电现象, 一直是威胁航天器安全运行的重要因素之一. 考虑入射电子在介质中的电荷沉积、能量沉积分布以及介质中的非线性暗电导和辐射诱导电导, 建立了介质深层充电的单极性电荷输运物理模型. 通过求解电荷连续性方程和泊松方程, 可以得出不同能量 (0.1–0.5 MeV) 电子辐射下, 低密度聚乙烯 (厚度为1 mm) 介质中的电荷输运特性. 计算结果表明, 不同能量的电子辐射下, 介质充电达到平衡时, 最大电场随入射能量的增加而减小; 同一能量辐射下, 最大电场随束流密度的增大而增加. 入射电子能量较低时 (≤ 0.3 MeV) , 最大电场随束流密度的变化趋势基本相同. 具体表现为: 当束流密度大于3× 10^-9 A/m²时, 最大场强超过击穿阈值2×10⁷ V/m, 发生静电放电 (ESD) 的可能性较大. 随着入射电子能量的增加, 发生静电放电 (ESD) 的临界束流密度增大, 在能量为0.4 MeV时, 临界束流密度为6×10^-8 A/m². 当能量大于等于0.5 MeV时, 在束流密度为10^-9–10^-6 A/m²的范围内, 均不会发生静电放电 (ESD) . 该物理模型对于深入研究深层充放电效应、评估航天器在空间环境下 深层带电程度及防护设计具有重要的意义. 关键词： 高能电子辐射 低密度聚乙烯(LDPE) 介质深层充电 电导特性     

微波大气击穿阈值的理论研究

本文通过对使用有效场强(或均方根场强)得到的微波大气击穿阈值表达式进行讨论, 指出其推导中所做的假设及这些假设应用到微波大气击穿过程中存在的问题. 然后分别使用解析理论和数值模拟对微波大气击穿过程中的有效电子温度变化过程和击穿阈值进行研究, 并将其与直流电场进行比较. 分析发现在高气压下, 电子能量转移频率高, 有效电子温度随电场大幅振荡, 由于电离频率随有效电子温度的增长率大于电子能量损失随有效电子温度的增长率, 因此在高气压时, 微波大气击穿阈值低于使用有效场强的击穿阈值. 通过大量分析, 给出了理论推导和数值模拟得到的微波大气击穿阈值拟合表达式.     

高功率微波作用下O-离子解吸附产生种子电子过程

种子电子是高功率微波大气击穿的根源, 研究高功率微波大气击穿时, 一般假设背景大气中存在种子电子, 此假设在低层大气环境中会给模拟结果带来较大误差. 本文建立了高功率微波强电场作用下O^-离子解吸附碰撞过程物理模型, 基于传统的空碰撞模型, 提出了改进的蒙特卡罗仿真方法, 编写了三维仿真程序, 对高功率微波作用下O^-离子的解吸附过程进行了仿真, 分析了O^-离子平均能量随时间的变化过程以及O^-离子与空气分子的碰撞过程, 得到了不同压强、场强、频率和击穿体积条件下种子电子平均产生时间. 理论与仿真结果表明, 随着频率增大, 种子电子平均产生时间变大, 随着击穿体积、场强以及压强增大, 种子电子平均产生时间变小. 最后, 考虑O^-离子与空气分子解吸附碰撞提供种子电子条件下, 给出了大气击穿时间理论与实验对比结果, 发现高功率微波频率较低时, 该种子电子产生机理可以解释实验结果, 而高功率微波频率较高时, 该机理下种子电子平均产生时间过长而与实验数据不符.     

氮等离子体枪中电子输运行为的蒙特卡罗模拟

采用蒙特卡罗模拟,对氮等离子体枪的两个环形电极间的氮气辉光放电过程中电子的输运过程进行研究.计算在不同平均电场与粒子数密度的比值(E/N)下,电子与氮分子发生不同碰撞的概率、出射电子的平均能量、方向角分布和电子的能量分布.结果表明,电子能量近似服从玻尔兹曼分布.随着E/N的升高,电子平均能量升高,发生激发、离化、电离和离化电离碰撞的概率增大;非均匀分布的电场使分子获得更高的离化率,同时显著增强出射电子的能量.模拟结果为等离子体应用设计提供了参考依据.     

氩直流辉光放电等离子体中电子运动及能量的模拟

采用自动调节时间步长的蒙特卡罗模拟,对平行板放电系统中的氩气直流辉光放电系统中的等离子体区内电子的运动过程进行了跟踪和抽样。统计结果表明:在我们的实验条件下,等离子体中的电子在电场作用下出现明显的轴向漂移;在40000次抽样中,出现能量为E的电子数目随能量E增大呈下降趋势,场强增大将引起能量分布展宽和电子平均能量增加;即使场强达到15V·cm-1,等离子体激发和电离仍是很少的;场强和气压都能明显改变电子的平均自由程。     

约化场强对氮-氧混合气放电等离子体演化特性的影响

采用零维等离子体动力学模型,计算了不同约化场强条件下N₂/O₂放电等离子体的演化特性.结果表明,平均电子能量与约化场强有着近似的线性关系,在约化场强为100 Td时,平均电子能量约为2.6 eV、最大电子能量达35 eV;约化场强是影响电子能量函数分布的主要因素.气体放电过程结束后,振动激发态氮分子的粒子数浓度不再变化,电子激发态的氮分子、原子和氧原子的粒子数浓度达到一峰值后开始降低;放电结束后的氧原子通过复合反应生成臭氧.约化场强升高,由于低能电子减少的影响,振动激发态氮分子的粒子数浓度降低,当约化场强由50 Td增加75 Td,100 Td时,粒子数浓度由3.83×10¹¹ cm^-3降至1.98×10¹¹ cm^-3和1.77×10¹¹ cm^-3,其他粒子浓度则相应增大. 关键词： 等离子体 约化场强 粒子演化 数值模拟     

极性半导体膜中的束缚极化子

采用线性组合算符法和幺正变换方法,研究极性晶体膜中束缚极化子的基态能量、自陷能随膜厚d的变化关系.得出束缚极化子的自陷能由两部分组成:第一部分是由于电子-体LO声子相互作用所引起的(E_e-LO^tr)极化子效应;第二部分则是电子-SO声子相互作用引起的.后者又包含两部分,分别是电子与极性膜中两支表面声子相互作用的贡献(E_e-SO^tr(+),E_e-SO^tr(-)).通过对KCl半导体膜的数值计算表明,E_e-LO^tr随膜厚d的增加而增加;但是E_e-SO^tr、极化子的振动频率以及电子-声子相互作用所产生的总自陷能E_e-ph^tr随膜厚d的增加而减少,当膜厚大于5nm时,总自陷能E_e-ph^tr趋于一稳定值.另外,由于束缚势的存在,使极化子的振动频率增大,这主要是由于束缚势的存在,使电子-声子间的相互作用增强,极化子效应增大而引起的.     

FDTD方法分析高功率微波对大气的电离与击穿

 应用麦克斯韦方程和电子流体方程,利用时域有限差分方法(FDTD)计算模拟了高功率微波(HPM)对大气的电离与击穿;该方法用瞬时电场代替等效电场,时刻更新大气电离击穿过程中的电离频率和碰撞频率,消除了近似解析法未考虑大气电离击穿过程中电场幅度衰减而引起的误差,计算得到击穿阈值大小随海拔高度的变化趋势与文献所得的变化趋势相吻合,其值略大于近似解析解;并通过仿真计算分析了HPM脉冲幅值、脉宽以及海拔高度等参数对大气击穿的影响。     

A model for a uniform steady-state vacuum arc with a hot anode

A model is formulated and evaluated for a Uniform electrical discharge sustained in vapor evaporated from an arc-heated anode. The plasma potential is positive with respect to both the cathode and anode. For a Cu anode, the anodic vapor dominates the plasma for current densities exceeding 8 kA/m². The anode heating potential is approximately 6.5 V, and the dominant cooling mechanism is evaporation for current densities exceeding 20 kA/m². Over the range 10 to 10000 kA/m², the electron density increases from 8×10¹⁷ to 5×10²³ m^-3, while the ionization fraction rises from 0.3% to 4%. At the lower end of this current range the electrical resistivity of 4 mΩ-m is determined primarily by electron-neutral collisions, while with increasing current the resistivity decreases to 0.7 mΩ-m, with electron-ion collisions contributing an equal share. This hot-anode vacuum arc may have potential for industrial application as a macroparticle-free high-deposition-rate coating source     

Excitation energy and angular momentum dependence of nuclear level densities and spin cut-off factor in SPA and SPA + RPA approaches

We investigate the excitation energy (E^*) and angular momentum (J) dependence of nuclear level density and spin cut-off factor (σ) within microscopic approaches based on SPA and its extension SPA + RPA representation of the grand partition function for quadrupole-quadrupole interaction model Hamiltonian. For ¹¹⁰Sn, we find that excitation energy dependence of the total level density obtained within these approaches is significantly different. On the other hand, these approaches yield similar behaviour for J-dependence of the level density at fixed values of E^*. Values of σ_{SPA + RPA} at low E^* are found to be slightly smaller than σ_SPA but they tend to become almost the same at higher E^* (> 30 MeV). We also find that Bethe's formula for fixed-J level density based on the spin cut-off approximation can be used to compute (E^*, J) near the yrast line provided one uses an appropriate value of the spin cut-off factor.     

Predicted emittance and brightness of the pseudospark electron-beam

The emittance and brightness of the electron beam generated during the hollow cathode phase of pseudospark operation are calculated using the two-dimensional hybrid fluid-particle model previously developed to study the time and space development of the plasma in a pseudospark discharge. Two distinct energy components exist in the electron beam; a high-energy component with an energy equivalent to the full discharge voltage and another, broad, low-energy component. In the 100 ns following breakdown and for the conditions of the calculations, the emittance of the high energy component decreases by an order of magnitude and the brightness of the high energy component reaches almost 10¹⁰ A/m² rad². This work demonstrates the feasibility of using the model to guide the optimization of the pseudospark electron beam properties and shows that the optimum beam properties are achieved after the plasma has filled the hollow cathode and begun to expand radially in the main gap     

Importance of high local cathode spot pressure on the attachment ofthermal arcs on cold cathodes

The importance of having high local cathode spot pressures for the self-sustaining operation of a thermal arc plasma on a cold cathode is theoretically investigated. Applying a cathode sheath model to a Cu cathode, it is shown that cathode spot plasma pressures ranging 7.4-9.2 atm and 34.2-50 atm for electron temperatures of ~1 eV are needed to account for current densities of 10⁹ and 10¹⁰ A·m^-2, respectively. The study of the different contributions from the ions, the emission electrons, and the back-diffusing plasma electrons to the total current and heat transfer to the cathode spot has allowed us to show the following. 1) Due to the high metallic plasma densities, a strong heating of the cathode occurs and an important surface electric field is established at the cathode surface causing strong thermo-field emission of electrons. 2) Due to the presence of a high density of ions in the cathode vicinity, an important fraction of the total current is carried by the ions and the electron emission is enhanced. 3) The total current is only slightly reduced by the presence of back-diffusing plasma electrons in the cathode sheath. For a current density j_tot=10⁹ A·m^-2 , the current to the cathode surface is mainly transported by the ions (76-91% of j_tot while for a current density j_tot = 10¹⁰ A·m^-2, the thermo-field electrons become the main current carriers (61-72% of j_tot). It is shown that the cathode spot plasma parameters are those of a high pressure metallic gas where deviations from the ideal gas law and important lowering of the ionization potentials are observed     

A two-component model for the electron distribution function in ahigh-current pseudospark or back-lighted thyratron

Temperature, energy, and densities of two electron distribution function components, including an isotropic bulk part and an anisotropic beam, are analyzed for a hydrogen pseudospark and/or back-lighted thyratron switch plasma with a peak electron density of 1-3×10¹⁵ cm^-3 and peak current density of ≈10⁴ A/cm². Estimates of a very small cathode-fall width during the conduction phase and high electric field strengths lead to the injection of an electron beam with energies ⩾100 eV and density of 10¹³-10¹⁴ cm^-3 into a Maxwellian bulk plasma. Collisional and radiative processes of monoenergetic beam electrons, bulk plasma electrons and ions, and atomic hydrogen are modeled by a set of rate equations, and line intensity ratios are compared with measurements. Under these high-current conditions, for an initial density n_H2=10¹⁶ cm^-3 and electron temperature of 0.8-1 eV, the estimated beam density is ≈10¹³ -10¹⁴ cm^-3. These results suggest the possibility of producing in a simple way a very high-density electron beam     

A lower bound for Coulomb energies

The indirect Coulomb energy for particles of charge e is usually approximated by E ≈ -Ce^2/3 ∫ (x)^4/3d(x), where (x) is the single-particle charge density. We prove that, for a suitable constant C, this form for E is actually a universal lower bound.     

Electron spectroscopy observation of the 1s2s2p(S) state in atomic fluorine by means of electron impact excitation of the hf molecule

The energy of the 1s2s²2p⁶²S) core excited state in atomic fluorine is determined by studying a satellite in the electron beam excited KVV Auger electron spectrum of HF. The satellite is assigned to an atomic autoionization transition 1s2(²2p⁶(²S) → 1ss²2p⁴ (¹D). The energy of the 1s2s²2P⁶(²s) initial state has been determined to be 676.5 eV. Transitions to the 1s²2s²2p⁴³p(³P)and 1s²2s²2P⁴(¹S) states are also observed. The intensity for these transitions is very low.     

流体-亚稳态原子传输混合模型模拟空心阴极放电特性

利用流体-亚稳态原子传输混合模型研究了氩气矩形空心阴极放电稳态时的参数. 数值计算得到了压强为10 Torr时的电势、电子、离子和亚稳态氩原子密度以及电子平均能量的分布. 结果表明电子和离子密度峰值为4.7×10¹² cm^-3, 亚稳态原子密度峰值为2.1×10¹³ cm^-3. 本文同时对流体-亚稳态原子传输混合模型和单一流体模型模拟得到的放电参数进行了比较. 结果表明, 分步电离是新电子产生的重要来源, 亚稳态原子对空心阴极放电特性有重要影响. 与单一流体模型相比, 混合模型计算得到的电子密度升高, 阴极鞘层宽度和电子平均能量降低. 关键词： 空心阴极放电 流体-亚稳态原子传输模型 电子密度 分步电离     

1000 V p-GaN混合阳极AlGaN/GaN二极管

GaN材料具有优异的电学特性,如大的禁带宽度(3.4 eV)、高击穿场强(3.3 MV/cm)和高电子迁移率(600 cm~2/(V·s)). AlGaN/GaN异质结由于压电极化和自发极化效应,产生高密度(1×10~(13)cm~(-2))和高迁移率(2000 cm~2/(V·s))的二维电子气(2DEG),在未来的功率系统中, AlGaN/GaN二极管具有极大的应用前景.二极管的开启电压和击穿电压是影响其损耗和功率处理能力的关键参数,本文提出了一种新型的具有高阻盖帽层(high-resistance-cap-layer, HRCL)的p-GaN混合阳极AlGaN/GaN二极管来优化其开启电压和击穿特性.在p-GaN/AlGaN/GaN材料结构基础上,通过自对准的氢等离子体处理技术,在沟道区域形成高阻盖帽层改善电场分布,提高击穿电压,同时在阳极区域保留p-GaN结构,用于耗尽下方的二维电子气,调控开启电压.制备的p-GaN混合阳极(p-GaN HRCL)二极管在阴阳极间距Lac为10μm时,击穿电压大于1 kV,开启电压+1.2 V.实验结果表明, p-GaN混合阳极和高阻GaN盖帽层的引入,有效改善AlGaN/GaN肖特基势垒二极管电学性能.     

The ΔNγ electromagnetic transition form factors GM(q) and GE(q)

A parameter-free, nonperturbative calculation of the ΔNγ electromagnetic transition amplitudes GM*(q2), GE*(q2), and the resonant multipole ratio REM(q2)≡E1+3/2(q2)/M1+3/2(q2) is performed in terms of the well-known nucleon isovector Sachs form factor GMV. Our methods are fully relativistic with conservation of the electromagnetic current guaranteed. We find that GM*(q2) decreases more rapidly than the nucleon dipole form factor when −q21 GeV2/c2 and that REM(q2) remains small even for very high four-momentum transfer implying that the perturbative QCD prediction REM(q2)→1 is purely asymptotic and is valid only for extremely high |q2|.