Fluid Simulation of the Conduction Phase of the Plasma Erosion Opening Switch

The conduction phase of the plasma erosion opening switch (PEOS) is studied using a 1?-D electromagnetic two-fluid code. The focus of this work is on understanding how two effects, a current-limiting model of electron emission, and the magnetic insulation of electrons at the cathode, determine current conduction in the plasma. Simulations are performed in the parameter regimes of the Gamble I, POP, and PBFA II pulsed power generators, and previous low-density, short-rise time simulations of the PEOS. Fluid code results are compared to a 1-D analytic theory and to the Gamble I and POP experiments. Good agreement between theory and simulation, but mixed agreement between simulation and experiment is found. Experimental B-field measurements on POP show weaker j × B compression than the simulation. Current penetration and plasma current channels qualitatively similar to experimental observation are found in the Gamble I regime. However, magnetic insulation of electrons emitted from the cathode bunches the electron flow into narrower current channels than observed experimentally. In several cases, the presence of an electron-scattering or energy-loss mechanism near the cathode must be invoked to overcome magnetic insulation and widen the current channels.     

Observation of tungsten particles in vacuum arcs by laser inducedfluorescence

We have clarified the relation between the decay of tungsten ion density in the vicinity of current zero and vacuum arc mode in high current period by using a laser induced fluorescence method in tungsten vacuum arcs of 60 Hz sinusoidal current with the peak value of 3.3, 6.7, and 9.8 kA. In the case of 6.7 kA, the arc mode was the anode spot mode. Because of the generation of the anode spot, the tungsten ion density near the anode was higher than near the cathode and the density near the anode was about ten times as high as the case of 3.3 kA which was the diffuse mode. In the case of 9.8 kA, which was the intense arc mode, the density near the anode was not significantly different from the case of 6.7 kA. The density near the cathode was higher than near the anode and tungsten ions were observed till about 30 μs after current zero while they disappeared at current zero in the other cases     

Oscillations of an electron stream with a virtual cathode

The results of an investigation of the oscillations of a virtual cathode and the hysteresis of the state for a one-dimensional electron stream in a flat drift space are given. It is shown that in the case of a given mean space charge density and a given injection current different oscillatory regimes can exist in a stream with a virtual cathode.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 74–77, October, 1987.In conclusion, the authors thank L. V. Shelomentsev for carrying out the numerical calculations for the nonrelativistic stream on the computer.     

Cathode erosion rate in high-pressure arcs: influence of swirling gas flow

Cathode erosion in a 400-A arc burning in oxygen at 4-atm pressure was measured at different gas dynamic conditions near the cathode. The flow pattern near the cathode was changed by varying the swirl of the gas entering the discharge chamber. Experiments showed that the erosion rate depends on the swirl in a very strong way. Model calculations of velocity distribution showed that the gas velocity near the cathode increases as the swirl intensifies. According to our model, it leads to an increase of the net evaporation rate (evaporation rate minus rate of previously evaporated particles returning back to the cathode).     

PEMFC阴极流道中阻块高度的敏度分析与优化

在质子交换膜燃料电池(PEMFC)的阴极流道中安装阻块可以强化氧气传输,提升电池性能,但不同位置的阻块高度对电池性能的影响程度不同。本文建立了一个三维、两相、稳态的PEMFC数值模型,对阴极流道安装有阻块的PEMFC进行了数值模拟,并结合全因子设计法研究了不同位置的阻块高度对电池性能的影响程度。此外,基于敏度分析结果,采用遗传算法对阻块高度进行了优化。结果表明:越靠近出口位置的阻块,其高度变化对低电压下的电池功率的影响程度越大。增加靠近出口位置的阻块高度可以促进催化层反应物的均匀分布,提高催化剂的利用率,最大程度地提升电池性能。当组块高度H1和高度增量ΔH分别为0.9537 mm和0.009 mm时,PEMFC性能最佳,与未安装阻块相比,电池最大净功率提高了18.45%。     

“强光一号”锥形丝阵内爆特性研究

在强光一号装置开展小倾角单层锥形钨丝阵实验, 获得了锥形丝阵在消融阶段, 拉链内爆阶段和滞止阶段的辐射参数和辐射图像. 消融阶段的物理图像映证了使用火箭模型对锥形丝阵消融过程及先驱等离子体形成的分析. 结合分幅像、针孔像和条纹像, 解释了锥形丝阵在内爆箍缩阶段阴极部分首先发生内爆并产生硬X射线成分的原因. 观察到的锥形丝阵的拉链内爆速度达到2.59× 10⁷ cm/s, 拉链内爆及滞止的持续时间达到40 ns, 远大于相近尺寸的柱形丝阵. 通过特殊构型丝阵内爆特性的研究, 为进一步验证基于柱形丝阵建立的Z-pinch理论模型, 完善MHD模型的普适性提供必要的实验数据.     

注入和输运对单层有机发光器件复合发光的影响

通过分析聚合物电致发光器件中载流子注入、输运、激子的解离与复合过程,提出了激子解离与复合的理论模型. 基于电流连续性方程和Poisson方程,给出了激子复合密度、电流密度及复合效率表达式.研究两电极与有机层之间在Ohmic和Fowler-Nordheim接触条件下载流子迁移率对器件中激子的复合密度及外加电压和注入势垒对器件电流和复合效率的影响.结果表明:1)在一个较宽的注入势垒和迁移率范围内,复合密度不是由两个注入电极的相对注入比决定而是由有机层电子和空穴迁移率之比所支配;2)固定阴极势垒,而阳极势垒由小变大时,器件电流由接触限制向空间电荷限制转变;3)复合效率随外电压升高先增加,当电压达一临界值时而陡降,存在一个最佳的注入势垒值.结果说明:电极与有机层的能带匹配及有机层间的迁移率匹配对器件复合发光有着极其重要的影响.其计算值与所报道的理论结果相符.     

Dynamics of an electron beam with a virtual cathode in a vircator without a magnetic field and with drift space filled by neutral gas

The physical processes in a vircator without an external magnetic field and with a drift space filled by neutral gas are studied. Three typical dynamic regimes are found: (1) the suppression of virtual cathode oscillations, (2) pulse generation, in which the suppression and resumption of virtual cathode oscillations are observed, and (3) continuous virtual cathode generation. The frequency dynamics of the vircator’s output radiation is investigated for the last case, and it is shown that the generation frequency of the vircator can rise when the gas pressure exceeds a certain critical value.     

Interaction of a surface glow discharge with a gas flow

A surface glow discharge in a gas flow is of particular interest as a possible tool for controlling the flow past hypersonic aircrafts. Using a hydrodynamic model of glow discharge, two-dimensional calculations for a kilovolt surface discharge in nitrogen at a pressure of 0.5 Torr are carried out in a stationary gas, as well as in a flow with a velocity of 1000 m/s. The discharge structure and plasma parameters are investigated near a charged electrode. It is shown that the electron energy in a cathode layer reaches 250–300 eV. Discharge is sustained by secondary electron emission. The influence of a high-speed gas flow on the discharge is considered. It is shown that the cathode layer configuration is flow-resistant. The distributions of the electric field and electron energy, as well as the ionization rate profile in the cathode layer, do not change qualitatively under the action of the flow. The basic effect of the flow’s influence is a sharp decrease in the region of the quasineutral plasma surrounding the cathode layer due to fast convective transport of ions.     

Zündspannung und normaler Katodenfall der Glimmentladung für Edelgase und reine Molybdänkatoden

Breakdown voltage near PASCHEN -minimum and normal cathode fall of the glow discharge were calculated for inert gases and molybdenum cathodes on the basis of a simple theory. The method used was similar to TAKEISHI 's one. The calculated breakdown voltages are in good agreement with measured values, if back scattering of the y_i -electrons and the distance which electrons travel from the cathode without ionization is taken into account. In the case of the normal cathode fall the values calculated with the same assumptions for inert gases and molybdenum cathodes are much higher (up to 50%) than the well known experimental values. It was concluded, that the y_i-process in the glow discharge ist not the only important one. Comparing the calculated and experimental values of the normal cathode fall we obtain the result, that photons and/or ions from the glow release about 40–60% of all electrons at the cathode.     

偏滤器运行模式对托卡马克边缘区等离子体平行流的影响

我们使用一维流体模型,根据在不同偏滤器运行模式下静压强沿着磁力线方向的分布变化,讨论了偏滤器运行模式对托卡马克边缘区等离子体平行流的影响.低再循环模式下,静压强从X点 (X-point)附近的刮削层区域开始明显下降,变化趋势与密度变化趋势一致;等离子体平行流的马赫数在偏滤器区域逐步变大,变化从平缓到迅速.高再循环模式下,静压强在靶板附近的区域迅速下降,在其他区域变化非常小;等离子体平行流的马赫数仅在靠近靶板附近的区域迅速变大,在其他区域变化平缓.在弱脱靶模式下的静压强变化与高再循环模式下类似,不过静压强在X-point附近的刮削层区域开始出现下降的趋势,导致等离子体平行流的马赫数在X-point处的值比在高再循环模式下大.强脱靶模式下,静压强在刮削层区域开始明显下降,在远离靶板的偏滤器区域,静压强迅速下降的地方,观察到高马赫数等离子体平行流.静压强迅速下降引起动压强迅速上升来维持总的压强守恒是在强脱靶状态 下产生高马赫数平行流的一种可能驱动机理.     

Mechanism of ion flow generation in vacuum arcs

The ecton model of the cathode spot is used to analyze the main parameters of ion flow in vacuum arcs (ion erosion, mean charge, and velocity). It is shown that the arc plasma is formed as a result of microexplosions at the cathode surface, induced by the Joule heating by the high-density current of explosive electron emission. Ionization processes are localized in a narrow region of the order of a micrometer near the cathode and the ionization composition of the plasma subsequently remains unchanged. Under the action of the electron pressure gradient, ions acquire directional velocities of the order of 10⁶ cm/s even over small distances of the order of several micrometers.     

Nonrelativistic planar double-stream equilibria in magneticallyinsulated diodes

The study of magnetically insulated electron flow is a topic of relevance to the startup state of magnetrons, cross-field amplifiers, and magnetically insulated line oscillators, as well as to the operation of magnetically insulated transmission lines and ion diodes. In a recent paper, Christenson and Lau explored double-stream equilibria in magnetically insulated diodes analytically for the case of vanishing injection electron velocities at the cathode and numerically for the case of nonzero injection velocities. Here, the analytical results are generalized to the case of nonzero injection velocities to allow the calculation of the equilibrium current density and electron sheath thickness as functions of the voltage, magnetic field, and gap width. An interesting outcome of the analysis is that it is shown that space-charge limited electron emission from the cathode is no longer possible for equilibria in which the electrons are monoenergetic with vanishing canonical momenta. In addition, for the special case of zero injection energies, but not necessarily zero electric field at the cathode, closed-form expressions are derived for the orbital quantities of the electrons-positions and velocities-as a function of time measured from the moment of emission from the cathode. Unfortunately, a spatial variation is not apparently available in closed form; however, one can express these quantities alternatively in terms of a generalized potential with an implicit spatial variation     

Soret-driven convection of binary mixture in a horizontal porous layer in the presence of a heat or concentration source

Soret-driven convection of a binary mixture in a shallow porous layer is analyzed. The analysis focuses on the behavior of the system in the presence of a concentration or heat source. In the long-wavelength limit, regimes are found in which the flow regions near the source and at the periphery are separated by narrow annular transition regions. It is also shown that the outward concentration flux from the source is dominated by convection, whereas heat can be transferred from the source both by convection and by diffusion. Multistability between these two regimes is possible.     

Complex analysis of microparticles deposited from arc-discharge plasma on vacuum-chamber walls

A complex study of the microparticles and condensate in the form of films deposited from an arcplasma flow on vacuum-chamber walls is performed. The study includes determination of the chemical and phase composition of the particles, their morphology, differential thermal analysis, electron microscopy, and also electron paramagnetic resonance and infrared spectroscopy. The plasma flow propagates in electric and magnetic fields. The formed structures are a result of the interaction of particles emitted from the cathode spot. In this case, the formation and existence of current layers in the plasma flow are assumed. The retrograde rotation of the cathode spot and plasma flow are analyzed. The conditions for the formation of fractal structures from plasma are determined.     

Inhomogeneous extended hollow cathode discharge for raising the current density in a forevacuum plasma source of a ribbon electron beam

The plasma parameters and the emissivity of a ribbon electron beam source based on a discharge with an inhomogeneous extended hollow cathode are measured. A constriction in the cathode cavity increases the plasma density near the emitting area boundary, which adds to the electron current density in the beam. The reason for the above effect is the formation of the plasma density distribution nonuniform across the cavity with a maximum in the middle. This maximum is caused primarily by a plasma electron flow from the constriction, which is generated by the electric field and is directed toward a slit emission-extracting aperture.     

Das Eindringen der Glimmentladung in Spalte

The fractional area of the cathode covered by the negative glow in low pressure hydrogen discharges has been studied using a special co - planar anode - cathode configuration. The cathode was a steel bar of 2×2×64 cm, sectionalised into 32 insulated parts. An identical electrode mounted directly above the cathode at a variable separation facilitated the simulation of screening effects of insulating or conducting walls and of hollow cathode conditions. The depth of penetrationξ of the discharge into this gap was studied for various conditions. In general for the insulating wall condition,ξ is approximately proportional to the applied voltage and depends on the gap width and pressure. The electron current in the negative glow parallel to the cathode surface as well as the cathode current density at any distancex from the anode vary exponentially withη=ξ-?. The potential of the negative glow in the gap decreases slowly with? to the “normal” cathode fall at?=ξ. In the case of the conducting wall, the discharge can penetrate to any depth, provided the wall is maintained at a potential ≧ the “normal” cathode fall. For a hollow cathodeξ is small, but the current density near the anode reaches extremely high values.     

磁绝缘传输线压力平衡模型的修正

 由于电子的发射,在磁绝缘传输线（MITL）的阴极表面附近会形成一层电子鞘层。当传输线工作在稳定态时,电子鞘层的边缘满足压力平衡。在此基础上,考虑了电子鞘层内部的碰撞,利用PIC计算方法对MITL的电压、阴极电流和阳极电流的关系进行修正,使得阴阳极电流在饱和流条件下与模拟计算结果更加符合,最大误差由9%降到3%。碰撞系数关于电压的拟合曲线适合于工作电压为2～7 MV的理想MITL。     

Consequences of balance equations applied to the diffuse plasma ofvacuum arcs

By means of balance equations, the partial currents in vacuum arcs are represented by some simple coefficients, and it is shown that the surprising properties of the expanding diffuse arc plasma can easily be explained if the plasma temperature near the cathode is 4.5±0.5 eV (in the case of Cu), i.e., by a factor three times higher than that measured and presumed in most studies. Evidence for this high temperature comes from both the plasma energy balance and the Saha equation     

Two-dimensional models of expanding vacuum arc plasmas

Multifluid equations describing the plasma of vacuum arcs expanding anisotropically from a cathode spot are given and discussed. Some first and preliminary results from an approximate analytical integration of such a system of equations, based on the representation of all plasma parameters by asymptotic power series, are presented and evaluated. Though the dependence of the plasma parameters on the direction (angle ϑ) is weak, the inclusion of angular terms and angular equations changes some results significantly, compared with the experiences from usually 1-D models. While the basic radial dependence (r) of the terms ~r^-2k/5, k=0,1,2..., remains the same as in 1-D theories, we now get an angular dependence ~ϑ² of the parameters in a first approximation, with positive factors, i.e., a flattening of the whole plasma distribution. However, with analytical solutions of higher flexibility, cos-like distributions are possible as well. The final kinetic energy of the plasma ions in the case of Cu arc spots is caused by the electric field within the plasma (contributing approximately 27%), the ion pressure gradient (~24%), and the electron-ion friction (plasma resistivity, ~49%) in directions near to the discharge axis. Again, similar to the results from 1-D models. With an approach to the cathode surface (ϑ→π/2), the plasma flow becomes ambipolar. Finally, some further possibilities which are available with the help of this model are discussed