Picosecond-controlled switching of high-voltage gas discharge

It is experimentally demonstrated that a nitrogen-filled discharge gap of a high-voltage oscillator can be switched by an electron beam with a time spread no exceeding ～25 ps relative to the accelerating voltage pulse front. The regime of high-precision control is obtained in the case of a homogeneous potential distribution in the gap, at a microsecond-long voltage buildup to a level of ～90% of the self-induced breakdown. The period of induced conductivity in the discharge gap corresponds to the beam current pulse duration.     

The normal current density effect in barrier discharge

The evolution of the glow of a homogeneous barrier discharge during a single current pulse have been experimentally studied. It is demonstrated that the behavior of this discharge is analogous to the normal current density effect in a dc discharge.     

Switching of the vacuum-arc discharge direct current

Interruption of the direct current of a vacuum-arc discharge (VAD) is analyzed with three methods: the connection of a shunt circuit with a capacitor to the discharge chamber, the overlaying of a pulse nonhomogeneous axisymmetric magnetic field upon the discharge, and the combined action of the magnetic field and the shunt circuit. The probability of interruption of the current I = 500 A as a function of the magnetic field induction is obtained for various shunt capacitances within the range 25 ≤ C ≤ 2500 microfarad (μF). It is shown that the probability of interruption for a given magnetic field induction increases as the capacitance is raised and for the given capacitance it increases as the induction is increased. The magnitude of the magnetic system current I _m to ensure the current interruption with the probability P = 1 is obtained as a function of the shunt capacitance. The impact of the shunt circuit parameters upon the arc current is analyzed. The time-domain plot of the arc current for the shunt capacitor C = 2500 μF was estimated. Its results agree with the experiment. It is shown that the combined use of the magnetic field and the capacitive shunt circuit is an effective method of VAD direct current interruption.     

First-principles study of repeated current switching in a bimolecular device

Using nonequilibrium Green’s functions in combination with the first-principles density-functional theory, we investigate electronic transport properties of a bimolecular device consisting of two parallel placed phenalenyl molecules. When the two molecules get close enough, the currents of this bimolecular device could switch repeatedly by the mechanical strain. The deeper analysis indicates that the overlapping region size sensibly alters the coupling and charge transfer between the two parallel π-conjugated molecules is a very important factor for this behavior.     

离子源放电电流对真空室压力影响机理研究

揭示了真空室压力随离子源放电电流增长，而且在不同的气流速率时的增长率不同这一现象，并对其机理进行了分析，认为真空室压力随放电电流的增长主要是离化粒子束流密度的增长引起的，当然离化粒子的平均能量的增长也起了作用，而在不同气流速率时该压力增长率的不同则主要是因为在不同的气流速率时离化率不同。     

Particle simulation of discharge current oscillation in Hall thrusters

Twenty kHz-range discharge current oscillation with a large amplitude is observed in Hall thrusters, causing unstable operation. In our previous studies, the effects of the anode orifice configuration on the current oscillation have been investigated. Using the results of neutral particle simulations, the relationship between the neutral distribution and the propellant inlet condition was analyzed. In the present study, to confirm the effect of the neutral distribution and investigate the ionization process in detail, axisymmetric analysis in the acceleration channel was carried out using a hybrid-PIC code. As a result, the periodic ionization and current oscillation were obtained. The oscillation amplitude for a smaller orifice is greater due to the higher ionization rate. The results agree with the effect of the neutral distribution predicted by the simulations.     

Spin-transfer switching current distribution and reduction in magnetic tunneling junction-based structures

Spin transfer switching current distribution within a cell and switching current reduction were studied at room temperature for magnetic tunnel junction-based structures with resistance area product (RA) ranged from 10 to 30 /spl Omega/-/spl mu/m/sup 2/ and TMR of 15%-30%. These were patterned into current perpendicular to plane configured nanopillars having elliptical cross sections of area /spl sim/0.02 /spl mu/m/sup 2/. The width of the critical current distribution (sigma/average of distribution), measured using 30 ms current pulse, was found to be 3% for cells with thermal factor (KuV/k/sub B/T) of 65. An analytical expression for probability density function p(I/I/sub c0/) was derived considering a thermally activated spin transfer model, which supports the experimental observation that the thermal factor is the most significant parameter in determining the within-cell critical current distribution. Spin-transfer switching current reduction was investigated through enhancing effective spin polarization factor /spl eta//sub eff/ in magnetic tunnel junction-based dual spin filter (DSF) structures. The intrinsic switching current density (J/sub c0/) was estimated by extrapolating experimental data of critical current density (J/sub c/) versus pulse width (/spl tau/), to a pulse width of 1 ns. A reduction in intrinsic switching current density for a dual spin filter (DSF: Ta/PtMn/CoFe/Ru/CoFeB/Al2O3/CoFeB/spacer/CoFe/PtMn/Ta) was observed compared to single magnetic tunnel junctions (MTJ: Ta/PtMn/CoFe/Ru/CoFeB/Al2O3/CoFeB/Ta). J/sub c/ at /spl tau/ of 1 ns (/spl sim/J/sub c0/) for the MTJ and DSF samples were 7/spl times/10/sup 6/ and 2.2/spl times/10/sup 6/ A/cm/sup 2/, respectively, for identical free layers. Thus, a significant enhancement of the spin transfer switching efficiency is seen for DSF structure compared to the single MTJ case.     

Simulation of the current switching by a metal-insulator transition

A one-dimensional simulation of the nonlinear diffusion of magnetic field through a shield made of the solid solution (V_1?X Cr_X)₂O₃ is carried out. Also simulated is the current switching from this shield to load when the former undergoes a metal-insulator transition. Pronounced steepening of a current pulse edge in the load circuit is demonstrated to be possible.     

Thermally stimulated discharge current studies on PMMA-PVAc blends

Thermally stimulated discharge current (TSDC) studies have been carried out on blends of polymethyl methacrylate (PMMA) and polyvinyl acetate (PVA). The effects of polarization temperature and field on the TSDC peaks of polyblends indicate that the polarization in the polyblends is due to charge-carrier trapping in deep traps which leads to induced dipole formation. The results of a.c. dielectric bridge measurements are also compared with the TSDC results. The dielectric relaxation parameters are also reported.     

Increased analytical precision in the hollow cathode discharge emission source by improved discharge current control.

A significant improvement in the precision of the hollow cathode as an emission source is reported. Precision of 1% or less has been observed several times over periods of several hours. An average long-term stability of 4.3% for Ll and 6.0% for Na in the emission signal from microsamples (less than 50 nL) deposited in the hollow cathode discharge source is reported. The improved precision is attributed primarily to the introduction of electronics that hold the discharge current more nearly constant and to the shielding of all wiring to the source from the power supply. A current-controlled switch that is capable of driving a hollow cathode discharge in either dc or pulsed mode is described. This switch is capable of generating current pulses as short as 2 microseconds through a resistive load and greatly improves the discharge stability and repeatability at turn-on in both the dc and pulsed modes. The characteristics of pulses produced by this switch are presented; however, analytical performance is reported only for the dc mode. Temporal current plots are presented for the new instrumentation and compared to plots taken with commonly used current-controlled power supplies. Instrumentation, operation, and sample preparation procedures are described. Typical temporal profiles of the emission signal from microsamples deposited in AI and stainless steel hollow cathodes are given.     

Influence of the magnetostatic coupling in magnetization switching driven by spin-polarized current

The influence of the magnetostatic coupling (MC) on magnetization reversal processes driven by spin-polarized current has been studied by means of a micromagnetic model. The spin transfer torque is included as an additional term in the Gilbert equation, following previous theoretical calculations by Slonczewski. The MC plays a crucial role and it speeds the magnetization switching process.     

An analog of the normal current density effect in barrier discharge

We have experimentally studied the regimes of homogeneous barrier discharge operation in air at reduced pressure and observed behavior analogous to the normal current density effect in a dc discharge.     

管件长度对电磁成形线圈放电电流的影响

为研究电磁成形过程线圈放电电流的变化规律,在放电回路分析的基础上,采用试验分析与代数解析相结合的方法研究了管件长度对线圈放电电流的影响.试验结果表明:随着管件长度的增加,线圈放电电流的幅值和频率均增加.通过解析方法定性分析管件长度对线圈放电电流的影响,其结果与试验结果一致.管件长度不同时,放电回路的等效电感不同,管件长度与等效电感成反比.将管件和成形线圈等效为圆柱线圈可定性分析电磁成形系统的电感和互感.     

静电放电电流校准及Matlab仿真研究

针对静电放电模拟器的静电放电电流波形参数校准的原理及方法,根据IEC 61000-4-2:2008标准中给定的理想静电放电电流方程对静电电流波形进行Matlab仿真,通过仿真分析方程中各个时间参数对电流波形的影响,结果表明静电放电模拟器实际的放电回路的分布参数对静电放电电流波形有直接影响.电流波形的第一峰值和上升时间随着t1增大而减小,随着t2的增大而增大,放电电流第二峰值随着t3的增大而减小,随着t4的增大I30、I60也随之增大.通过控制放电回路分布参数有助于提高校准结果的准确度,仿真结果对实际的静电放电电流的校准工作有一定参考价值.     

Thermally stimulated discharge current studies of acrylic-acid-doped polystyrene films

Thermally stimulated discharge currents in thin cylindrical films (30 μm thick) of polystyrene either undoped or doped with acrylic acid were measured under various polarizing conditions. It was found that the undoped samples give rise to a single peak around 105 °C which is due to the release of space charges present in the material. In the doped samples this peak is shifted to 58 °C and another peak appears at around 150 °C. The peak at the lower temperature is due to the superposition of the dipolar peak of the dopant and the space-charge-limited peak of the matrix, whereas the peak at the higher temperature has been attributed to the space charge of the material retained in the double bonds of the dopant, as a result of interaction between the dopant and the matrix.     

Ionization mechanism in a direct current discharge plasma in an argon-oxygen mixture

We present the results of calculating the charged particle formation and decay rates in a dc discharge positive column in argon-oxygen mixture. The calculations are based on a combined solution to the Boltzmann equation; vibration kinetics equations for the ground state of the O₂ molecule; and chemical kinetics equations for Ar and O atoms, O₂ molecule, and their excited states under the experimentally determined reduced electric field strength, E/N, for various gas phase compositions, currents, and pressures. The role is clarified of particular components in the ionization processes. Electron and positive ion decay is shown to take place predominantly at the reactor walls on which they fall due to joint diffusion. The negative oxygen ions affect the diffusion rate.     

Discharge current and ultrashort avalanche electron beam current in a volume nanosecond gas discharge in inhomogeneous electric field

The moment of generation of an ultrashort avalanche electron beam (UAEB) relative to the discharge current pulse front has been determined in a volume discharge formed upon nanosecond breakdown of an air gap at atmospheric pressure in an inhomogeneous electric field. The UAEB current reaches maximum on the front of the discharge current pulse, ～100 ps before the peak of this current. Bias currents with amplitude above 1 kA have been observed. The amplitude of this current increases due to the charging of a capacitor formed by the flat metal anode and a dense discharge plasma expanding from the cathode.     

Substantial reduction of critical current for magnetization switching in an exchange-biased spin valve

Great interest in current-induced magnetic excitation and switching in a magnetic nanopillar has been caused by the theoretical predictions of these phenomena. The concept of using a spin-polarized current to switch the magnetization orientation of a magnetic layer provides a possible way to realize future 'current-driven' devices: in such devices, direct switching of the magnetic memory bits would be produced by a local current application, instead of by a magnetic field generated by attached wires. Until now, all the reported work on current-induced magnetization switching has been concentrated on a simple ferromagnet/Cu/ferromagnet trilayer. Here we report the observation of current-induced magnetization switching in exchange-biased spin valves (ESPVs) at room temperature. The ESPVs clearly show current-induced magnetization switching behaviour under a sweeping direct current with a very high density. We show that insertion of a ruthenium layer between an ESPV nanopillar and the top electrode effectively decreases the critical current density from about 10(8) to 10(7) A cm(-2). In a well-designed 'antisymmetric' ESPV structure, this critical current density can be further reduced to 2 x 10(6) A cm(-2). We believe that the substantial reduction of critical current could make it possible for current-induced magnetization switching to be directly applied in spintronic devices, such as magnetic random-access memory.     

Novel zero voltage switching dual-switch forward converter with ripple current cancellation

A zero voltage switching (ZVS) dual-switch forward converter with ripple current cancellation is presented. In the proposed converter, active clamp circuit is used to clamp the voltage stresses and to realise ZVS of all switching devices. Active clamp boost converter with power factor correction is used in the front stage of the proposed converter to draw a sinusoidal line current from the AC source and to maintain a constant voltage at the DC bus. The second stage of the proposed converter is a dual-switch forward converter with current doubler rectifier to obtain the isolated low output voltage. Active clamp circuit used in the DC/DC converter can recycle the energy stored in the leakage inductor and magnetising inductor so that the voltage stresses on the switches are limited and the ZVS feature is realised. The current doubler rectifier offers the ripple current cancellation at the output capacitor and reduces the current stress of the transformer secondary winding. The circuit configuration and principle of operation are analysed and discussed in detail. Experimental results with a laboratory prototype based on a 90-260 V_rms input and 12 V/30 A output were provided to verify the effectiveness of the proposed converter.     

Amorphous carbon films in direct current magnetron sputtering from regenerative sooting discharge

We present results of carbon coatings on metal substrates in cylindrical hollow cathode (CHC) direct current magnetron sputtering. This is a new technique for making amorphous carbon films by CHC magnetron sputtering from a regenerative sooting discharge. The carbon films are deposited on Cu and Al substrates in a Ne atmosphere and compared with the films of carbon soot on the same materials produced from a conventional 80A arc discharge between graphite electrodes in a He background gas. Raman spectroscopy reveals the existence of graphite and diamond-like structures from the arc discharge while in CHC magnetron sputtering, graphite-like structures are dominant. X-ray diffraction data of samples from the arc discharge show nano-size precipitates of Al₄C₃ of rhombohedral and hexagonal form for the aluminium sample and probable formation of diamond and hexagonal carbon in copper whilst in magnetron sputtering we obtain amorphous carbon films. Scanning electron microscope images of the surface show a collection of loose agglomerates of carbon particles in the arc discharge whereas, for magnetron sputtering, structures are regular with smooth edges and fine grains.