Electrical breakdown of the space vacuum

The insulating properties of the vacuum of space are examined in this paper. Generic electrode configurations are introduced to calculate the leakage current to the electrodes and the breakdown potential of the “space-vacuum” as a function of altitude above ground. The condition that defines the breakdown threshold is dependent on the size of the sheath, r_L-B, relative to the interelectrode separation, r_c. Results for the leakage currents and breakdown voltage are presented for both cases r_L-B>r_e and r_L-Be, and discussed in the context of space power systems and the SPEAR Program in particular     

Experiments with Enhanced Mode Thermionic Converters

Reduction of the ionization and scattering losses associated with ignited mode cesium diodes is essential for high thermal-to-electrical conversion efficiency. Use of an auxiliary electrode in conjuction with a noble gas in the interelectrode space should permit more efficient ion generation for space charge neutralization. The characteristics of a thermionic triode utilizing a ring electrode and a dispenser cathode emitter have been studied as a function of xenon pressure, cesium reservoir temperature, spacing, electrode temperature and pulse parameters (i.e., potential, duration and repetition rate) applied to the auxiliary electrode. Pulsed operation significantly enhanced output power with uniform discharges appearing to be sustained at emitter-collector spacings as low as 0.5 mm.     

On the mechanism of the runaway of electrons in a gas: The upper branch of the paschen curve

Basing on the simulation results, it is shown that the Townsend mechanism of electron multiplication in a gas at sufficiently large interelectrode distances is valid at least up to such large values of E/p at which relativistic electrons are generated. Correspondingly, the runaway electron producing in a gas is determined not by the local criteria accepted presently, but by the ratio of interelectrode distance and the characteristic electron multiplication length. It is shown that the critical discharge voltage U, at which the runaway electrons appear in a given gas, is a function of the product of the interelectrode distance by the gas pressure. This function (U-pd dependence) defines not only well-known Paschen curve but also an additional branch, which describes the absence of a self-sustained discharge at a high voltages sufficiently rapidly supplied across the electrodes. Critical discharge voltage dependence for helium and xenon are presented.     

Nasicon sensor for NO detection in the automotive exhaust

Development of a sensor where both electrodes are exposed to the same gas offers simple design with no need for a separate reference gas with a complicated sealing system. Such sensors are still in research stage. The NASICON electrolyte, which has high ionic conductivity at the temperature of the exhaust gas mixture, allows such sensors to work without a heater, making a simple and miniaturised construction technique. Using two asymmetric electrodes, where each electrode has a different catalytic activity at a given temperature, can result in anemf, which is a function of partial pressure of NO. A NASICON sensor with SmFeO₃ as a sensing electrode and Au as an oxygen electrode was calibrated as a NO sensor and tested in oxidising atmosphere in simulated exhaust gases. The sensor responded quickly with reproducibleemf values. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Free‐Electron Gas Spectrum Uniqueness in the Mixture of Noble Gases

Gas mixtures can reach the Maxwell's specter shape in case of low‐ionized mono‐atomic mixtures in the weak electric field. The parameters pertaining to the Maxwell spectrum of free electrons' gas straightforwardly settle on the insulating characteristics of the examined gas mixture at the fundamental level. In this paper, a condition for breakdown has been accomplished taking as a starting point the ionization coefficients derived accordingly, as well as the conditions for breakdown in keeping with the Townsend mechanism. The dc breakdown voltage value of noble gases mixture has been measured in the experimental part of the paper. The hypothesis that the free‐electron gas spectrum is unique in the noble gas mixture and is of Maxwell's type has been verified. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Particle‐in‐Cell Simulation for the Effect of Segmented Electrodes Near the Exit of an Aton‐Type Hall Thruster on Ion Focusing Acceleration

The effect of floating conductive electrodes near the channel exit of an Aton‐type Hall thruster on ion focusing acceleration is studied by simulating the two‐dimensional plasma flow with a fully kinetic Particle‐in‐Cell method for the gas flow rate j_a ranged in 1～3 mg/s. Numerical results show that low‐emissive electrodes can reduce plume divergence if the electrode length is less than 2 mm due to the low secondary electron emissive characteristic, but widen plume in all the gas flow rate range if the electrode length is greater than 2mm since the conductive property of segmented electrodes trends to make equipotential lines convex toward channel exit and is even parallel to the wall surface in the near‐wall region. Further investigation predicts that the combination of high emissive dielectric wall and segmented low‐emissive dielectric wall is a promising way to reduce plume divergence (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Non-equilibrium Green's function approach to the quasiparticle transport and magnetoresistance in hybrid magnetic tunnel junctions

A generalized approach to study quasiparticle transport across hybrid magnetic tunnel junctions (MTJs) is formulated using the non-equilibrium Green's function technique. This formalism allows for arbitrary thicknesses of the electrodes and the central scattering region comprising of materials with multiple electronic bands, and incorporates the many body interactions present in the electrode regions. While the method can be used to study the transport characteristics of various types of MTJs, we have used it to study the tunneling characteristics and magnetoresistance (MR) of MTJs in which s-f interaction is present at the electrode layers. It is also used to study the transport characteristics of MTJs with hybrid electrodes and double barrier. The magnetic correlation present in the electrodes is found to strongly influence the TMR. Eventhough the magnetic correlation in general suppress the TMR, the TMR is found to be enhanced strongly for certain band occupations of the electrodes. We observe a fall of TMR with increase in the number of layers in the insulating region. Band occupation of the metallic layer present at the middle of the insulating layers in the double barrier MTJ is found to be important in deciding its tunnel characteristics. Origin of the different types of behavior of TMR is analyzed in terms of the spin-dependent tunnel currents.     

Fabrication of combinatorial nm-planar electrode array for high throughput evaluation of organic semiconductors

We have fabricated a combinatorial nm-planar electrode array by using photolithography and chemical mechanical polishing processes for high throughput electrical evaluation of organic devices. Sub-nm precision was achieved with respect to the average level difference between each pair of electrodes and a dielectric layer. The insulating property between the electrodes is high enough to measure I-V characteristics of organic semiconductors. Bottom-contact field-effect-transistors (FETs) of pentacene were fabricated on this electrode array by use of molecular beam epitaxy. It was demonstrated that the array could be used as a pre-patterned device substrate for high throughput screening of the electrical properties of organic semiconductors.     

Characteristics of a localized gas discharge

The rf capacitive gas discharge with an interelectrode distance of 15–200 µm is studied, and possible forms of its existence are established. The conditions for the formation of discrete microdischarges with a preset geometry, which are localized near some elements of the surfaces of the electrodes, are formulated.     

Generation of thermonuclear energy by fusing hydrogen and lithium atoms

A method of designing a thermonuclear reactor based on the modified Cockroft-Walton accelerator, where the lithium-proton fusion was first observed, is considered. It is proposed that the reactor have the form of a spherical capacitor with a point lithium cathode used as the inner electrode and a spherical anode, as the outer electrode. The interelectrode space is filled with hydrogen. A high-voltage electric pulse applied to the electrodes is used as a driver. The reactor parameters providing an ion temperature of 100 keV and a proton flux of 8.6×10¹⁵ W/cm² to the cathode are determined. The basic elements of a system generating thermonuclear fusion energy, including those of the energy conversion chamber with a fusion chamber inside, are listed, and possible applications of the system are indicated.     

Electrofluorescence polarity in a molecular diode

The kinetic equations describing the transmission of an electron in the molecular compound “electrode 1–molecule–electrode 2” (1M2 system) are derived using the method of a nonequilibrium density matrix. The steady-state transmission regime is considered, for which detailed analysis of the kinetics of electrofluorescence formation in systems with symmetric and asymmetric couplings between the molecule and the electrodes is carried out. It is shown that the optically active state of the molecule is formed as a result of electron hops between the molecule and each of the electrodes, as well as due to inelastic interelectrode tunneling of the electron. The electrofluorescence power for a molecular diode (asymmetric 1M2 system) depends on the polarity of the voltage bias applied to the electrodes. The polarity is explained using a model in which the optically active part of the molecule (chromophore group) is represented by the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Two mechanisms of the emergence of polarity are revealed. One mechanism is associated with nonidentical Stark shifts of the HOMO and LUMO levels relative to the Fermi levels of the electrodes. The second mechanism is associated with the fact that the rates of an electron hopping between HOMO (LUMO) and one of the electrodes are much higher than the rates of such a hopping with the other electrode. The conditions in which each mechanism can be implemented experimentally are indicated.     

The correlation between corona current distribution and collection of fine particles in a laboratory-scale electrostatic precipitator

An analysis of the electrostatic gas cleaning fundamental phenomenon shows an essential influence of discharge electrode construction on the gas cleaning process efficiency.In the physical model tests there were used rigid discharge electrodes with corona emitting elements of various geometries. Different constructions of discharge electrode were tested in the aspect of discharge current uniform distribution on collecting electrode surfaces. Measurements of discharge current distribution has been carried out for discharge electrodes with different spike shapes and in different electric field geometry. The research aim was to determine the optimal discharge electrode construction ensuring high collection efficiency of fine particles. Collection efficiency measurements of selected fly ash samples (from coal fired boilers) were carried out on a laboratory testing bench in a horizontal electrostatic precipitator model.     

Experimental and theoretical analysis of bias ionization by?α-particles in a nitrogen laser

Nitrogen laser performance with TE configuration and wedge electrodes is analyzed with background ionization in the laser discharge channel by α particles at a low exposition rate. With the bias ionization, the laser power presents two peaks as a function of gas pressure, with one at the normal low pressure, without bias ionization, and the other at high pressure generated by bias ionization. A simple theoretical model has been developed in a trial to understand this behavior. This model was first tested in later results for a TE configuration nitrogen laser, with flat electrodes, without and with bias ionization. It has been observed that due to the competition between electrode shielding by positively charged α particles and bulk ionization by impact, the laser energy is suppressed with pressure below 50 Torr and enhanced above it.     

Silicon carbide as electrode material of a pseudospark switch

Through the last years, the pseudospark switch, a low-pressure gas discharge switch with hollow cathode geometry, became established as a promising element of pulsed power technology and a serious alternative to other high-power switches. The use of a novel electrode material silicon carbide yields performance improvements in two main areas. Quenching phenomena, a long-standing problem for several applications, are suppressed completely and the switch lifetime can be distinctly increased, approaching that of thyratrons for operation with high repetition rate. As a crow-bar switch, the lifetime is nearby unlimited due to cold electrode usage. Spatial and temporal resolved spectroscopy revealed new insight into the extraordinary discharge behavior of silicon carbide electrodes. The radial plasma expansion from the central bore hole to the outer electrode regions, forming vesicular shells of different ionization stages of Si and C, are described in detail. The remaining problem, a significant loss of deuterium gas during discharge, has been long-term tested and is assumed to be the outcome of absorption in the silicon carbide electrodes. An envisaged promising remedy is presented     

高效集成电光调制器和开关的电极与光波导的优化设计

集成电光调制与开关的效率取决于光波导中电光重叠因子Γ的大小，直接影响此因子的有电极宽度、电极间距、光模场尺寸、以及电极与光波导的相对位置等参量。本文利用集成电光调制器电极电场的分布规律，研究了各种结构中影响电光重叠因子的诸参量之间的关系；根据实际工艺条件，应用系统优化方法尽可能对各种参量进行优化计算，得到了实现高效调制和开关的全部参量的优化设计结果。     

Discharge between Liquid Jet and Metallic Electrodes

Experimental investigation of the low-temperature plasma of a gas discharge initiated in an interelectrode gap where one electrode is a liquid jet cathode and the other is a metallic anode has been carried out. Data are presented for the physical and spectral characteristics of the discharge and plasma composition. The electron concentration and temperature, as well as the vibrational and rotational temperatures of the heavy component have been estimated. The form and type of the discharge and also gas-hydrodynamic and thermal processes at the interface are described.     

Bias‐driven super‐insulating state in prismane single‐molecule contacts

Quantum transport in a single‐molecule contact made of a prismane cluster C₈ attached to quasi‐one‐dimensional gold (100) electrodes is calculated using the ab initio methodology based on the density‐functional theory and the nonequilibrium Green's functions formalism. Varying the junction length L we calculate the length dependence of the zero‐bias conductance G (L) and, for a set of the interelectrode distances, the current–voltage (I –V) characteristics. It is shown that the G (L) dependence is strongly nonmonotonic with a sharp dip at some value of L. With increase in L, the I –V curves change their shape from monotonic curves to curves with a negative differential resistance area and, for a larger L, the junction exhibits the super‐insulating state, i.e., within some applied bias voltage range the current through the junctions is about two orders of magnitude less than the current outside this bias range. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of electrode microstructure on gas-phase diffusion in solid oxide fuel cells

The relation between electrode microstructure and gas diffusion has been investigated with different morphologies of Pt electrodes by using AC impedance techniques. The measurements were carried out at temperatures of 873–1273 K and oxygen partial pressure (P_O2) of 0.01–1 atm.Gas-phase diffusion was observed only for high-performance electrodes at the high-temperature (1073–1273 K) and low-oxygen-partial-pressure regions (<0.1 atm P_O2). Considering the physical and electrochemical characteristics of impedance arcs, it was found that the arc at the frequency of below 1 Hz was related to gas conversion resistance, while the arc at the frequency of around 10 Hz represented pore diffusion resistance through the current-collecting part. For a thick electrode with a low porosity, however, gas diffusion resistance through pores of an electrode was observed at a frequency of around 100 Hz.From the results of a comparison of electrode performances with different electrode microstructures, electrochemical reaction sites (ERS) are supposed to be located at the peripheral line of Pt and YSZ as well as the Pt/YSZ interfaces where reaction gas can easily diffuse.     

Development of electrostatic paper separation and feed mechanism

A new paper separation and feed mechanism is proposed to realize a highly reliable paper handling system for printers and copiers. The paper-separation system consisted of a pair of parallel electrodes and a paper pile between the electrodes. Electrostatic separation of a piece of paper was possible from the top of the pile when the applied voltage exceeded the threshold needed to generate an electrostatic force larger than the weight of the paper. The threshold voltage was on the order of several kilovolts, and it agreed with the numerical value calculated using the finite element method (FEM) for the electrostatic field. Based on these basic investigations, a prototype mechanism for paper separation and feeding was constructed. It consisted of a roller-type separation electrode coated with an insulating film, a biased charger roller in contact with the separation roller to charge the insulating film on the separation roller, a ground electrode, and a paper pile situated between the electrodes. When an electrostatic field was applied between the biased charger roller and the ground electrode on which the paper pile was mounted, only the top sheet of paper separated, adhering electrostatically to the roller. The sheet was then fed rotating the separation roller. Using this system, reliable paper separation and feed was realized and a feed speed over 600 mm/s was demonstrated.     

The low-pressure transition from diffusion-controlled to multipacting gas breakdown

Summary The transition region from diffusion-controlled to multipacting discharge conditions in a parallel geometry, when the gas pressure is decreased and attains values below the so-called mean-free-path limit, is investigated using a Monte Carlo simulation technique. An ideal gas, without excitation states at energies below the ionization level and with a velocity-independent electron-molecule collision frequency, is considered. The results are plotted as breakdown voltages, properly normalized to the electron energy for which the secondary-electron emission yield of the electrodes is unity,vs. the gas pressure, given as the ratio of the electron collision frequency over the applied electric field frequency. Curves for various sets of dimensionless variables, representing size, electrode and gas parameters, are given and interpreted in terms of the changing relative importance of electron collisions with gas molecules and with electrodes, when the gas pressure is decreased and vacuum conditions are approached.