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At least 1,369 records · Page 76
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Lifetime of hydrogenated composite cathodes in a vacuum arc ion source

The paper reports on a study of the mass-charge state of the plasma produced in a vacuum arc discharge with composite cathodes which were copper-disk coated with a hydrogenated Zr film of thicknesses 9, 22, and 35 μm. The cathodes allow the generation of multicomponent gas and metal ion beams with a hydrogen ion content from several to several tens of percent. Also investigated is the dependence of the H ion fraction in a beam on the Zr film thickness during erosion to the point of disappearance of Zr peaks in mass-charge spectra. The ability of the vacuum arc system to produce H ions is analyzed by analyzing the cathode lifetime as a function of the film thickness and pulse repetition frequency.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Vacuum arc plasma generation and thin film deposition from a TiB{sub 2} cathode

We have studied the utilization of TiB{sub 2} cathodes for thin film deposition in a DC vacuum arc system. We present a route for attaining a stable, reproducible, and fully ionized plasma flux of Ti and B by removal of the external magnetic field, which leads to dissipation of the vacuum arc discharge and an increased active surface area of the cathode. Applying a magnetic field resulted in instability and cracking, consistent with the previous reports. Plasma analysis shows average energies of 115 and 26 eV, average ion charge states of 2.1 and 1.1 for Ti and B, respectively, and a plasma ion composition of approximately 50% Ti and 50% B. This is consistent with measured resulting film composition from X-ray photoelectron spectroscopy, suggesting a negligible contribution of neutrals and macroparticles to the film growth. Also, despite the observations of macroparticle generation, the film surface is very smooth. These results are of importance for the utilization of cathodic arc as a method for synthesis of metal borides.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Emission current from a single micropoint of explosive emission cathode

Explosive emission cathodes (EECs) are widely used due to their large current. There has been much research on the explosive electron emission mechanism demonstrating that a current density of 10{sup 8}–10{sup 9 }A/cm{sup 2} is necessary for a micropoint to explode in several nanoseconds and the micropoint size is in micron-scale according to the observation of the cathode surface. This paper, however, makes an effort to research the current density and the micropoint size in another way which considers the space charge screening effect. Our model demonstrates that the relativistic effect is insignificant for the micropoint emission due to the small size of the micropoint and uncovers that the micron-scale size is an intrinsic demand for the micropoint to reach a space charge limited current density of 10{sup 8}–10{sup 9 }A/cm{sup 2}. Meanwhile, our analysis shows that as the voltage increases, the micropoint emission will turn from a field limited state to a space charge limited state, which makes the steady-state micropoint current density independent of the cathode work function and much less dependent on the electric field and the field enhancement factor than that predicted by the Fowler-Nordheim formula.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Self-pulsing in a low-current hollow cathode discharge: From Townsend to glow discharge

We investigate the self-pulsing phenomenon of a low current cavity discharge in a cylindrical hollow cathode in pure argon. The waveforms of pulsed current and voltage are measured, and the time-averaged and time-resolved images of hollow cathode discharge are recorded by using high-speed intensified charge coupled device camera. The results show that the self-pulsing is a mode transition between low-current stage of Townsend discharge and high-current stage of glow discharge. During the self-pulsing, the current rising time relates to the dissipation of space charges, and the decay time relates to the reconstruction of the virtual anode by the accumulation of positive ions. Whether or not space charges can form and keep the virtual anode is responsible for the discharge mode and hence plays an important role in the self-pulsing phenomenon in low current hollow cathode discharge.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Scaled-down particle-in-cell simulation of cathode plasma expansion in magnetically insulated coaxial diode

The expansion of cathode plasma in magnetically insulated coaxial diode (MICD) is investigated in theory and particle-in-cell (PIC) simulation. The temperature and density of the cathode plasma are about several eV and 10{sup 13}–10{sup 16 }cm{sup −3}, respectively, and its expansion velocity is of the level of few cm/μs. Through hydrodynamic theory analysis, expressions of expansion velocities in axial and radial directions are obtained. The characteristics of cathode plasma expansion have been simulated through scaled-down PIC models. Simulation results indicate that the expansion velocity is dominated by the ratio of plasma density other than the static electric field. The electric field counteracts the plasma expansion reverse of it. The axial guiding magnetic field only reduces the radial transport coefficients by a correction factor, but not the axial ones. Both the outward and inward radial expansions of a MICD are suppressed by the much stronger guiding magnetic field and even cease.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

The importance of the cathode plume and its interactions with the ion beam in numerical simulations of Hall thrusters

Numerical simulations with a 2-D axisymmetric multi-fluid plasma code illustrate the significance of the near-plume interactions in investigations of the anomalous electron transport in Hall thrusters. In our simulations, the transport of electrons is modeled using an anomalous collision frequency, ν{sub anom}, yielding ν{sub anom} ≈ ω{sub ce} (i.e., the electron cyclotron frequency) in the near-plume region. We first show that restricting the anomalous collision frequency in this region to only within the ion beam, where the current density of ions is large, does not alter the plasma discharge in the Hall thruster as long as the interaction between the beam and the cathode plume is captured properly. These simulations suggest that electron transport occurs largely inside the beam. A second finding is on the significance of accounting for the ion acoustic turbulence (IAT), now known to occur in the vicinity of the cathode exit. We have included in our simulations a model of the IAT-driven anomalous collision frequency based on Sagdeev's model for saturation of the ion-acoustic instability. This implementation has allowed us to achieve excellent agreement with experimental measurements in the near plume of the H6 Hall thruster. Low frequency plasma oscillations similar in both magnitude and frequency to those found in the H6 thruster are recovered in our simulations when the model for the anomalous collision frequency in the cathode plume is included.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Note: Arc discharge plasma source with plane segmented LaB{sub 6} cathode

A plane cathode composed of close-packed hexagonal LaB{sub 6} (lanthanum hexaboride) segments is described. The 6 cm diameter circular cathode is heated by radiation from a graphite foil flat spiral. The cathode along with a hollow copper anode is used for the arc discharge plasma production in a newly developed linear plasma device. A separately powered coil located around the anode is used to change the magnetic field strength and geometry in the anode region. Different discharge regimes were realized using this coil.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Ab initio study of vacancy formation in cubic LaMnO{sub 3} and SmCoO{sub 3} as cathode materials in solid oxide fuel cells

Doped LaMnO{sub 3} and SmCoO{sub 3} are important solid oxide fuel cell cathode materials. The main difference between these two perovskites is that SmCoO{sub 3} has proven to be a more efficient cathode material than LaMnO{sub 3} at lower temperatures. In order to explain the difference in efficiency, we need to gain insight into the materials’ properties at the atomic level. However, while LaMnO{sub 3} has been widely studied, ab initio studies on SmCoO{sub 3} are rare. Hence, in this paper, we perform a comparative DFT + U study of the structural, electronic, and magnetic properties of these two perovskites. To that end, we first determined a suitable Hubbard parameter for the Co d–electrons to obtain a proper description of SmCoO{sub 3} that fully agrees with the available experimental data. We next evaluated the impact of oxygen and cation vacancies on the geometry, electronic, and magnetic properties. Oxygen vacancies strongly alter the electronic and magnetic structures of SmCoO{sub 3}, but barely affect LaMnO{sub 3}. However, due to their high formation energy, their concentrations in the material are very low and need to be induced by doping. Studying the cation vacancy concentration showed that the formation of cation vacancies is less energetically favorable than oxygen vacancies and would thus not markedly influence the performance of the cathode.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Fractional-dimensional Child-Langmuir law for a rough cathode

This work presents a self-consistent model of space charge limited current transport in a gap combined of free-space and fractional-dimensional space (F{sup α}), where α is the fractional dimension in the range 0 < α ≤ 1. In this approach, a closed-form fractional-dimensional generalization of Child-Langmuir (CL) law is derived in classical regime which is then used to model the effect of cathode surface roughness in a vacuum diode by replacing the rough cathode with a smooth cathode placed in a layer of effective fractional-dimensional space. Smooth transition of CL law from the fractional-dimensional to integer-dimensional space is also demonstrated. The model has been validated by comparing results with an experiment.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Pulsed hollow-cathode ion lasers: pumping and lasing parameters

Optimal discharge conditions have been experimentally found for ion lasers excited in the hollow-cathode discharge plasma by microsecond current pulses by pumping working atoms in secondkind collisions with ions and metastable buffer-gas atoms. Measurements of the output power of krypton ion and zinc-, cadmium-, mercury-, thallium-, copper-, and gallium-vapour lasers in tubes with cathodes of different diameters showed that the pulse power reaches several tens of watts, and the average power obtained with cathodes 2 cm in diameter and a length of 40 cm or more approaches 1 W. Lasing in most media is observed simultaneously at several lines (the multi-wavelength regime). Lasing on a three-component (He - Kr - Hg) mixture is realised in the multi-wavelength regime at blue, red, and IR lines.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Co-free gradient lithium-rich cathode for high-energy batteries with optimized cyclability

Lithium-rich layered oxides (LLOs) hold the promise for high-energy battery cathodes. However, its application has been hindered by voltage decay associated with irreversible reactions at high voltages despite decades of intensive efforts. Here, we first theoretically studied the molecular orbitals of Mn-based Li-rich configurations. We found that the π-bond ring formed within the LiMn 6 structure could participate in stable redox reactions as one unit, but Co could disrupt its symmetry. We thus designed and synthesized Co-free concentration-gradient LLOs (CF-CG-LLOs) materials. The combination of concentration gradient and Co removal leads to exceptional capacity retention without any fading over 100 cycles of the pouch cell. More importantly, it exhibits an extraordinarily low voltage decay of 0.15 mV/cycle, accompanied by a high Coulombic efficiency of 99.86%. This concept and demonstration of CF-CG-LLO cathodes reveal a viable avenue toward low-cost, high-energy-density battery cathodes.

25 ENERGY STORAGE↗
DOE OSTI2020

A high-capacity cathode for rechargeable K-metal battery based on reversible superoxide-peroxide conversion

Abstract As a promising low-cost energy storage device, the development of a rechargeable potassium-ion battery (KIB) is severely hindered by the limited capacity of cathode candidates. Regarded as an attractive capacity-boosting strategy, triggering the O-related anionic redox activity has not been achieved within a sealed KIB system. Herein, in contrast to the typical gaseous open K-O2 battery (O2/KO2 redox), we originally realize the reversible superoxide/peroxide (KO2/K2O2) interconversion on a KO2-based cathode. Controlled within a sealed cell environment, the irreversible O2 evolution and electrolyte decomposition (induced by superoxide anion (O2−) formation) are effectively restrained. Rationally controlling the reversible depth-of-charge at 300 mAh/g (based on the mass of KO2), no obvious cell degradation can be observed during 900 cycles. Moreover, benefitting from electrolyte modification, the KO2-based cathode is coupled with a limited amount of K-metal anode (merely 2.5 times excess), harvesting a K-metal full-cell with high energy efficiency (∼90%) and long-term cycling stability (over 300 cycles).

25 ENERGY STORAGE↗
DOE OSTI2016

Optimizing AlF{sub 3} atomic layer deposition using trimethylaluminum and TaF{sub 5}: Application to high voltage Li-ion battery cathodes

Atomic layer deposition (ALD) of conformal AlF{sub 3} coatings onto both flat silicon substrates and high-voltage LiNi{sub 0.5}Mn{sub 0.3}Co{sub 0.2}O{sub 2} (NMC) Li-ion battery cathode powders was investigated using a Al(CH{sub 3}){sub 3}/TaF{sub 5} precursor combination. This optimized approach employs easily handled ALD precursors, while also obviating the use of highly toxic HF(g). In studies conducted on planar Si wafers, the film's growth mode was dictated by a competition between the desorption and decomposition of Ta reaction byproducts. At T ≥ 200 °C, a rapid decomposition of the Ta reaction byproducts to TaC led to continuous deposition and high concentrations of TaC in the films. A self-limited ALD growth mode was found to occur when the deposition temperature was reduced to 125 °C, and the TaF{sub 5} exposures were followed by an extended purge. The lower temperature process suppressed conversion of TaF{sub x}(CH{sub 3}){sub 5−x} to nonvolatile TaC, and the long purges enabled nearly complete TaF{sub x}(CH{sub 3}){sub 5−x} desorption, leaving behind the AlF{sub 3} thin films. NMC cathode powders were coated using these optimized conditions, and coin cells employing these coated cathode particles exhibited significant improvements in charge capacity fade at high discharge rates.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Study of the prebreakdown stage of a gas discharge in a diode with point cathode by laser probing

The prebreakdown stage of a gas discharge in a diode with strongly overloaded cathode is studied by laser methods (by simultaneous use of multiframe interferometry and shadow and schlieren photographing) at atmospheric pressure. The spatial resolution of the methods is about 20 μm. A probing pulse of a laser (LS-2151 Nd: YAG laser with a half amplitude duration of 70 ps and a pulse energy of up to 40 mJ) is synchronized with a voltage pulse with accuracy of about 1 ns. High field strength at the cathode is achieved due to the use of thin individual metal tips on the electrodes. It is shown that the initial stage of breakdown of a discharge gap is accompanied by the emergence of a dense plasma cloud at the end of a tip with electron density of about 5 × 10{sup 19} cm{sup –3} with a size of tens of microns, as well as by a sharp increase in the total current through the diode. After the emergence of a dense plasma cloud at the end of a cathode tip, a similar cloud is formed on the surface of the anode; sometime later, these clouds join together and form a tubular current channel. The dynamics of the breakdown, as well as the parameters of the plasma are studied by the abovementioned techniques in three independent optical channels.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Study of the feasibility of distributed cathodic arc as a plasma source for development of the technology for plasma separation of SNF and radioactive wastes

One of the key problems in the development of plasma separation technology is designing a plasma source which uses condensed spent nuclear fuel (SNF) or nuclear wastes as a raw material. This paper covers the experimental study of the evaporation and ionization of model materials (gadolinium, niobium oxide, and titanium oxide). For these purposes, a vacuum arc with a heated cathode on the studied material was initiated and its parameters in different regimes were studied. During the experiment, the cathode temperature, arc current, arc voltage, and plasma radiation spectra were measured, and also probe measurements were carried out. It was found that the increase in the cathode heating power leads to the decrease in the arc voltage (to 3 V). This fact makes it possible to reduce the electron energy and achieve singly ionized plasma with a high degree of ionization to fulfill one of the requirements for plasma separation of SNF. This finding is supported by the analysis of the plasma radiation spectrum and the results of the probe diagnostics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Simulation of oscillatory processes in a beam-plasma system with a virtual cathode in gas-filled interaction space

Physical processes occurring in an intense electron beam with a virtual cathode in an interaction space filled with neutral gas are studied in a two-dimensional model. A mathematical model is proposed for investigating complicated self-consistent processes of neutral gas ionization by the beam electrons and the dynamics of an electron beam and heavy positive ions in the common space charge field with allowance for the two-dimensional motion of charged particles. Three characteristic dynamic regimes of the system are revealed: complete suppression of oscillations of the virtual cathode as a result of neutralizing its space charge by positive ions; the pulsed generation regime, in which the ions dynamics repeatedly suppresses and restores the virtual cathode oscillations; and the continuous generation regime with an anomalously high level of noisy oscillations.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Specific features of an electric discharge operating between an electrolytic anode and a metal cathode

Results are presented from experimental studies of a high-current electric discharge operating between an St45 steel cathode and a service water anode in a wide range of air pressures. Peculiarities of discharge ignition and specific features of cathode and anode spots were revealed. The behavior of the current density on a service water anode was investigated for the first time. Comparison of the current densities j on the steel cathode and service water anode shows that, in the parameter range under study, Hehl's law is not satisfied on the water anode. The two-dimensional distribution of the potential inside and on the surface of the service water anode was measured.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Expansion of the cathode spot and generation of shock waves in the plasma of a volume discharge in atmospheric-pressure helium

The expansion of the cathode spot and the generation of shock waves during the formation and development of a pulsed volume discharge in atmospheric-pressure helium were studied by analyzing the emission spectra of the cathode plasma and the spatiotemporal behavior of the plasma glow. The transition of a diffuse volume discharge in a centimeter-long gap into a high-current diffuse mode when the gas pressure increased from 1 to 5 atm and the applied voltage rose from the statistical breakdown voltage to a 100% overvoltage was investigated. Analytical expressions for the radius of the cathode spot and its expansion velocity obtained in the framework of a spherically symmetric model agree satisfactorily with the experimental data.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗