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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,531 records · Page 85
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Dark current in multilayer stabilized amorphous selenium based photoconductive x-ray detectors

We report on experimental results which show that the dark current in n-i-p structured, amorphous selenium films is independent of i-layer thickness in samples with consistently thick blocking layers. We have observed, however, a strong dependence on the n-layer thickness and positive contact metal chosen. These results indicate that the dominant source of the dark current is carrier injection from the contacts and any contribution from carriers thermally generated in the bulk of the photoconductive layer is negligible. This conclusion is supported by a description of the dark current transients at different applied fields by a model which assumes only carrier emission over a Schottky barrier. This model also predicts that while hole injection is initially dominant, some time after the application of the bias, electron injection may become the dominant source of dark current.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Non-inductive current start-up assisted by energetic electrons in Q-shu University experiment with steady-state spherical tokamak

After intensive discharge cleaning of the chamber wall, non-inductive current start-up experiments have been successfully performed in QUEST in moderate vertical fields of about 1.0-1.5 mT with positive n-index. Simultaneously, with increasing plasma current, an asymmetric toroidal flow of energetic electrons was observed and direct measurements of current driven by this asymmetric flow were taken with a newly developed Langmuir probe technique. A numerical study of the energetic electron orbits indicates that the total current is enough to play a dominant role in the formation of a closed flux surface in QUEST.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Optical transition radiation used in the diagnostic of low energy and low current electron beams in particle accelerators

Optical transition radiation (OTR) plays an important role in beam diagnostics for high energy particle accelerators. Its linear intensity with beam current is a great advantage as compared to fluorescent screens, which are subject to saturation. Moreover, the measurement of the angular distribution of the emitted radiation enables the determination of many beam parameters in a single observation point. However, few works deals with the application of OTR to monitor low energy beams. In this work we describe the design of an OTR based beam monitor used to measure the transverse beam charge distribution of the 1.9-MeV electron beam of the linac injector of the IFUSP microtron using a standard vision machine camera. The average beam current in pulsed operation mode is of the order of tens of nano-Amps. Low energy and low beam current make OTR observation difficult. To improve sensitivity, the beam incidence angle on the target was chosen to maximize the photon flux in the camera field-of-view. Measurements that assess OTR observation (linearity with beam current, polarization, and spectrum shape) are presented, as well as a typical 1.9-MeV electron beam charge distribution obtained from OTR. Some aspects of emittance measurement using this device are also discussed.

43 PARTICLE ACCELERATORS↗
DOE OSTI2013

Transient and steady-state dark current mechanisms in amorphous selenium avalanche radiation detectors

A theoretical model for describing bias-dependent transient and steady-state behaviors of dark current in amorphous selenium (a-Se) avalanche detector structures has been developed. The analytical model considers bulk thermal generation current from mid-gap sates, transient carrier depletion, and carrier injection from the electrodes incorporating avalanche multiplication. The proposed physics-based dark current model is compared with the published experimental results on three potential a-Se avalanche detector structures. The steady-state dark current is the minimum for the structures that have effective blocking layers for both holes and electrons. The transient decay time to reach a plateau decreases considerably with increasing electric field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Transport phenomena in intrinsic semiconductors and insulators at high current densities: Suppression of the broken neutrality drift

It is shown that, in addition to the diffusion and broken neutrality drift (BND) modes well-known for insulators and very lightly doped semiconductors, the quasineutral drift (QND) mode is possible. The transition from the BND to QND mode is accompanied by the appearance of a portion with a very sharp current rise in the current-voltage characteristic. This effect is observed in a new type of semiconductor detectors (CIDs, Current Injected Detectors) of high-intensity neutron and proton radiation, suggested, in particular, for Large Hadron Collider. The effect is unambiguously attributed now to the presence of radiation-induced deep centers in a semiconductor. It is shown, however, in this paper that the effect of a very sharp rise in current upon a slight increase in voltage is even possible when there are no deep centers. An equation adequately describing the possible transport modes in intrinsic semiconductors and insulators is derived. The results of an analytical study are confirmed by an adequate simulation.

36 MATERIALS SCIENCE↗
DOE OSTI2014

A high-current electron gun for the electron beam ion trap at the National Superconducting Cyclotron Laboratory

The Electron Beam Ion Trap (EBIT) in NSCL’s reaccelerator ReA uses continuous ion injection and accumulation. In order to maximize capture efficiency and minimize breeding time into high charge states, the EBIT requires a high-current/high current-density electron beam. A new electron gun insert based on a concave Ba-dispenser cathode has been designed and built to increase the current transmitted through the EBIT’s superconducting magnet. With the new insert, stable EBIT operating conditions with 0.8 A of electron beam have been established. The design of the electron gun is presented together with calculated and measured perveance data. In order to assess the experimental compression of the electron beam, a pinhole CCD camera has been set up to measure the electron beam radius. The camera observes X-rays emitted from highly charged ions, excited by the electron beam. Initial tests with this camera setup will be presented. They indicate that a current density of 640 A/cm{sup 2} has been reached when the EBIT magnet was operated at 4 T.

43 PARTICLE ACCELERATORS↗
DOE OSTI2013

The role of localized junction leakage in the temperature-dependent laser-beam-induced current spectra for HgCdTe infrared focal plane array photodiodes

We have performed the study on the dependence of laser beam induced current (LBIC) spectra on the temperature for the vacancy-doped molecular beam epitaxy grown Hg{sub 1−x}Cd{sub x}Te (x = 0.31) photodiodes by both experiment and numerical simulations. It is found that the measured LBIC signal has different distributions for different temperature extents. The LBIC profile tends to be more asymmetric with increasing temperature below 170 K. But the LBIC profile becomes more symmetric with increasing temperature above 170 K. Based on a localized leakage model, it is indicated that the localized junction leakage can lead to asymmetric LBIC signal, in good agreement with the experimental data. The reason is that the trap-assisted tunneling current is the dominant leakage current at the cryogenic temperature below 170 K while the diffusion current component becomes dominant above the temperature of 170 K. The results are helpful for us to better clarify the mechanism of the dependence of LBIC spectra on temperature for the applications of HgCdTe infrared photodiodes.

36 MATERIALS SCIENCE↗
DOE OSTI2013

First demonstration of an asymmetric kinetic equilibrium for a thin current sheet

The modeling of steady state collisionless asymmetric tangential current layers is a challenging and poorly understood problem. For decades now, this difficulty has been limiting numerical models to approximate equilibria built with locally Maxwellian current layers and theoretical analyses to the very restricted Harris equilibrium. We show how the use of any distribution functions depending only on local macroscopic quantities results in a strong alteration of the current layer internal structure, which converges toward an unpredictable quasi-steady state with emission of ion scale perturbations. This transient can be explained in terms of ion kinetic and electron fluid physics. We demonstrate, for the first time, the validity of an asymmetric kinetic equilibrium model as well as its usability as an initial condition of hybrid kinetic simulations. This offers broad perspectives for the current sheet modeling, for which the early phase of instabilities can be studied within the kinetic formalism.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Power spectra and auto correlation analysis of hyperfine-induced long period oscillations in the tunneling current of coupled quantum dots

We outline power spectra and auto correlation analysis performed on temporal oscillations in the tunneling current of coupled vertical quantum dots. The current is monitored for ∼2325 s blocks as the magnetic field is stepped through a high bias feature displaying hysteresis and switching: hallmarks of the hyperfine interaction. Quasi-periodic oscillations of ∼2 pA amplitude and of ∼100 s period are observed in the current inside the hysteretic feature. Compared to the baseline current outside the hysteretic feature the power spectral density is enhanced by up to three orders of magnitude and the auto correlation displays clear long lived oscillations about zero.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

Spin-current noise from fluctuation relations

We present fluctuation relations that connect spin-polarized current and noise in mesoscopic conductors. In linear response, these relations are equivalent to the fluctuation-dissipation theorem that relates equilibrium current-current correlations to the linear conductance. More interestingly, in the weakly nonlinear regime of transport, these relations establish a connection between the leading-order rectification spin conductance, the spin noise susceptibility and the third cumulant of spin current fluctuations at equilibrium. Our results are valid even for systems in the presence of magnetic fields and coupled to ferromagnetic electrodes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Current dependence of spin torque switching rate based on Fokker-Planck approach

The spin torque switching rate of an in-plane magnetized system in the presence of an applied field is derived by solving the Fokker-Planck equation. It is found that three scaling currents are necessary to describe the current dependence of the switching rate in the low-current limit. The dependences of these scaling currents on the applied field strength are also studied.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Graphene electron cannon: High-current edge emission from aligned graphene sheets

High-current field emitters are made by graphene paper consist of aligned graphene sheets. Field emission luminance pattern shows that their electron beams can be controlled by rolling the graphene paper from sheet to cylinder. These specific electron beams would be useful to vacuum devices and electron beam lithograph. To get high-current emission, the graphene paper is rolled to array and form graphene cannon. Due to aligned emission array, graphene cannon have high emission current. Besides high emission current, the graphene cannon is also tolerable with excellent emission stability. With good field emission properties, these aligned graphene emitters bring application insight.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Current sheets and pressure anisotropy in the reconnection exhaust

A particle-in-cell simulation shows that the exhaust during anti-parallel reconnection in the collisionless regime contains a current sheet extending 100 inertial lengths from the X line. The current sheet is supported by electron pressure anisotropy near the X line and ion anisotropy farther downstream. Field-aligned electron currents flowing outside the magnetic separatrices feed the exhaust current sheet and generate the out-of-plane, or Hall, magnetic field. Existing models based on different mechanisms for each particle species provide good estimates for the levels of pressure anisotropy. The ion anisotropy, which is strong enough to reach the firehose instability threshold, is also important for overall force balance. It reduces the outflow speed of the plasma.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Effects of pressure anisotropy on magnetospheric magnetohydrodynamics equilibrium of an internal ring current system

Effects of pressure anisotropy on magnetospheric magnetohydrodynamics equilibrium are studied analytically and numerically, where the plasma is confined by only poloidal magnetic field generated by an internal ring current. The plasma current due to finite pressure can be divided into two components; one remains at isotropic pressure and the other arises from pressure anisotropy. When p{sub ⊥}, the pressure perpendicular to the magnetic field, is larger than p{sub ∥}, the pressure parallel to the magnetic field, those two components of plasma current tend to cancel each other to reduce the total amount of plasma current. Equilibrium beta limit is also examined, where the beta is a ratio of the plasma pressure to the magnetic pressure. The equilibrium beta limit decreases as the pressure anisotropy becomes strong. The beta value is strictly limited by ellipticity of the equilibrium equation when p{sub ∥}>p{sub ⊥}. On the other hand, when p{sub ⊥}>p{sub ∥}, although the tendency of the beta limit agrees with the ellipticity condition of the equilibrium equation, equilibria with a hyperbolic region can be obtained by iterative procedure with practically reasonable convergence criteria.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Large and stable emission current from synthesized carbon nanotube/fiber network

In order to obtain a large and stable electron field emission current, the carbon nanotubes have been synthesized on carbon fibers by cold wall chemical vapor deposition method. In the hierarchical nanostructures, carbon fibers are entangled together to form a conductive network, it could provide excellent electron transmission and adhesion property between electrode and emitters, dispersed clusters of carbon nanotubes with smaller diameters have been synthesized on the top of carbon fibers as field emitters, this kind of emitter distribution could alleviate electrostatic shielding effect and protect emitters from being wholly destroyed. Field emission properties of this kind of carbon nanotube/fiber network have been tested, up to 30 mA emission current at an applied electric field of 6.4 V/μm was emitted from as-prepared hierarchical nanostructures. Small current degradation at large emission current output by DC power operation indicated that carbon nanotube/fiber network could be a promising candidate for field emission electron source.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Neoclassical effects on the stabilization of tearing mode by current modulation

The neoclassical effects on the stabilization of tearing modes by current modulation have been investigated. Neoclassical effects enhance the resistivity and reduce the resistive diffusion time of the modulation current. Therefore, the oscillating current can penetrate deeper into the plasma. With an oscillating loop voltage, the plasma oscillates radially at the Ware-pinch velocity. These neoclassical effects improve the efficiency of tearing mode stabilization by the current modulation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Magnetization stability analysis of the Stoner-Wohlfarth model under a spin-polarized current with a tilted polarization

The stationary-state solutions of magnetization dynamics under a spin-polarized current that was polarized in an arbitrary direction were investigated by solving the Landau-Lifshitz-Gilbert-Slonczewski equation for a single-domain magnet. Taking into consideration the uniaxial magnetic anisotropy, the equilibrium directions of the magnetization vectors were analytically obtained by solving an algebraic cubic equation. It was found that one to three pairs of magnetization equilibrium states existed, depending on the current intensity and the direction of the spin polarization. By numerically analyzing the stabilities of these equilibrium states, the threshold switching current for the reversing the magnetic vector was obtained under different current polarization configurations, which may be useful for use in future spintronics devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

The physical nature of the phenomenon of positive column plasma constriction in low-pressure noble gas direct current discharges

The essence of the positive-column plasma constriction for static (the diffusion mode) and dynamic ionization equilibrium (the stratificated and constricted modes) is analyzed. Two physical parameters, namely, the effective ionization rate of gas atoms and the ambipolar diffusion coefficient of electrons and ions, determine the transverse distribution of discharge species and affect the current states of plasma. Transverse constriction of the positive column takes place as the gas ionization level (discharge current) and pressure increase. The stratified mode (including the constricted one) is observed between the two adjacent types of self-sustained discharge phases when they coexist together at the same time or in the same place as a coherent binary mixture. In the case, a occurrence of the discharge phase with more high electron density presently involve a great decrease in the cross-section of the current channel for d.c. discharges. Additional physical factors, such as cataphoresis and electrophoresis phenomena and spatial gas density inhomogeneity correlated with a circulatory flow in d.c. discharges, are mainly responsible for the current hysteresis and partially constricted discharge.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗