1,477 Matching Results

Search Results

Advanced search parameters have been applied.

Modeling and Optimization of Deflection Slits for Fast-Pulsing of a Low Energy Ion Beam

Description: This paper discusses research on modeling and optimization of deflection slits for fast-pulsing of a low energy ion beam. The authors also investigate simulated slit configurations that reduce the length of the fields along the beam axis, thus optimizing the pulsing mechanism.
Date: March 30, 2006
Creator: Bosca, Ryan & Weathers, Duncan L.
Partner: UNT Honors College

The Effect of Electric Fields on Cathodoluminescence from Phosphors

Description: When external electric fields are applied to phosphors the cathodoluminescence (CL) at low beam energies is strongly affected. This experiment has been carried out on a variety of common phosphors used in cathode ray tube applications, and the electron beam energy, beam current, and electric field dependence of the CL are thoroughly characterized. It is found that the general features of these effects, particular y the strong polarity and beam energy dependence, are consistent with a model which assumes that the main effect of the electric fields is to alter the populations of electrons `and holes at the phosphor surface. This in turn, modulates the non-radiative energy losses that strongly affect the low-beam-energy CL efficiency. Because the external fields are applied without any direct contact to the phosphor material, the large changes seen in the CL decay rapidly as the beam-created electrons and holes polarize, shielding the externally applied bias. These results have important implications for designing phosphors which might be efficient at low electron energies.
Date: January 14, 1999
Creator: Seager, C.H.
Partner: UNT Libraries Government Documents Department

Flow-Based Detection of Bar Coded Particles

Description: We have developed methods for flow control, electric field alignment, and readout of colloidal Nanobarcodes{copyright}. Our flow-based detection scheme leverages microfluidics and alternate current (AC) electric fields to align and image particles in a well-defined image plane. Using analytical models of the particle rotation in electric fields we can optimize the field strength and frequency necessary to align the particles. This detection platform alleviates loss of information in solution-based assays due to particle clumping during detection.
Date: June 24, 2005
Creator: Rose, K A; Dougherty, G M & Santiago, J G
Partner: UNT Libraries Government Documents Department

Brain Activity in Rats Exposed to Short-Term External Electrical Fields

Description: The effects of external electric fields (EEF) on brain activity in anesthetized rats were studied. The field strengths used, 9 kV/m and 5 kV/m, both D.C. and A.C. (60Hz) were in the range of those measured beneath current overhead transmission lines. Brain activity was monitored from surface electrodes and from electrodes stereotaxically implanted in the posterior-lateral portion of the hypothalamus. It was found that 9 kV/m and 5 kV/m EEF's both D.C. and A.C. brought about statistically significant changes in hypothalamic activity, however, the effects were bi-directional, (i.e. increases and decreases). Only seven of the 60 animals exposed showed changes in the EEG recorded with surface electrodes. The data clearly indicate that (1) anesthetized animals do respond to a change in the external electric field around them, (2) the hypothalamus may contain special electro-receptors that, in turn, may alter various other physiological processes, and (3) the data indicates the need for further research to help government agencies to establish more adequate safety guidelines.
Date: May 1985
Creator: Hines, Gregory M. (Gregory Manuel)
Partner: UNT Libraries

A Study of Quantum Electron Dynamics in Periodic Superlattices under Electric Fields

Description: This thesis examines the quantum dynamics of electrons in periodic semiconductor superlattices in the presence of electric fields, especially uniform static fields. Chapter 1 is an introduction to this vast and active field of research, with an analysis and suggested solutions to the fundamental theoretical difficulties. Chapter 2 is a detailed historical review of relevant theories, and Chapter 3 is a historical review of experiments. Chapter 4 is devoted to the time-independent quantum mechanical study of the electric-field-induced changes in the transmission properties of ballistic electrons, using the transfer matrix method. In Chapter 5, a new time-dependent quantum mechanical model free from the fundamental theoretical difficulties is introduced, with its validity tested at various limiting cases. A simplified method for calculating field-free bands of various potential models is designed. In Chapter 6, the general features of "Shifting Periodicity", a distinctive feature of this new model, is discussed, and a "Bloch-Floquet Theorem" is rigorously proven. Numerical evidences for the existence of Wannier-Stark-Ladders are presented, and the conditions for its experimental observability is also discussed. In Chapter 7, an analytical solution is found for Bloch Oscillations and Wannier-Stark-Ladders at low electric fields. In Chapter 8, a new quantum mechanical interpretation for Bloch Oscillations and Wannier-Stark-Ladders is derived from the analytical result. The extension of this work to the cases of time-dependent electric fields is also discussed.
Date: May 1996
Creator: Yuan, Daiqing
Partner: UNT Libraries

Effects of External Electric Fields on Light Transmittance in Isolated Crayfish Nerves

Description: Acute effects of a pulsed external electric field (PEEF) at 20 V/cm and a d.c. EEF at 90 V/cm on light transmittance in an isolated compound crayfish nerve was measured. In a third series, the nerve was pre-treated with the Na+ channel blocker tetrodotoxin (TTX). A PEEF produced an irreversible increase in the variation of light transmittance in normal nerves but a reversible increase in TTX treated nerves. This data was statistically insignificant. The d.c. EEFs produced a reversible and statistically significant enhancement of variation in light transmittance in both untreated and TTX-treated nerves. The findings may be due to either (1) an alteration in the ion/fluid flux within the nerve or (2) a physical alteration of protein molecules in the membranes.
Date: December 1995
Creator: Northcutt, Brian S. W.
Partner: UNT Libraries

Local field and quantum effects for current perpendicular to planes in multilayers

Description: The calculation of giant-magnetoresistance and in general, of electron transport for multilayers in the case of current perpendicular to the planes (CPP) requires both the two-point conductivity and the solution to the local field problem. In this paper we present a solution to the local field problem at an interface using two approaches. In the first approach we find the semiclassical solution for the local field when there is a band mismatch between two sides of an interface, and examine the deviation of the total resistance from the result of ``self-averaging``, in the lowest order of the value of the potential step. In the second approach, we solve for the quantum correction to the local field through a numerical iterative scheme. The oscillations due to the quantum correction are surprisingly large, but their correction to the total resistance is remarkably small. Our results imply that the ``self-averaging`` of the resistance, which is usually assumed in analysis of CPP, is only approximate. 8 refs., 2 figs., 2 tabs.
Date: December 31, 1996
Creator: Zhang, X.G. & Butler, W.H.
Partner: UNT Libraries Government Documents Department

Asymmetrical polarization of spherical colloidal particle double layer in electrical fields

Description: It has long been accepted that the drastic increase in the viscosity of electrorheological fluids subjected to electric fields is due to interactions between the induced dipole moments of the colloidal particles in the suspensions. Computer simulations of dipole-dipole interactions have produced chain and column formations of particles in the direction of the applied field that agree well with experimental observations. In an aqueous suspension of colloidal particles, however, the polarization of particles is very sensitive to the frequency of the applied electric field, and the dynamics of the particles can differ drastically from the dynamics that would be expected from a simple induced dipole model. In certain frequency regimes, instead of forming chains and columns in the direction of a linear applied field, the particles are dynamically unstable and circulate in bands that tilt at a significant angle relative to the direction of the field. This indicates that the interactions between the particles are no longer symmetrical with respect to the applied electric field. The authors have proposed a phenomenological model to explain the mechanism of this symmetry breaking. They have carried out a computer simulation of colloidal particle dynamics resulting from their model, and the results of the simulation agree well with experimental observations. When a particle is spinning, the diffusion of the ions in the double layer becomes asymmetrical with respect to the electric field, with the ions diffusing faster in the direction of spinning than against it. This causes the induced dipole moment of the particle to be misaligned with respect to the applied field. The authors earlier simulation results show that this misalignment between the polarization of the particle and the applied electric field plays a crucial role in the formation of a band of particles tilted at an angle relative to the direction of ...
Date: April 1, 1995
Creator: Hu, Y.; Kumar, V. & Fraden, S.
Partner: UNT Libraries Government Documents Department

Extension of a thin-wire algorithm for wires moved laterally within a mesh

Description: It was shown that the accuracy of results for wires moved laterally from mesh edges can be greatly improved by taking account of the behavior of the field in the vicinity of the wire. Also, the distance to the end of the wire can be adjusted within a cell by using a general second-order difference form for the derivative. Making the wire location completely independent of the mesh would require the additional ability to tilt the wire with respect to the edges. This seems to be a considerably more difficult problem than moving the wires laterally, since the component of the mesh field parallel to the wire gets mixed with the larger radial electric field due to charge on the wire. Simply averaging the mesh fields did not seem to work well, except in the case where the wire was tilted in one coordinate plane, and the mesh fields above and below the plane of the wire, on edges orthogonal to the wire normal, could be averaged. Further study is needed to develop a more general capability to tilt a wire with respect to the mesh.
Date: November 1, 1998
Creator: Burke, G J & Steich, D J
Partner: UNT Libraries Government Documents Department

Effects of an electric field on water uptake in single roots of intact onion plants

Description: Water uptake by a single root of an onion plant (Allium cepa) was measured potentially before, during and following exposure of shoots to an external electric field (EEF). The field strength used was 9kV/m DC and AC (60-Hz) brought about a statistically significant increase (44-71%) in water uptake if the shoot chamber was at almost 100% humidity.
Date: August 1982
Creator: Sardarabadi, Bahram M. (Bahram Moory)
Partner: UNT Libraries

Breakdown During High-Field Bias-Temperature Stress

Description: Measurements of dielectric breakdown during high-field electrical stress are typically performed at or near room temperature via constant voltage or current stress methods. In this summary they explore whether useful information might also be obtained by performing current measurements during a temperature ramp at high electric field.
Date: August 5, 1999
Creator: Fleetwood, D.M.; Krisch, K.S. & Sexton, F.W.
Partner: UNT Libraries Government Documents Department

A simple drift-diffusion model for calculating the neutralization time of H- in xe gas for choppers placed in the LEBT

Description: The neutralization of H{sup -} beam with a gas like Xe is an important part of low energy beam transport (LEBT). It is well known that choppers which use an electric field when placed in the LEBT strongly affects the neutralization of H{sup -}. The question then naturally arises as to whether a magnetic chopper has a better neutralization time than an electric chopper. To answer this question, a simple 1-space, 1 time drift-diffusion model of H{sup -} beam in Xe gas has been used to calculate the neutralization times for the following scenarios: (a) a region initially cleared of Xe+ ions with an electric field but partially neutralized outside, (b) a region within and outside the chopper which is initially partially neutralized.
Date: March 1, 2010
Creator: Tan, Cheng-Yan
Partner: UNT Libraries Government Documents Department

PetaScale calculations of the electronic structures ofnanostructures with hundreds of thousands of processors

Description: Density functional theory (DFT) is the most widely used ab initio method in material simulations. It accounts for 75% of the NERSC allocation time in the material science category. The DFT can be used to calculate the electronic structure, the charge density, the total energy and the atomic forces of a material system. With the advance of the HPC power and new algorithms, DFT can now be used to study thousand atom systems in some limited ways (e.g, a single selfconsistent calculation without atomic relaxation). But there are many problems which either requires much larger systems (e.g, >100,000 atoms), or many total energy calculation steps (e.g. for molecular dynamics or atomic relaxations). Examples include: grain boundary, dislocation energies and atomic structures, impurity transport and clustering in semiconductors, nanostructure growth, electronic structures of nanostructures and their internal electric fields. Due to the O(N{sup 3}) scaling of the conventional DFT algorithms (as implemented in codes like Qbox, Paratec, Petots), these problems are beyond the reach even for petascale computers. As the proposed petascale computers might have millions of processors, new computational paradigms and algorithms are needed to solve the above large scale problems. In particular, O(N) scaling algorithms with parallelization capability up to millions of processors are needed. For a large material science problem, a natural approach to achieve this goal is by divide-and-conquer method: to spatially divide the system into many small pieces, and solve each piece by a small local group of processors. This solves the O(N) scaling and the parallelization problem at the same time. However, the challenge of this approach is for how to divide the system into small pieces and how to patch them up without the trace of the spatial division. Here, we present a linear scaling 3 dimensional fragment (LS3DF) method which uses a novel ...
Date: April 1, 2006
Creator: Wang, Lin-Wang; Zhao, Zhengji & Meza, Juan
Partner: UNT Libraries Government Documents Department


Description: Expressions are presented for the electric and magnetic fields due to a pulse of charge, which may be oscillating transversely while moving down an infinitely long highly conducting pipe of circular cross section. The expressions are evaluated at large distances from the pulse and the fields are shown to decrease algebraically in the distance behind the pulse. In the absence of transverse oscillations the longitudinal electric field varies as the inverse three-halves power of the distance; in the presence of oscillations the dominant field component is the transverse magnetic field, which decreases as the inverse one-half power. In the long-range limit the amplitude of the fields is proportional to the square root of the wall resistivity. The phase of the field associated with the oscillating pulse is shown to be the phase of the pulse at the time when it passed the point of observation.
Date: February 28, 1966
Creator: Morton, P.L.; Neil, V.K. & Sessler, A.M.
Partner: UNT Libraries Government Documents Department


Description: We discuss the possibility of using synchrotron radiation to form electron rings having a very high electric field to hold the ions inside the ring. The formulas describing bow the energy and the dimension of the ring change under the effect of synchrotron radiation are derived, and a numerical example is given.
Date: May 13, 1970
Creator: Pellegrini, C.
Partner: UNT Libraries Government Documents Department

Compact Analytic Expression for the Electric Field of a 2DElliptical Charge Distribution Inside a Perfectly Conducting CircularCylinder

Description: By combining the method of images with calculus of complex variables, we provide a simple expression for the electric field of a two-dimensional (2D) static elliptical charge distribution inside a perfectly conducting cylinder. The charge distribution need not be concentric with the cylinder.
Date: May 29, 2007
Creator: Furman, M.A.
Partner: UNT Libraries Government Documents Department

Measuring Helical FCG Voltage with an Electric Field Antenna

Description: A method of measuring the voltage produced by a helical explosive flux compression generator using a remote electric field antenna is described in detail. The diagnostic has been successfully implemented on several experiments. Measured data from the diagnostic compare favorably with voltages predicted using the code CAGEN, validating our predictive modeling tools. The measured data is important to understanding generator performance, and is measured with a low-risk, minimally intrusive approach.
Date: August 1, 2011
Creator: White, A D; Anderson, R A; Javedani, J B; Reisman, D B; Goerz, D A; Ferriera, A J et al.
Partner: UNT Libraries Government Documents Department

A proof-of-principle experiment of the ferroelectric tuner for the 1.3 GHz gun cavity

Description: A novel ferroelectric frequency tuner was developed by the Ornega-P company and was tested at the Brookhaven National Laboratory on a 1.3 GHz RF cavity at room temperature. The tuner is based on the ferroelectric property of having a permittivity variable with an applied electric field. The achievable frequency tuning range can be estimated from the reactive impedance change due to an applied voltage via a S{sub 11} measurement at the tuner port. The frequency shift can be measured directly with a S{sub 21} measurement across the gun cavity with the tuner connected and activated. The frequency change due to an applied 5 kV obtained from the two methods is in reasonable agreement. The reactive impedance measurement yields a value in the range between 3.2 kHz and 14 kHz, while 9 kHz is the result from the direct measurement. The detail description of the experiment and the analysis will be discussed in the paper.
Date: May 4, 2009
Creator: Hahn,H.; Choi, E.; Shchelkunov, S. V.; Hirshfield, J.; Kazakov, S. & Shschelkunov, S.
Partner: UNT Libraries Government Documents Department

Toward a simple molecular understanding of sum frequency generation at air-water interfaces

Description: Second-order vibrational spectroscopies successfully isolate signals from interfaces, but they report on intermolecular structure in a complicated and indirect way. Here we adapt a perspective on vibrational response developed for bulk spectroscopies to explore the microscopic fluctuations to which sum frequency generation (SFG), a popular surface-specific measurement, is most sensitive. We focus exclusively on inhomogeneous broadening of spectral susceptibilities for OH stretching of HOD as a dilute solute in D{sub 2}O. Exploiting a simple connection between vibrational frequency shifts and an electric field variable, we identify several functions of molecular orientation whose averages govern SFG. The frequency-dependence of these quantities is well captured by a pair of averages, involving alignment of OH and OD bonds with the surface normal at corresponding values of the electric field. The approximate form we obtain for SFG susceptibility highlights a dramatic sensitivity to the way a simulated liquid slab is partitioned for calculating second-order response.
Date: January 13, 2009
Creator: Noah-Vanhoucke, Joyce; Smith, Jared D. & Geissler, Phillip L.
Partner: UNT Libraries Government Documents Department