966 Matching Results

Search Results

Advanced search parameters have been applied.

RF Driven Multicusp H- Ion Source

Description: An rf driven multicusp source capable of generating 1-ms H{sup -} beam pulses with a repetition rate as high as 150 Hz has been developed. This source can be operated with a filament or other types of starter. There is almost no lifetime limitation and a clean plasma can be maintained for a long period of operation. It is demonstrated that rf power as high as 25 kW could be coupled inductively to the plasma via a glass-coated copper-coil antenna. The extracted H{sup -} current density achieved is about 200 mA/cm{sup 2}.
Date: June 1, 1990
Creator: Leung, K.N.; DeVries, G.J.; DiVergilio, W.F.; Hamm, R.W.; Hauck, C.A.; Kunkel, W.B. et al.
Partner: UNT Libraries Government Documents Department

Comment on flux creep with logarithmic U(j) dependence

Description: The numerical calculation by Wang and Dong [Phys. Rev. B 49, 698 (1994)] of flux density profiles across a slab sample exposes a possible misunderstanding of the solution given earlier by vinokur, Feigel`man and Geshkenbein, but also predicts a kink in the magnetization relaxation curve which Schnack and Griessen have already shown to be erroneous.
Date: February 1, 1994
Creator: Gilchrist, J.; Schnack, H.G. & van der Beek, C.J.
Partner: UNT Libraries Government Documents Department

Double Barrier Resonant Tunneling Transistor with a Fully Two Dimensional Emitter

Description: A novel planar resonant tunneling transistor is demonstrated. The growth structure is similar to that of a double-barrier resonant tunneling diode (RTD), except for a fully two-dimensional (2D) emitter formed by a quantum well. Current is fed laterally into the emitter, and the 2D--2D resonant tunneling current is controlled by a surface gate. This unique device structure achieves figures-of-merit, i.e. peak current densities and peak voltages, approaching that of state-of-the-art RTDs. Most importantly, sensitive control of the peak current and voltage is achieved by gating of the emitter quantum well subband energy. This quantum tunneling transistor shows exceptional promise for ultra-high speed and multifunctional operation at room temperature.
Date: July 13, 2000
Creator: MOON,J.S.; SIMMONS,JERRY A.; RENO,JOHN L.; BACA,WES E.; BLOUNT,MARK A.; HIETALA,VINCENT M. et al.
Partner: UNT Libraries Government Documents Department

Cathode limited charge transport and performance of thin-film rechargeable lithium batteries

Description: Several types of thin-film rechargeable batteries based on lithium metal anodes and amorphous V{sub 2}O{sub 5} (aV{sub 2}O{sub 5}), LiMn{sub 2}O{sub 4}, and LiCoO{sub 2} cathodes have been investigated in this laboratory. In all cases, the current density of these cells is limited by lithium ion transport in the cathodes. This paper, discusses sources of this impedance in Li-aV{sub 2}O{sub 5} and Li-LiMn{sub 2}O{sub 4} thin-film cells and their effect on cell performance.
Date: November 1, 1994
Creator: Bates, J.B.; Hart, F.X.; Lubben, D.; Kwak, B.S. & van Zomeren, A.
Partner: UNT Libraries Government Documents Department

State of the Art Power-in Tube Niobium-Tin Superconductors

Description: Powder-in-Tube (PIT) processed Niobium-Tin wires are commercially manufactured for nearly three decades and have demonstrated a combination of very high current density (presently up to 2500 A mm{sup -2} non-Cu at 12 T and 4.2 K) with fine (35 {micro}m), well separated filaments. We review the developments that have led to the present state of the art PIT Niobium-Tin wires, discuss the wire manufacturing and A15 formation processes, and describe typical superconducting performance in relation to magnetic field and strain. We further highlight successful applications of PIT wires and conclude with an outlook on possibilities for further improvements in the performance of PIT Niobium-Tin wires.
Date: June 1, 2008
Creator: Godeke, A.; Ouden, A. Den; Nijhuis, A. & ten Kate, H.H.J.
Partner: UNT Libraries Government Documents Department

Characterization of an RF-Driven Plasma Ion Source for Heavy Ion Fusion

Description: We are testing a high-current-density high-brightness Argon-Ion Source for Heavy Ion Fusion applications. The 100-kV 20-{micro}s source has produced up to 5 mA of Ar{sup +} in a single beamlet. The extraction current density is 100 mA/cm2. We have measured the emittance of the beamlet, and the fraction of Ar{sup ++} ions under several operating conditions. We present measurements of the extracted current density as a function of RF power and gas pressure ({approx} 2 mT), current density uniformity, and energy dispersion (due to charge exchange). We are testing a 80-kV 61-hole multi-beamlet array that will produce a total current > 200 mA. In the current experiments the beamlets are not merged into a single beam. A 500-kV experiment where the beamlets will be merged to a produce 0.5-A beam is being planned.
Date: May 13, 2003
Creator: Westenskow, G A; Hall, R P; Halaxa, E & Kwan, J W
Partner: UNT Libraries Government Documents Department

Supercurrents in HgBa{sub 2}CaCu{sub 2}O{sub 6+{delta}} and TlBa{sub 2}CaCu{sub 2}O{sub 7} Epitaxial Thin Films

Description: The availability of high-quality epitaxial thin films of HgBa{sub 2}CaCu{sub 2}O{sub 6+{delta}} (Hg-1212) and TlBa{sub 2}CaCu{sub 2}O{sub 7} (Tl-1212) with high critical current densities (J{sub c}) has made it possible to examine and compare the J{sub c} of these species. Results reveal that the J{sub c} of 1212 species have very similar temperature behavior at low fields, strongly suggesting that the 30-K shift in critical temperature (T{sub c}) induced by the exchange of Hg and Tl in the 1212 structure is due largely to a change in charge carrier density.
Date: August 23, 1999
Creator: Gapud, A.A.; Wu, J.Z.; Fang, L.; Yan, S.L.; Xie, Y.Y.; Siegal, M.P. et al.
Partner: UNT Libraries Government Documents Department

Forces in a thin cosine(n{theta}) helical wiggler

Description: We commence with the derivation of the Lorentz force density on a surface of discontinuity based on the expressions of fields and currents previously derived (Appendix A). Applying such Lorentz body forces to the equilibrium condition of an infinitesimal surface area yields a set of differential equations for the local total force. In attempting to solve such differential equations it may prove to be useful and prudent to reduce their complexity by first transforming all fields, current densities and Lorentz forces to a coordinate system that is aligned with the direction of the current flow. A Frenet--Serret rotating unit vector coordinate system may serve such a purpose and will reduce the 3 components of the Lorentz force to 2. We proceed with obtaining such a conversion through the use of differential geometry, although a more straight forward approach may exist through the use of surface developability and coordinate transformation. Following a solution to the force equations we continue with and example of a nested set of a combined function dipole and quadrupole that employ an identical periodicity {omega}. The expressions for the self force and the mutual force on each magnet element are obtained. Finally, by reducing the periodicity {omega} to zero we obtain the force expressions for long (2D) multipole magnets including both the self and interactive forces.
Date: March 1, 1995
Creator: Caspi, S.
Partner: UNT Libraries Government Documents Department

Relationships between processing temperature and microstructure in isothermal melt processed Bi-2212 thick films

Description: The microstructure and phase assemblage of isothermal melt processed (IMP) Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub y} (Bi-2212) thick films have been evaluated. Results from compositional analysis and phase identification indicate that the characteristics of the partial melt greatly influence the microstructural and chemical development of the thick films. The highest critical current densities were obtained in films processed below 800{degrees}C where the partial melt uniformly coats the substrate without excessive phase segregation.
Date: May 1, 1995
Creator: Holesinger, T.G.; Phillips, D.S.; Willis, J.O. & Peterson, D.E.
Partner: UNT Libraries Government Documents Department

Accurate estimation of the RMS emittance from single current amplifier data

Description: This paper presents the SCUBEEx rms emittance analysis, a self-consistent, unbiased elliptical exclusion method, which combines traditional data-reduction methods with statistical methods to obtain accurate estimates for the rms emittance. Rather than considering individual data, the method tracks the average current density outside a well-selected, variable boundary to separate the measured beam halo from the background. The average outside current density is assumed to be part of a uniform background and not part of the particle beam. Therefore the average outside current is subtracted from the data before evaluating the rms emittance within the boundary. As the boundary area is increased, the average outside current and the inside rms emittance form plateaus when all data containing part of the particle beam are inside the boundary. These plateaus mark the smallest acceptable exclusion boundary and provide unbiased estimates for the average background and the rms emittance. Small, trendless variations within the plateaus allow for determining the uncertainties of the estimates caused by variations of the measured background outside the smallest acceptable exclusion boundary. The robustness of the method is established with complementary variations of the exclusion boundary. This paper presents a detailed comparison between traditional data reduction methods and SCUBEEx by analyzing two complementary sets of emittance data obtained with a Lawrence Berkeley National Laboratory and an ISIS H{sup -} ion source.
Date: May 31, 2002
Creator: Stockli, Martin P.; Welton, R.F.; Keller, R.; Letchford, A.P.; Thomae, R.W. & Thomason, J.W.G.
Partner: UNT Libraries Government Documents Department

Development of Biaxially Textured YBa{sub 2}Cu{sub 3}O{sub 7} Coated Conductors in the U.S.

Description: Two new processes have been under development since 1991 that promise a new, cost-effective way to manufacture flexible, high current density wires made from YBa{sub 2}Cu{sub 3}O{sub 7} (YBCO). The key is to prepare a textured substrate, or ''template,'' on which the YBCO may be deposited as a biaxially aligned thick film. Ion beam assisted deposition (IBAD) of yttria stabilized zirconia or magnesium oxide on alloy tapes enables a final superconducting layer with grain-to-grain, in-plane alignment to within 3-5 degrees. Similar results are achieved on rolling-assisted, biaxially textured substrates (RABiTS) using a variety of oxide layers on textured nickel tapes. The performance of research lengths of prototype wires in strong magnetic fields at 65 K already exceeds that of NbTi and Nb{sub 3}Sn in liquid helium. A scalable, ex-situ process for the YBCO coating has been demonstrated on both types of substrates. Consistent values of critical current density (J{sub c }) greater than 1 x 10{sup 6} A/cm{sup 2} are now obtained on RABiTS, and J{sub c}'s in excess of 2 x 10{sup 6} A/cm{sup 2} have been obtained on both substrates. A nonmagnetic variation of RABiTS (Ni-13% Cr) has also been shown to yield Jc greater than 1.5 x 10{sup 6} A/cm{sup 2} . Six private companies in the U.S. are scaling up YBCO coated conductors for power and physics applications.
Date: November 13, 1999
Creator: Christen, D.K.; Hawsey, R.A. & Kroeger, D.M.
Partner: UNT Libraries Government Documents Department

Hybrid modeling of the formation and structure of thin current sheets in the magnetotail

Description: Hybrid simulations are used to investigate the formation of a thin current sheet inside the plasma sheet of a magnetotail-like configuration. The initial equilibrium is subjected to a driving electric field qualitatively similar to what would be expected from solar wind driving. As a result, we find the formation of a raw current sheet, with a thickness of approximately the ion inertial length. The current density inside the current sheet region is supplied largely by the electrons. Ion acceleration in the cross-tail direction is absent due since the driving electric field fails to penetrate into the equatorial region.
Date: July 1, 1996
Creator: Hesse, M.; Winske, D. & Birn, J.
Partner: UNT Libraries Government Documents Department

Processing and properties of high J{sub c} grain boundaries in melt textured YBa{sub 2}Cu{sub 3}O{sub x}

Description: Conventional polycrystalline HTS possess extremely low transport critical current density, which has been attributed to grain boundary related deficiencies. We have developed a processing method to eliminate secondary phases and to reduce the amount of microcracks at the grain boundaries in melt-textured YBCO superconductors. In this process, a fraction of the liquid phase is removed during melt-texturing which lead to cleaner grain boundaries. These boundaries have been found to carry high currents (3 {times} 10{sup 4}A/cm{sup 2} at self field and 1.5 {times} 10{sup 2}A/cm{sup 2} at 1.5T) at 77 K in spite of relatively high misorientation angles. Also, the weak dependence of J{sub c} on magnetic field indicates that these grain boundaries are strongly coupled. In addition, the linear I-V characteristics of these samples suggest that flux flow along these boundaries may be the J{sub c} limiting mechanism. These results give promise to manufacturing of polycrstalline YBCO superconductor with high current carrying capability.
Date: November 1, 1994
Creator: Salama, K.; Parikh, A.S.; Kunchur, M.N. & Christen, D.K.
Partner: UNT Libraries Government Documents Department

Discharges with high bootstrap current fraction on Tore Supra

Description: Bootstrap current is regarded as a serious candidate for non-inductively driving a significant fraction of the total current. High bootstrap fraction discharges have already been achieved and analysed in several tokamaks, including JT-60, DIII-D and TFTR. Tore Supra (R=2. 36 m, a=0.80 m) is particularly suited for the study of non-inductive discharges and long pulse operation. It is equipped with several of non-inductive current drive/heating systems including Lower Hybrid Current Drive (LHCD), Fast Wave Electron Heating (FWEH), and in the future Electron Cyclotron Heating. Fully non-inductive discharges with enhanced confinement (LHEP mode) have already been obtained in Tore Supra with LHCD. High {Beta}p ({le}1.6) regimes current nave also been achieved in the presence of FWEH. In particular, a discharge with 70% of the total current generated by the bootstrap current was observed. In this context, non-inductive current density profile determination is essential for understanding current drive experiments and ultimately for implementing current profile control. This paper briefly describes two methods developed on Tore Supra to determine the non-inductive current density profiles. The agreement between the two methods has been tested by applying them to ohmic discharges. These methods are then applied to the high bootstrap fraction discharges heated by FWEH. The non-inductive current density profile of these discharges are carried out. and the results are finally compared to several models of bootstrap current including Hirsman`s with low collisionality, matrix formulation and both Kessel and Houlberg matrix formulation.
Date: December 31, 1995
Creator: Joffrin, E.; Saoutic, B.; Basiuk, V.; Forest, C. & Houlberg, W.A.
Partner: UNT Libraries Government Documents Department

Effects of temperature ramp rate during heat treatment on hysteresis loss and critical current density of internal tin processed wires

Description: It has been shown that temperature ramp rates utilized in heat treatment schedules for internal tin processed Nb{sub 3}Sn wires substantially influence both hysteresis loss and critical current density J{sub c} of the wires, i.e. a slow ramp rate (e.g. 6{degree}C/h) favors a higher J{sub c} while a fast ramp (e.g. 60{degree}C/h)results in a low hysteresis loss of the wire.
Date: April 1, 1995
Creator: Suenaga, M. & Sabatini, R.L.
Partner: UNT Libraries Government Documents Department

The Algebraic Reconstruction Technique (ART)

Description: Projections of charged particle beam current density (profiles) are frequently used as a measure of beam position and size. In conventional practice only two projections, usually horizontal and vertical, are measured. This puts a severe limit on the detail of information that can be achieved. A third projection provides a significant improvement. The Algebraic Reconstruction Technique (ART) uses three or more projections to reconstruct 3-dimensional density profiles. At the 200 MeV H-linac, we have used this technique to measure beam density, and it has proved very helpful, especially in helping determine if there is any coupling present in x-y phase space. We will present examples of measurements of current densities using this technique.
Date: July 1, 1997
Creator: Raparia, D.; Alessi, J. & Kponou, A.
Partner: UNT Libraries Government Documents Department

On the theory of critical currents and flux flow in superconductors by the mechanism of plastic deformation of the flux-line lattice

Description: In this paper the author will discuss how the nature of the stress state in the flux-line lattice (FLL) of superconductors arises from the distribution, density, geometry, and strength of pinning centers. Under certain conditions this stress causes the onset of plastic deformation in the FLL for values of the current density below that required for flux-flow by general depinning. He will describe an analytic framework, based on a theory of plasticity of the FLL, which describes the flux-flow characteristics, including the possibility of thermally-activated flow and flux creep.
Date: July 1, 1999
Creator: Welch, D.O.
Partner: UNT Libraries Government Documents Department

Quantum Well Width Dependence of Threshold Current Density in InGaN Lasers

Description: The quantum confined Stark effect was found to result in a strong quantum well width dependence of threshold current density in strained group-III nitride quantum well lasers. For an In{sub 0.2}Ga{sub 0.8}N/GaN structure with quantum well width in the neighborhood of 3.5nm, our analysis shows that the reduction in spontaneous emission loss by the electron-hole spatial separation outweighs the corresponding reduction in gain to produce a threshold current density minimum.
Date: March 16, 1999
Creator: Amano, H.; Chow, W.W.; Han, J. & Takeuchi, T.
Partner: UNT Libraries Government Documents Department

The effect of chromate concentration on the repassivation of corroding aluminum

Description: Current density maps of anodically polarized pure aluminum in chloride solutions were measured and the effect of chromate/dichromate buffer additions monitored. The higher the polarized potential the more chromate was required to repassivate the corroding surface. Small pits repassivated easily, crevice corrosion events were the last to repassivate. Open circuit potential measurements showed the presence of meta-stable pitting at chloride concentrations of 0.3M. The lifetime and magnitude of these metastable pits was reduced on the addition of 0.05M chromate buffer.
Date: February 9, 1999
Creator: Jeffcoate, C.S.; Isaacs, H.S.; Hawkins, J. & Thompson, G.E.
Partner: UNT Libraries Government Documents Department

Efficient Injection of Electron Beams into Magnetic Guide Fields

Description: Preliminary experimental and modeling study of injection and transport of high current electron beams in current-neutralized background gas has been performed. Initial analysis of the results indicates that high current triaxial ring diode operates very reproducibly in the pinch mode. High current density beam can be injected efficiently into the drift region, using azimuthal guide field with reduced intensity near the injection region. This was shown to improve the effectiveness of capturing the beam for the transport. The transport length was insufficient to measure losses, such as would arise from scattering with the background gas.
Date: June 8, 1999
Creator: Chorny, V.; Cooperstein, G.; Dubyna, V.; Frolov, O.; Harper-Slaboszewicz, V.; Hinshelwood, D. et al.
Partner: UNT Libraries Government Documents Department

Composite electrodes for advanced electrochemical applications. Quarterly report for the period July 1 - September 30, 1999

Description: The electrochemical industry is one of the most highly energy-intensive industries today. However, there have been no significant advances in the electrodes that these industries use. The dimensionally stable anode (DSA), which ELTECH introduced under a license agreement, has been the industry standard for the past twenty-five years. But, DSAs are nearing the end of their technological prevalence. The principal problems with DSAs include high capital and operating cost, and the proprietary nature of the technology. In addition, DSAs experience problems that include contamination of the process solution by anode materials, failure when the electrocatalytic coating peels from under attack, generally low anode performance due to inherent limitations in operating current density, and short anode lifetime because of corrosion. The proposed innovation combines the low electrical resistance of copper with the corrosion resistance of electrically conductive diamond to achieve energy-efficient, long-lifetime electrodes for electrochemistry. The proposed work will ultimately develop a composite electrode that consists of a copper substrate, a conductive diamond coating, and a catalytic precious metal coating. The scope of the current work includes preparation, testing, and evaluation of diamond-coated titanium electrodes.
Date: October 1, 1999
Creator: Kovach, Chris
Partner: UNT Libraries Government Documents Department

Diffraction Grating Structures in Solar Cells

Description: Sub-wavelength periodic texturing (gratings) of crystalline-silicon (c-Si) surfaces for solar cell applications can be designed for maximizing optical absorption in thin c-Si films. We have investigated c-Si grating structures using rigorous modeling, hemispherical reflectance, and internal quantum efficiency measurements. Model calculations predict almost {approximately}100% energy coupling into obliquely propagating diffraction orders. By fabrication and optical characterization of a wide range of 1D & 2D c-Si grating structures, we have achieved broad-band, low ({approximately} 5%) reflectance without an anti-reflection film. By integrating grating structures into conventional solar cell designs, we have demonstrated short-circuit current density enhancements of 3.4 and 4.1 mA/cm{sup 2} for rectangular and triangular 1D grating structures compared to planar controls. The effective path length enhancements due to these gratings were 2.2 and 1.7, respectively. Optimized 2D gratings are expected to have even better performance.
Date: December 1, 2000
Creator: ZAIDI,SALEEM H.; GEE,JAMES M. & RUBY,DOUGLAS S.
Partner: UNT Libraries Government Documents Department

INTENSE PROTON CORE AND HALO BEAM PROFILE MEASUREMENT: BEAM LINE COMPONENT MECHANICAL DESIGN

Description: The 6.7-MeV, 100-mA proton beam being produced in the Low Energy Demonstration Accelerator (LEDA) RFQ will be injected into a 52-magnet lattice in order to study the formation of beam halo [1]. The LEDA RFQ beam has a rms size of 1 mm. At nine longitudinal locations along the lattice an assembly that incorporates both a wire scanner and a halo-scraper assembly will be placed to make current density measurements of the beam.
Date: October 1, 2000
Creator: VALDIVIEZ, R.; PATTERSON, N. & AL, ET
Partner: UNT Libraries Government Documents Department