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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

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

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
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

Emission, plasma formation, and brightness of a PZT ferroelectric cathode

Description: We have measured an 36-A-cm{sup {minus}2} current emission density over the surface area of an 11.4-cm{sup 2}-area Lead-Titanate- Zirconate (PZT) ferroelectric cathode with a pulsed anode-cathode (A-K) potential of 50 kV. We have also observed currents above those predicted by classical Child-Langmuir formula for a wide variety of cases. Since a plasma within the A-K gap could also lead to increase current emission we are attempting to measure the properties of the plasma near the cathode surface at emission time. In other measurements, we have observed strong gap currents in the absence of an A-K potential. Further, we continue to make brightness measurements of the emitted beam and observe spatially non-uniform emission and large shot-to-shot variation. Measurements show individual beamlets with a brightness as high 10{sup 11} Am{sup {minus}2} rad{sup {minus}2}.
Date: April 27, 1995
Creator: Sampayan, S.; Caporaso, G.; Trimble, D. & Westenskow, G.
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

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

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

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

Initial experiments of RF gas plasma source for heavy ionfusion

Description: The Source Injector Program for the US Heavy Ion Fusion Virtual National Laboratory is currently exploring the feasibility of using RF gas plasma sources for a HIF driver. This source technology is presently the leading candidate for the multiple aperture concept, in which bright millimeter size beamlets are extracted and accelerated electrostatically up to 1 MeV before the beamlets are allowed to merge and form 1 A beams. Initial experiments have successfully demonstrated simultaneously high current density, {approx} 100 mA/cm{sup 2} and fast turn on, {approx} 1 {micro}s. These experiments were also used to explore operating ranges for pressure and RF power. Results from these experiments are presented as well as progress and plans for the next set of experiments for these sources.
Date: May 22, 2002
Creator: Ahle, L.; Hall, R.; Molvik, A.W.; Chacon-Golcher, E.; Kwan, J.W.; Leung, K.N. 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

Improvement in brightness of multicusp-plasma ion source

Description: The beam brightness of a multicusp-plasma ion source has been substantially improved by optimizing the source configuration and extractor geometry. The current density of a 2 keV He{sup +} beam extracted from a 7.5-cm-diameter source operating at 2.5 kW RF power is {approx}100 mA/cm{sup 2}, which is {approx}10 times larger than that of a beam extracted from a 5-cm-diameter source operating at 1 kW RF power. A smaller focused beam spot size is achieved with a counter-bored extractor instead of a conventional (''through-hole'') extractor, resulting another order of magnitude improvement in beam current density. Measured brightness can be as high as 440 A/cm{sup 2}Sr, which represents a 30 times improvement over prior work.
Date: May 24, 2002
Creator: Ji, Q.; Jiang, X.; King, T-J.; Leung, K-N.; Standiford, K. & Wilde, S.B.
Partner: UNT Libraries Government Documents Department

New Particle-in-Cell Code for Numerical Simulation of Coherent Synchrotron Radiation

Description: We present a first look at the new code for self-consistent, 2D simulations of beam dynamics affected by the coherent synchrotron radiation. The code is of the particle-in-cell variety: the beam bunch is sampled by point-charge particles, which are deposited on the grid; the corresponding forces on the grid are then computed using retarded potentials according to causality, and interpolated so as to advance the particles in time. The retarded potentials are evaluated by integrating over the 2D path history of the bunch, with the charge and current density at the retarded time obtained from interpolation of the particle distributions recorded at discrete timesteps. The code is benchmarked against analytical results obtained for a rigid-line bunch. We also outline the features and applications which are currently being developed.
Date: May 1, 2010
Creator: Balsa Terzic, Rui Li
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

A new generation Nb3Sn wire, and the prospects for its use inparticle accelerators

Description: The US DOE has initiated a Conductor Development Program aimed at demonstrating a high current density, cost effective Nb3Sn conductor for use in accelerator magnets. The first goal, an increase in current density by 50%, has been achieved in a practical conductor. The program is focused at present on achieving the second goal of reduced losses. The different approaches for achieving these goals will be discussed, and the status will be presented. Magnet technology R&D has been proceeding in parallel with the conductor development efforts, and these two technologies are reaching the level required for the next step--introduction into operating accelerator magnets. An obvious point for introducing this technology is the LHC interaction region magnets, which require large apertures and high fields (or high field gradients). By upgrading the interaction region magnets, machine performance can be enhanced significantly without replacing the arc magnets, which represent most of the cost of an accelerator. Design requirements generated by recent studies and workshops will be reviewed, and a roadmap for the development of the next-generation interaction region magnets will be presented.
Date: September 30, 2003
Creator: Scanlan, R.M.; Dietderich, D.R. & Gourlay, S.A.
Partner: UNT Libraries Government Documents Department


Description: Pushing accelerator magnets beyond 10 T holds a promise of future upgrades to machines like the Tevatron at Fermilab and the LHC at CERN. Exceeding the current density limits of NbTi superconductor, Nb{sub 3}Sn is at present the only practical superconductor capable of generating fields beyond 10 T. Several Nb{sub 3}Sn pilot magnets, with fields as high as 16 T, have been built and tested, paving the way for future attempts at fields approaching 20 T. High current density conductor is required to generate high fields with reduced conductor volume. However this significantly increases the Lorentz force and stress. Future designs of coils and structures will require managing stresses of several 100's of MPa and forces of 10's of MN/m. The combined engineering requirements on size and cost of accelerator magnets will involve magnet technology that diverges from the one currently used with NbTi conductor. In this paper we shall address how far the engineering of high field magnets can be pushed, and what are the issues and limitations before such magnets can be used in particle accelerators.
Date: May 1, 2005
Creator: Caspi, Shlomo & Ferracin, Paolo
Partner: UNT Libraries Government Documents Department

Fabrication of a Short-Period Nb3Sn Superconducting Undulator

Description: Lawrence Berkeley National Laboratory develops high-field Nb{sub 3}Sn magnets for HEP applications. In the past few years, this experience has been extended to the design and fabrication of undulator magnets. Some undulator applications require devices that can operate in the presence of a heat load from a beam. The use of Nb{sub 3}Sn permits operation of a device at both a marginally higher temperature (5-8K) and a higher J{sub c}, compared to NbTi devices, without requiring a larger magnetic gap. A half-undulator device consisting of 6 periods (12 coil packs) of 14.5 mm period was designed, wound, reacted, potted and tested. It reached the short sample current limit of 717A in 4 quenches. The non-Cu Jc of the strand was over 7,600 A/mm{sup 2} and the Cu current density at quench was over 8,000 A/mm{sup 2}. Magnetic field models show that if a complete device was fabricated with the same parameters one could obtain beam fields of 1.1 T and 1.6 T for pole gaps of 8 mm and 6 mm, respectively.
Date: June 1, 2007
Creator: Dietderich, Daniel; Dietderich, Daniel; Godeke, Arno; Prestemon, Soren; Pipersky, Paul T.; Liggins, Nate L. et al.
Partner: UNT Libraries Government Documents Department

How an antenna launches its input power into radiation: thepattern of the Poynting vector at and near an antenna

Description: In this paper I first address the question of whether theseat of the power radiated by an antenna made of conducting members isdistributed over the "arms" of the antenna according to $ - \bf J \cdotE$, where $\bf J$ is the specified current density and $\bf E$ is theelectric field produced by that source. Poynting's theorem permits only aglobal identification of the total input power, usually from a localizedgenerator, with the total power radiated to infinity, not a localcorrespondence of $- \bf J \cdot E\ d^3x $ with some specific radiatedpower, $r^2 \bf S \cdot \hat r\ d\Omega $. I then describe a modelantenna consisting of two perfectly conducting hemispheres of radius\emph a separated by a small equatorial gap across which occurs thedriving oscillatory electric field. The fields and surface current aredetermined by solution of the boundary value problem. In contrast to thefirst approach (not a boundary value problem), the tangential electricfield vanishes on the metallic surface. There is no radial Poyntingvector at the surface. Numerical examples are shown to illustrate how theenergy flows from the input region of the gap and is guided near theantenna by its "arms" until it is launched at larger \emph r/a into theradiation pattern determined by the value of \emph ka.
Date: May 18, 2005
Creator: Jackson, J.D.
Partner: UNT Libraries Government Documents Department

Multi-staged, InAsSb mid-infrared lasers and light-emitting diodes, grown by MOCVD

Description: Due to lower nonradiative rates, mid-infrared (2-6 micron) lasers with strained, narrow bandgap, Sb-based active regions have the potential to operate at lower current density and higher temperature than competing devices. Superior performance may be achieved through the {open_quotes}band structure engineered{close_quotes} reduction of Auger recombination and the implementation of multi-stage (or {open_quotes}cascaded{close_quotes}) active regions. We describe the first lasers and LEDs utilizing strained InAsSb, multi-stage active regions. An (n)InAs / (p)GaAsSb semimetal layer is incorporated into each stage as an internal electron-hole source. To date, 2-stage LEDs and 2-stage lasers have been demonstrated. Our multi-stage devices were grown by MOCVD.
Date: September 1, 1997
Creator: Kurtz, S.R.; Allerman, A.A.; Biefeld, R.M. & Baucom, K.C.
Partner: UNT Libraries Government Documents Department


Description: OAK-B135 Advanced Tokamak (AT) research in DIII-D focuses on developing a scientific basis for steady-state, high performance operation. For optimal performance, these experiments routinely operate with {beta} above the n = 1 no-wall limit, enabled by active feed-back control. The ideal wall {beta} limit is optimized by modifying the plasma shape, current and pressure profile. Present DIII-D AT experiments operate with f{sub BS} {approx} 50%-60%, with a long-term goal of {approx} 90%. Additional current is provided by neutral beam and electron cyclotron current drive, the latter being localized well away from the magnetic axis ({rho} {approx} 0.4-0.5). Guided by integrated modeling, recent experiments have produced discharges with {beta} {approx} 3%, {beta}{sub N} {approx} 3, f{sub BS} {approx} 55% and noninductive fraction f{sub NI} {approx} 90%. Additional control is anticipated using fast wave current drive to control the central current density.
Date: August 1, 2003
Partner: UNT Libraries Government Documents Department