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  • 1
    Electronic Resource
    Electronic Resource
    Exact solution for the singlet density distributions and second-order correlations of normal-mode coordinates for hard rods in one dimension (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 11390-11398 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We examine the distribution of normal-mode coordinates (defined via the eigenvectors of a chain of harmonic oscillators) for a system of purely repulsive hard rods in one dimension. We obtain an exact solution for the singlet density distribution, and separately for the covariances of the normal-mode coordinates. The hard-rod behavior is examined in terms of its deviation from the corresponding distributions for the system of harmonic oscillators. All off-diagonal covariances are zero in the hard-rod system, and the (on-diagonal) variances vary with the normal-mode wave number exactly as in the harmonic system. The detailed singlet normal-mode density distributions are very smooth but nonanalytic, and they differ from the (Gaussian) distributions of the corresponding harmonic system. However, all of the normal-mode coordinate distributions differ in roughly the same way when properly scaled by the distribution variance, and the differences vanish as 1/N in the thermodynamic limit of an infinite number of particles N. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479080
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  • 2
    Electronic Resource
    Electronic Resource
    Zinc-implantation-disordered (InGa)As/GaAs strained-layer superlattice diodes (1986)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 60 (1986), S. 1131-1134 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have examined the properties of (InGa)As/GaAs strained-layer superlattices (SLSs) that have been disordered by implantation of 5×1015/cm2, 250 keV 64Zn+ followed by controlled atmosphere annealing at 680 °C for 30 min. Ion channeling techniques indicate that the Zn-disordered regions of the SLS contain extensive crystalline damage after annealing. Simulations of the disordering process using an analytic ion range code predict that the electrical junction resulting from the implantation process is located outside the disordered region of the SLS in both the vertical and the lateral directions. Junction electroluminescence intensity for given drive current densities from the Zn-disordered SLS devices is comparable to that from reference Be-implantation-doped (SLS retained) devices and greatly exceeds that from heavily dislocated grown-junction mesa diodes in the homogeneous alloy of the average SLS composition; this result is consistent with the results of the simulations. This study demonstrates that implantation disordering can be as useful for strained-layer systems as for less severely mismatched heterojunction systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.337356
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  • 3
    Electronic Resource
    Electronic Resource
    Radiographic image analysis of cylindrical implosion experiments (invited) : Papers from the thirteenth topical conference on high temperature plasma diagnostics (2001)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 72 (2001), S. 643-650 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Radiography is a heavily used tool for diagnosing laser-based hydrodynamic experiments. A successful experiment relies on the gathering of data over a time window where the relevant physics occurs and on an accurate analysis of those data. Comparison of this experimental data to theory is often best done by generating simulated images from hydrodynamic calculations, including all necessary and important experimental details. Care must be taken to treat both the experimental and theoretical images identically in the analysis. Frequently, image filtering and enhancement routines are used to obtain interface location and perturbation information from the radiographic image. Previous techniques were found to be too sensitive to global image details. New procedures have been developed which utilize local operators that provide better edge or interface identification without bias. These procedures are benchmarked and validated using static radiographic targets of known configuration that mock up experimental situations of interest. The experiment and the image analysis development are described, including discussion of key contributions to the uncertainty of the results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1315643
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  • 4
    Electronic Resource
    Electronic Resource
    Optimization and control of a small angle ion source using an adaptive neural network controller (invited) : Proceedings of 5th international conference on ion sources (1994)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 65 (1994), S. 1411-1415 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: This project developed an automated controller based on an artificial neural network and evaluated its applicability in a real-time environment. This capability was developed within the context of a small angle negative ion source on the Discharge Test Stand at Los Alamos. The controller processes information obtained from the beam current wave form, developing a figure of merit (fom) to determine the ion source operating conditions. The fom is composed of the magnitude of the beam current, the stability of operation, and the quietness of the beam. Using no knowledge of operating conditions, the controller begins by making of rough scan of the four-dimensional operating surface. This surface uses as independent variables the anode and cathode temperatures, the hydrogen flow rate, and the arc voltage. The dependent variable is the fom described above. Once the rough approximation of the surface has been determined, the network formulates a model from which it determines the best operating point. The controller takes the ion source to that operating point for a reality check. As real data is fed in, the model of the operating surface is updated until the neural network's model agrees with reality. The controller then uses a gradient ascent method to optimize the operation of the ion source. Initial tests of the controller indicate that it is remarkably capable. It has optimized the operation of the ion source on six different occasions bringing the beam to excellent quality and stability.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1144978
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  • 5
    Electronic Resource
    Electronic Resource
    Magnetization reversal and magnetoresistance in a lateral spin-injection device (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 85 (1999), S. 6682-6685 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have investigated the magnetization reversal and magnetoresistance (MR) behavior of a lateral spin-injection device. The device consists of a two-dimensional electron gas (2DEG) system in an InAs quantum well and two ferromagnetic (Ni80Fe20) contacts: an injector (source) and a detector (drain). Spin-polarized electrons are injected from the first contact and propagating through InAs are collected by the second contact. By engineering the shape of the permalloy film distinct switching fields (Hc) from the injector and the collector have been observed by scanning Kerr microscopy and MR measurements. Magneto-optic Kerr effect (MOKE) hysteresis loops demonstrate that there is a range of magnetic field (20–60 Oe), at room temperature, over which magnetization in one contact is aligned antiparallel to that in the other. The MOKE results are consistent with the variation of the magnetoresistance in the spin-injection device. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.370504
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  • 6
    Electronic Resource
    Electronic Resource
    Characterization of ion-implantation doping of strained-layer superlattices. II. Optical and electrical properties (1986)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 60 (1986), S. 3641-3650 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have investigated the properties of Ga(AsP)/GaP strained-layer superlattices (SLSs) that have been doped by implantation of 1×1015/cm2, 75 keV Be+ followed by controlled-atmosphere annealing at 825 °C for 10 min. Our results indicate that doping of these strained-layer superlattices without disordering is a viable process. Liquid-helium temperature photoluminescence suggests a binding energy for the implanted acceptors of 50 meV, consistent with that of beryllium in GaP-based alloys. The implantation-doped regions exhibit room-temperature electrical activation of 15% and hole mobilities of 20 cm2/V s, consistent with the values expected for type-converted GaP-based alloys. SLS diodes fabricated by this process exhibit excellent rectification properties, with a forward turn-on voltage of approximately 1.8 V and low values of room-temperature reverse leakage current densities. Diodes formed from SLSs with original n-type doping of 1×1017/cm3 have typical reverse leakage current densities of 1×10−7 A/cm2 at −10 V, despite the depletion region penetrating approximately ten interfaces of the SLS at this bias. Deep-level transient spectroscopy demonstrates the existence of defect centers, whose densities and signatures are similar to those found in ion-implanted GaP. The implanted photodiodes exhibit a wavelength-dependent photoresponse characteristic of grown-junction SLS photodetectors in the same chemical system. Examination of the spatial response of the photodiodes to a tightly focussed (FWHM=2.45 μm) laser beam at a wavelength of 488 nm indicates that the photoresponse from the device is uniform to within 10% for regions away from the edges of the implanted regions. Modelling of the wavelength-dependent and the spatially dependent photoresponse allows an estimate of minority-carrier diffusion lengths for electrons and holes of 1.0 μm parallel to the SLS layers and 0.1 μm perpendicular to the SLS layers. The excellent electrical and optical properties of the implanted and annealed SLS materials implies additional device applications for these novel materials.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.337796
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  • 7
    Electronic Resource
    Electronic Resource
    The processes of formation and epitaxial alignment of SrTiO3 thin films prepared by metallo-organic decomposition (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 961-970 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The processes of formation and crystallization of thin films of SrTiO3 prepared by the method of metallo-organic decomposition have been studied with particular emphasis on the relationship between the thermal decomposition of the metallo-organic precursors and the eventual epitaxial alignment of the crystallized films. The films are deposited by spin coating onto single-crystalline silicon and SrTiO3 substrates, pyrolyzed on a hot plate at temperatures ranging from 200 to 450 °C, and subsequently heat treated in a quartz tube furnace at temperatures ranging from 300 to 1200 °C. Heat treatment at temperatures up to 450–500 °C results in the evaporation of solvents and other organic addenda, thermal decomposition of the metallo-organic (primarily metal-carboxylates) precursors, and formation of a carbonate species. This carbonate appears to be an intermediate phase in the reaction of SrCO3 and TiO2 to form SrTiO3. Relevant to this work is the fact that the carbonate species exhibits diffraction lines, indicating the formation of grains that can serve as seeds for the nucleation and growth of randomly oriented SrTiO3 crystallites, thereby leading to a polycrystalline film. Deposition on silicon substrates indeed results in the formation of polycrystalline SrTiO3. However, when the precursor solution is deposited on single-crystalline SrTiO3 substrates, the crystallization process involves a competition between two mechanisms: the random nucleation and growth of crystallites just described, and layer-by-layer solid phase epitaxy. Epitaxial alignment on SrTiO3 substrates can be achieved when the samples are heat treated at temperatures of 1100–1200 °C or at temperatures as low as 600–650 °C when the substrate is heated to about 1100 °C before spin coating.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.353310
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  • 8
    Electronic Resource
    Electronic Resource
    Preparations for deuterium–tritium experiments on the Tokamak Fusion Test Reactor* (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 1560-1567 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The final hardware modifications for tritium operation have been completed for the Tokamak Fusion Test Reactor (TFTR) [Fusion Technol. 21, 1324 (1992)]. These activities include preparation of the tritium gas handling system, installation of additional neutron shielding, conversion of the toroidal field coil cooling system from water to a FluorinertTM system, modification of the vacuum system to handle tritium, preparation, and testing of the neutral beam system for tritium operation and a final deuterium–deuterium (D–D) run to simulate expected deuterium–tritium (D–T) operation. Testing of the tritium system with low concentration tritium has successfully begun. Simulation of trace and high power D–T experiments using D–D have been performed. The physics objectives of D–T operation are production of ≈10 MW of fusion power, evaluation of confinement, and heating in deuterium–tritium plasmas, evaluation of α-particle heating of electrons, and collective effects driven by alpha particles and testing of diagnostics for confined α particles. Experimental results and theoretical modeling in support of the D–T experiments are reviewed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870707
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  • 9
    Electronic Resource
    Electronic Resource
    Correlations of heat and momentum transport in the TFTR tokamak (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 1300-1305 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Measurements of the toroidal rotation speed vφ(r) driven by neutral beam injection in tokamak plasmas and, in particular, simultaneous profile measurements of vφ, Ti, Te, and ne, have provided new insights into the nature of anomalous transport in tokamaks. Low-recycling plasmas heated with unidirectional neutral beam injection exhibit a strong correlation among the local diffusivities, χφ≈χi〉χe. Recent measurements have confirmed similar behavior in broad-density L-mode plasmas. These results are consistent with the conjecture that electrostatic turbulence is the dominant transport mechanism in the tokamak fusion test reactor tokamak (TFTR) [Phys. Rev. Lett. 58, 1004 (1987)], and are inconsistent with predictions both from test-particle models of strong magnetic turbulence and from ripple transport. Toroidal rotation speed measurements in peaked-density TFTR "supershots'' with partially unbalanced beam injection indicate that momentum transport decreases as the density profile becomes more peaked. In high-temperature, peaked-density plasmas the observed gradient scale length parameter ηtoti=d ln Ti/d ln ne correlates reasonably well with predictions of the threshold for exciting ion-temperature-gradient-driven turbulence (ITGDT), as would be expected for plasmas at marginal stability with respect to this strong transport mechanism. In L-mode plasmas where ITGDT is expected to be too weak to enforce marginal stability, ηtoti exceeds this threshold considerably. However, preliminary experiments have failed to observe a significant increase in ion heat transport when ηtoti was rapidly forced above ηc (the threshold for exciting ITGDT) using a perturbative particle source, as would have been expected for a plasma at marginal stability.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859545
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  • 10
    Electronic Resource
    Electronic Resource
    High-beta operation and magnetohydrodynamic activity on the TFTR tokamak (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 1287-1290 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Magnetohydrodynamic (MHD) activity within three zones (core, half-radius, and edge) of TFTR [Plasma Physics and Controlled Nuclear Fusion Research 1986 (IAEA, Vienna, 1987), Vol. 1, p. 51] tokamak plasmas are discussed. Near the core of the plasma column, sawteeth are often observed. Two types of sawteeth are studied in detail; one with complete, and the other with incomplete, magnetic reconnection. Their characteristics are determined by the shape of the q profile. Near the half-radius the m/n=3/2 and 2/1 resistive ballooning modes are found to correlate with a beta collapse. The pressure and the pressure gradient at the mode rational surface are found to play an important role in stability. MHD activity is also studied at the plasma edge during limiter H modes. The edge localized modes (ELM's) are found to have a precursor mode with a frequency between 50–200 kHz and a mode number m/n=1/0. The mode does not show a ballooning structure. While these instabilities have been studied on many other machines, on TFTR the studies have been extended to high pressure (plasma pressure greater than 4×105 Pa) and low collisionality [vi@B|(a/2)〈0.002, ve*(a/2)〈0.01].
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859544
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