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  • 1
    Electronic Resource
    Electronic Resource
    Proton spin-lattice relaxation dispersion of liquid crystal polymers: Characterization of local and collective motions (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 3794-3802 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper reports on studies of the longitudinal proton spin relaxation dispersion T1Z(ω) of a nematic main chain liquid crystal polymer (M¯n=30 000) over a very broad Larmor frequency range (1 kHz≤ω/2π≤120 MHz). Analysis of the experiments is achieved in terms of a density operator treatment employing the Redfield approximation. The results show that collective motions contribute to the proton spin relaxation process in the kilohertz regime, as found for low molar mass liquid crystals, whereas the conventional megahertz range is dominated by reorientation of individual molecules. The intramolecular motions consist of trans–gauche isomerization and phenyl ring flips. These motions are the fastest in the hierarchy of time with correlation times of 10−10 s in the nematic melt of the polymer at T≈460 K. The intermolecular (whole molecule) motions are interpreted as rotational diffusion in an orienting potential. They exhibit a T1Z(ω)∼ω0.65 dispersion in the megahertz range and have correlation times ranging from 10−9 to 10−7 s at this temperature. The slowest motions affecting longitudinal spin relaxation can be assigned to nematic order director fluctuations characterized by a broad distribution of thermally activated modes. Analysis of the dispersion profiles in the kilohertz regime provides the viscoelastic parameters of the main chain liquid crystal polymer. At T=460 K, an average elastic constant of K=8×10−11 N and an effective viscosity of η=1×103 Pa s have been determined. Using the experimentally accessible value for the short wavelength cutoff of the elastic modes, one obtains the mean-square amplitude of the director fluctuations 〈θ02(approximately-greater-than)=0.02, corresponding to a director order parameter of SOF=0.98. Thus, the contributions of the collective chain motions to the measured order parameters are negligible.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462960
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  • 2
    Electronic Resource
    Electronic Resource
    Energy deposition and hydrodynamic response of PBFA-II targets (abstract) : Eighth topical conference on high−temperature plasma diagnostics (1990)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 61 (1990), S. 3149-3149 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We are developing at PBFA II a laser-reflection-holographic-interferometer technique to measure the time- and space-resolved hydrodynamic response of planar targets to intense light-ion-beam energy deposition. The interpretation of these experiments will lead to an improved understanding of ion beam divergence. A linear analysis of thermoelastic response indicates that ion beam spatial and temporal profiles (including filamentation), as well as properties of the high-temperature ablation plasma, can be obtained from such experiments. For PBFA II we have used the hydrodynamic code TITAN to design the experiments. Single crystals of Al2O3 will be employed initially as targets, since their high Hugoniot elastic limit of ∼200 kbar will permit an elastic wave analysis. Single crystals of LiF will be used at energy deposition levels corresponding to megabar pressures. We will discuss the theory, previous electron beam, and planar plate impact experiments which have employed the diagnostic technique, advantages of arranging the Al2O3 and LiF as shock wave window materials, possible limitations on the technique, and PBFA-II results. This work was supported by U.S. DOE contract No. DE-AC04-76DP00789.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1141668
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  • 3
    Electronic Resource
    Electronic Resource
    Development of holographic interferometry techniques for application to PBFA-II ion source and target experiments (abstract) : Eighth topical conference on high−temperature plasma diagnostics (1990)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 61 (1990), S. 3150-3150 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We report on a second-generation pulsed laser holographic interferometry system (PLHIS) and its applications to the recording and measurement of light-ion diode and target plasmas on the Particle Beam Fusion Accelerator (PBFA II). Known as the Optical Diagnostics System (ODS), it combines an actively mode-locked and Q-switched, 0.15 ns, pulsed Nd:YAG laser with a 76 m pathlength Mach–Zehnder interferometer. This produces multiple, time-resolved holographic interferograms and shadowgrams from which we can obtain density, velocity, and qualitative information on diode and target plasmas. Background information on the first-generation PLHIS is presented to show the evolution of this diagnostic. The development in the laboratory and installation of the ODS at PBFA II is described, emphasizing the mechanical, optical, electrical, and safety engineering aspects. Included are some of the unique optical engineering problems with regard to current and proposed experiments as well as the limitations of this type of "long stand-off'' diagnostic system. This work was supported by U. S. DOE contract No. DE-AC04-76DP00789.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1141669
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  • 4
    Electronic Resource
    Electronic Resource
    Nanosecond time-resolved vacuum ultraviolet spectrometer for ion diode spectroscopy (1993)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 64 (1993), S. 2493-2495 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A 1-m normal incidence spectrometer has been modified for use as a diagnostic of ion diode plasmas. To improve instrumental sensitivity, an elliptical mirror images an anode surface plasma onto the entrance slit of an f/10 normal incidence spectrometer. The detector is a time-resolving copper iodide coated microchannel plate stripline framing camera with 60-μm resolution, limiting instrumental resolution to 1 A(ring) with a 600 l/mm grating in first order. Reflectivity of optics and photoelectron efficiency limit the spectral range from 400 to 2000 A(ring). With a 600-l/mm grating the detector spans a 600-A(ring) range. Applications of the instrument may include ion source divergence measurements from Doppler broadening, electric field measurements from Stark splittings or shifts, electron temperature from mean ionization state, and magnetic field measurements on high-power Z pinches from Zeeman splitting.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143909
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  • 5
    Electronic Resource
    Electronic Resource
    Extension of the tuning range of a femtosecond Ti:sapphire laser amplifier through cascaded second-order nonlinear frequency conversion processes (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 7704-7712 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The article presents single step and cascade methods for converting the femtosecond radiation from a 1 kHz repetition rate Ti:sapphire regenerative amplifier to the near-infrared (1.2–1.5 μm and 1.6–2.2 μm), visible (554–593 nm), and for the first time to our knowledge to the vacuum ultraviolet spectral region well below 190 nm. Using only solid state materials (nonlinear optical crystals) femtosecond pulses are generated through phase-matched mixing processes down to 172.7 nm. The developed scheme for femtosecond parametric conversion provides nearly bandwidth limited femtosecond pulses, whose duration is comparable or even shorter than that of the pump pulses. The temporal gain narrowing in the optical parametric amplifier is identified as a pulse compression mechanism on the femtosecond time scale leading to generation of infrared light pulses as short as ≈50 fs. As a whole our all-solid state laser complex provides independently tunable and synchronized sub-200 fs light pulses simultaneously at several wavelengths and practically without relative jitter. © 1994 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.357944
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  • 6
    Electronic Resource
    Electronic Resource
    A low-dimensional Galerkin method for the three-dimensional flow around a circular cylinder (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 6 (1994), S. 124-143 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A low-dimensional Galerkin method for the three-dimensional flow around a circular cylinder is constructed. The investigation of the wake solutions for a variety of basic modes, Hilbert spaces, and expansion modes reveals general mathematical and physical aspects which may strongly effect the success of low-dimensional simulations. Besides the cylinder wake, detailed information about the construction of similar low-dimensional Galerkin methods for the sphere wake, the boundary-layer, the flow in a channel or pipe, the Taylor–Couette problem, and a variety of other flows is given.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.868433
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  • 7
    Electronic Resource
    Electronic Resource
    High repetition rate traveling wave optical parametric generator producing nearly bandwidth limited 50 fs infrared light pulses (1994)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 65 (1994), S. 268-270 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A 1 kHz Ti:sapphire femtosecond regenerative amplifier system is used to pump a lithium triborate optical parametric generator in a type II noncritical phase matching configuration and a β-barium borate parametric amplifier. Infrared pulses near 1460 nm as short as 50 fs are achieved with a pulse duration/bandwidth product of only 0.44.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.112367
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  • 8
    Electronic Resource
    Electronic Resource
    Three-dimensional stability analysis of the periodic flow around a circular cylinder (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1279-1281 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The onset of three-dimensionality in the von Kármán vortex street behind a circular cylinder is investigated by carrying out the first global, nonparallel, three-dimensional stability analysis of the periodic flow. This flow is found to become unstable at a Reynolds number of 170 by a critical, three-dimensional Floquet mode with a spanwise wavelength of 1.8 diam. The spatial structure of this mode indicates that the onset of three-dimensionality is due to a near-wake instability and not caused by a stagnation-line or a boundary-layer instability.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858616
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  • 9
    Electronic Resource
    Electronic Resource
    Discrete shedding modes in the von Kármán vortex street (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 1846-1848 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In the laminar vortex street behind a circular cylinder, only few distinct shedding modes for a large range of aspect ratios and various end conditions are observed. The nth mode is characterized by a Strouhal number Stn and a shedding angle, θn, both of which vary continuously with the Reynolds number. The experimentally observed discontinuities in the Strouhal–Reynolds number relationship are transitions between these shedding modes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858807
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  • 10
    Electronic Resource
    Electronic Resource
    Selective sorption uptake kinetics of n-hexane on ZSM 5 - a new method for measuring anisotropic diffusivities (1993)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 97 (1993), S. 13685-13690 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100153a043
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