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  • Other Sources  (11)
  • Lasers and Masers  (5)
  • ASTROPHYSICS  (3)
  • Numerical Analysis  (3)
  • 2005-2009  (8)
  • 1970-1974  (3)
  • 1
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    The interaction of interstellar gas with stellar density wave packets (1974)
    In:  Other Sources
    Details
    Publication Date: 2011-08-16
    Description: We have numerically studied the interaction between interstellar gas and a gravitational perturbation produced by a fluctuation in the stellar mass distribution. For a simple equation of state, shock waves develop even if the gravitational perturbation is aperiodic and transient. In a medium which can change thermal phase, shocks do not form and the gas makes a transition directly to the cold, dense phase. The cold material has an anomalous velocity imparted to it by the gravitational perturbation and it becomes an accretion front moving through the hot gas and capturing it. The accretion fronts trigger thermal phase change on a large scale and can reproduce the extent of observed cold clouds. Furthermore their high mass is favorable to gravitational instability and subsequent star formation.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics; 36; 2, No; Nov. 197
    Format: text
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  • 2
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    Fine-scale structure of a cold hydrogen layer (1974)
    In:  Other Sources
    Details
    Publication Date: 2011-08-16
    Description: Neutral-hydrogen absorption-line interferometer observations are reported for five continuum radio sources situated behind an extensive layer of cold interstellar H I gas. In contrast to low-spatial-resolution emission-line observations reported earlier which show the large-scale uniformity of the layer, the high-spatial-resolution absorption-line observations reveal variations in the velocity and density within the layer. The new results together with the old indicate the apparent coexistence of a large-scale mechanism for triggering the formation of cold hydrogen and an instability mechanism leading to small-scale irregularities.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; 194; Dec. 1
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  • 3
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    An experimental test of the homogeneity of the interstellar medium (1974)
    In:  Other Sources
    Details
    Publication Date: 2011-08-16
    Description: Observations of nonthermal continuum radio emission at 750 MHz and hydrogen-line emission at 1420 MHz have been compared in order to test the extent to which cosmic-ray electrons, magnetic fields, and hydrogen gas coexist in the interstellar medium. If the interstellar medium is homogeneous in the sense that its constituents are mixed together and can interact, then regions of line and continuum emission should be spatially correlated. The measurements indicate that at most 28% of continuum emission in the Galaxy comes from such a homogeneous medium. The remaining nonthermal emission agrees in magnitude with the residual emission found by Berkhuijsen after subtraction of the polarized component of nonthermal emission. In addition, the data discussed here show that the major filamentary structures found in the two forms of emission rarely coincide spatially. Consequently, both the average and the exceptional structure in the medium indicate that the synchrotron-radiating magnetoplasma and the neutral hydrogen gas are not closely interacting in the present state of the medium.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; 194; Dec. 1
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  • 4
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    LISA Technology Development at GSFC (2008)
    In:  Other Sources
    Details
    Publication Date: 2019-07-19
    Description: The prime focus of LISA technology development efforts at NASA/GSFC has been in LISA interferometry, specifically in the area of laser frequency noise mitigation. Laser frequency noise is addressed through a combination of stabilization and common-mode rejection. Current plans call for two stages of stabilization, pre-stabilization to a local frequency reference and further stabilization using the constellation as a frequency reference. In order for these techniques to be used simultaneously, the pre-stabilization step must provide an adjustable frequency offset. Here, we report on a modification to the standard modulation/demodulation techniques used to stabilize to optical cavities that generates a frequency-tunable reference from a fixed-length cavity. This technique requires no modifications to the cavity itself and only minor modifications to the components. The measured noise performance and dynamic range of the laboratory prototype meets the LISA requirements.
    Keywords: Lasers and Masers
    Type: Committee on Space Research, 37th Scientific Assembly; Jul 13, 2008 - Jul 20, 2008; Montreal; Canada
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  • 5
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    LISA Parameter Estimation using Numerical Merger Waveforms (2008)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Coalescing supermassive black holes are expected to provide the strongest sources for gravitational radiation detected by LISA. Recent advances in numerical relativity provide a detailed description of the waveforms of such signals. We present a preliminary study of LISA's sensitivity to waveform parameters using a hybrid numerical/analytic waveform describing the coalescence of two equal-mass, nonspinning black holes. The Synthetic LISA software package is used to simulate the instrument response and the Fisher information matrix method is used to estimate errors in the waveform parameters. Initial results indicate that inclusion of the merger signal can significantly improve the precision of some parameter estimates. For example, the median parameter errors for an ensemble of systems with total redshifted mass of 10(exp 6) deg M solar mass at a redshift of z is approximately 1 were found to decrease by a factor of slightly more than two when the merger was included.
    Keywords: Numerical Analysis
    Type: 7th International LISA Symposium/Space Sciences Institute of California (IEEC); Jun 16, 2008 - Jun 20, 2008; Barcelona; Spain
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  • 6
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    Reliability of High Power Laser Diode Arrays Operating in Long Pulse Mode (2006)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Reliability and lifetime of quasi-CW laser diode arrays are greatly influenced by their thermal characteristics. This paper examines the thermal properties of laser diode arrays operating in long pulse duration regime.
    Keywords: Lasers and Masers
    Type: 2006 CLEO/QELS - Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference; May 21, 2006 - May 26, 2006; Long Beach, CA; United States
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  • 7
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    Improving Reliability of High Power Quasi-CW Laser Diode Arrays Operating in Long Pulse Mode (2006)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Operating high power laser diode arrays in long pulse regime of about 1 msec, which is required for pumping 2-micron thulium and holmium-based lasers, greatly limits their useful lifetime. This paper describes performance of laser diode arrays operating in long pulse mode and presents experimental data of the active region temperature and pulse-to-pulse thermal cycling that are the primary cause of their premature failure and rapid degradation. This paper will then offer a viable approach for determining the optimum design and operational parameters leading to the maximum attainable lifetime.
    Keywords: Lasers and Masers
    Type: Paper-B1P1 , 6th Annual NASA Earth Science Technology Conference - ESTC 2006; Jun 27, 2006 - Jun 29, 2006; College Park, MD; United States
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  • 8
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    Improving Lifetime of Quasi-CW Laser Diode Arrays for Pumping 2-Micron Solid State Lasers (2007)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Operating high power laser diode arrays in long pulse regime of about 1 msec, which is required for pumping 2-micron thulium and holmium-based lasers, greatly limits their useful lifetime. This paper describes performance of laser diode arrays operating in long pulse mode and presents experimental data on the active region temperature and pulse-to-pulse thermal cycling that are the primary cause of their premature failure and rapid degradation. This paper will then offer a viable approach for determining the optimum design and operational parameters leading to the maximum attainable lifetime.
    Keywords: Lasers and Masers
    Type: SPIE Defense and Security Symposium; Apr 09, 2007 - Apr 13, 2007; Orlando, FL; United States
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  • 9
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    Spherical Harmonic Decomposition of Gravitational Waves Across Mesh Refinement Boundaries (2005)
    In:  CASI
    Details
    Publication Date: 2019-07-11
    Description: We evolve a linearized (Teukolsky) solution of the Einstein equations with a non-linear Einstein solver. Using this testbed, we are able to show that such gravitational waves, defined by the Weyl scalars in the Newman-Penrose formalism, propagate faithfully across mesh refinement boundaries, and use, for the first time to our knowledge, a novel algorithm due to Misner to compute spherical harmonic components of our waveforms. We show that the algorithm performs extremely well, even when the extraction sphere intersects refinement boundaries.
    Keywords: Numerical Analysis
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  • 10
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    Improving Reliability of High Power Quasi-CW Laser Diode Arrays for Pumping Solid State Lasers (2005)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Most Lidar applications rely on moderate to high power solid state lasers to generate the required transmitted pulses. However, the reliability of solid state lasers, which can operate autonomously over long periods, is constrained by their laser diode pump arrays. Thermal cycling of the active regions is considered the primary reason for rapid degradation of the quasi-CW high power laser diode arrays, and the excessive temperature rise is the leading suspect in premature failure. The thermal issues of laser diode arrays are even more drastic for 2-micron solid state lasers which require considerably longer pump pulses compared to the more commonly used pump arrays for 1-micron lasers. This paper describes several advanced packaging techniques being employed for more efficient heat removal from the active regions of the laser diode bars. Experimental results for several high power laser diode array devices will be reported and their performance when operated at long pulsewidths of about 1msec will be described.
    Keywords: Lasers and Masers
    Type: SPIE Paper-5887-14 , SPIE 50th Annual Meeting; Jul 31, 2005 - Aug 04, 2005; San Diego, CA; United States
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