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  • 1
    Electronic Resource
    Electronic Resource
    Dynamic structure factor in a bidisperse polymer melt (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 1641-1649 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present calculations of the single-chain dynamic structure factor for a polymer melt composed of linear molecules of the same chemical identity but of two different chain lengths. The fluid is treated within a dynamical mean-field approach, in which each molecule is represented as a freely jointed chain moving among stochastic obstacles. The obstacles are of two types, each representing the obstruction of local conformational changes by one of the species present. The obstacle dynamics are determined self-consistently by equating the relaxation rate of an obstacle of a given type to the smallest conformational relaxation rate of the species that it represents. Calculation of the dynamic structure factor is mapped onto the solution of a random walk with dynamical disorder, in which a walker moves on a one-dimensional lattice with hopping rates that randomly fluctuate among three states. The relevant random walk problem is solved within the effective medium approximation, and the results are employed to examine the dependence of the dynamic structure factor on time, wave vector, chain lengths, and fluid composition. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469736
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    Paper (German National Licenses)
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  • 2
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    Dynamic structure factor in a bidisperse polymer melt (1995)
    American Institute of Physics
    Details
    Publication Date: 1995-07-22
    Print ISSN: 0021-9606
    Electronic ISSN: 1089-7690
    Topics: Chemistry and Pharmacology , Physics
    Published by American Institute of Physics
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    PAPER CURRENT
  • 3
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    Multifocal flat lens with left-handed metamaterial (2005)
    American Institute of Physics
    Details
    Publication Date: 2005-01-10
    Print ISSN: 0003-6951
    Electronic ISSN: 1077-3118
    Topics: Physics
    Published by American Institute of Physics
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    PAPER CURRENT
  • 4
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    A universal electromagnetic energy conversion adapter based on a metamaterial absorber (2014)
    Springer Nature
    Details
    Publication Date: 2014-09-09
    Electronic ISSN: 2045-2322
    Topics: Natural Sciences in General
    Published by Springer Nature
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    PAPER CURRENT
  • 5
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    Technique Developed for Optimizing Traveling-Wave Tubes (1999)
    In:  CASI
    Details
    Publication Date: 2018-06-05
    Description: A traveling-wave tube (TWT) is an electron beam device that is used to amplify electromagnetic communication waves at radio and microwave frequencies. TWT s are critical components in deep-space probes, geosynchronous communication satellites, and high-power radar systems. Power efficiency is of paramount importance for TWT s employed in deep-space probes and communications satellites. Consequently, increasing the power efficiency of TWT s has been the primary goal of the TWT group at the NASA Lewis Research Center over the last 25 years. An in-house effort produced a technique (ref. 1) to design TWT's for optimized power efficiency. This technique is based on simulated annealing, which has an advantage over conventional optimization techniques in that it enables the best possible solution to be obtained (ref. 2). A simulated annealing algorithm was created and integrated into the NASA TWT computer model (ref. 3). The new technique almost doubled the computed conversion power efficiency of a TWT from 7.1 to 13.5 percent (ref. 1).
    Keywords: Acoustics
    Type: Research and Technology 1998; NASA/TM-1999-208815
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 6
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    Cassini Radio Science Experiments on Saturn and Titan Preserved Because of Lewis Analysis (1999)
    In:  CASI
    Details
    Publication Date: 2018-06-05
    Description: The Cassini mission to Saturn is an international venture with participation from NASA, the European Space Agency, and the Italian Space Agency. The Cassini spacecraft was launched from Cape Canaveral in October 1997 and is scheduled to arrive at Saturn in July 2004. After arrival, the spacecraft will orbit Saturn about 60 times over a period of 4 years. During this time, the Cassini Radio Science Subsystem will be used to investigate the atmosphere and rings of Saturn and the atmosphere of its largest moon, Titan--which is larger than Mercury and is the only moon in our solar system with a dense atmosphere. A critical component in Cassini s Radio Science Subsystem is a traveling-wave tube (TWT) that was designed at the NASA Lewis Research Center and built by Hughes Electronic Dynamics Division (ref. 1). This TWT will amplify downlink microwave signals at a frequency of 32 GHz for the Deep Space Network and will be involved in a number of experiments. These include occultation experiments in which the microwave signal will be beamed through rings and atmospheres toward Earth. Researchers will analyze the received signals to determine the sizes and distributions of the particles in the rings and the structure and composition of the atmospheres. The Radio Science Subsystem also will also be used to more accurately determine the mass and size of Saturn and its moons, to investigate the solar corona, and to search for gravity waves from outside the solar system.
    Keywords: Astronomy
    Type: Research and Technology 1998; NASA/TM-1999-208815
    Format: text
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    NASA TECHNICAL REPORTS
  • 7
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    Three-Dimensional Simulation of Traveling-Wave Tube Cold-Test Characteristics Using MAFIA (1995)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The three-dimensional simulation code MAFIA was used to compute the cold-test parameters - frequency-phase dispersion, beam on-axis interaction impedance, and attenuation - for two types of traveling-wave tube (TWT) slow-wave circuits. The potential for this electromagnetic computer modeling code to reduce the time and cost of TWT development is demonstrated by the high degree of accuracy achieved in calculating these parameters. Generalized input files were developed for ferruled coupled-cavity and TunneLadder slow-wave circuits. These files make it easy to model circuits of arbitrary dimensions. The utility of these files was tested by applying each to a specific TWT slow-wave circuit and comparing the results with experimental data. Excellent agreement was obtained.
    Keywords: ELECTRONICS AND ELECTRICAL ENGINEERING
    Type: NASA-TP-3513 , E-9192 , NAS 1.60:3513
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 8
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    Ring-plane traveling-wave tube slow-wave circuit design simulations at V-Band frequencies (1995)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The V-Band frequency range of 59-64 GHz is a region of the millimeter-wave spectrum that has been designated for intersatellite communications. As a first effort to develop a high-efficiency V-band TWT, variations on a ring-plane slow-wave circuit were computationally investigated to develop an alternative to the more conventional ferruled coupled-cavity circuit. The ring-plane circuit was chosen because of its high interaction impedance, large beam aperture, and excellent thermal dissipation properties. Despite the high-power capabilities of the ring-plane TWT, disadvantages of low bandwidth and high voltage requirements have until now prevented its acceptance outside the laboratory. In this paper, we use the three-dimensional electromagnetic simulation code MAFIA to investigate methods of increasing the bandwidth and lowering the operating voltage. Dispersion, impedance, and attenuation calculations for various geometric variations and loading distributions were performed. Based on the results of the variations, a circuit termed the finned-ladder TWT slowwave circuit was designed and is compared here to the scaled ring-plane prototype and the conventional ferruled coupled-cavity TWT circuit over the V-band frequency range.
    Keywords: COMPUTER PROGRAMMING AND SOFTWARE
    Type: NASA-TM-106949 , E-9687 , NAS 1.15:106949 , 1995 International Conference on Plasma Science; Jun 05, 1995 - Jun 08, 1995; Madison, WI; United States
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 9
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    Assessing Spectrum Compatibility for Beyond-Line-of-Sight UAS Control and Non-Payload Communications (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: In order to provide for the safe integration of unmanned aircraft systems (UAS) into the National Airspace System (NAS), the control and non-payload communications (CNPC) link must be highly reliable. A specific requirement is that it must operate using aviation safety radiofrequency spectrum. Two types of links are required - line-of-sight (LOS) using terrestrial-based communications and beyond-line-of-sight (BLOS) using satellite communications. The 2012 World Radiocommunication Conference (WRC-12) provided a suitable allocation for LOS CNPC spectrum in the 5030 to 5091 MHz band which, when combined with a previously existing allocation fulfills the LOS spectrum requirement. The 5030 to 5091 MHz band is also allocated for BLOS CNPC, but since a significant portion of that band is required for LOS CNPC, additional BLOS spectrum is required. More critically, there are no satellites in operation or in development to provide such services in that band. Hence BLOS CNPC cannot be provided in protected aviation spectrum under current conditions. To fill this gap and enable integration of UAS into the NAS, it has been proposed to allow CNPC to operate over certain Fixed Satellite Service (FSS) bands in which many satellites currently provide commercial services. To enable this, changes in international regulation must be enacted. Agenda Item 1.5 of the 2015 WRC examines the possible regulatory changes needed. As part of the examination process, sharing between potential UAS using satellite communications for BLOS CNPC and other services allocated to the FSS bands being considered must be studied. This paper reviews the technical requirements and approach being undertaken for these sharing studies, with emphasis on study of interference from UAS into digital repeater links operating under the Fixed Service allocation. These studies are being conducted by NASA Glenn Research Center.
    Keywords: Aircraft Communications and Navigation; Air Transportation and Safety
    Type: NASA/TM-2014-218407 , E-18985 , GRC-E-DAA-TN14171 , Integrated Communications, Navigation and Surveillance (ICNS) 2014 Conference; Apr 08, 2014 - Apr 10, 2014; Herndon, VA; United States
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 10
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    Magnetic Tunnel Junction-Based On-Chip Microwave Phase and Spectrum Analyzer (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: A magnetic tunnel junction (MTJ)-based microwave detector is proposed and investigated. When the MTJ is excited by microwave magnetic fields, the relative angle between the free layer and pinned layer alternates, giving rise to an average resistance change. By measuring the average resistance change, the MTJ can be utilized as a microwave power sensor. Due to the nature of ferromagnetic resonance, the frequency of an incident microwave is directly determined. In addition, by integrating a mixer circuit, the MTJ-based microwave detector can also determine the relative phase between two microwave signals. Thus, the MTJ-based microwave detector can be used as an on-chip microwave phase and spectrum analyzer.
    Keywords: Electronics and Electrical Engineering
    Type: GRC-E-DAA-TN13791 , IEEE International Microwave Symposium; Jun 01, 2014 - Jun 06, 2014; Tampa Bay, FL; United States
    Format: application/pdf
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    NASA TECHNICAL REPORTS
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