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  • 1
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    Ion-acoustic waves in a plasma with negative ions (1991)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: Propagation and damping of ion-acoustic waves have been investigated in a Q-machine plasma consisting of K(+) positive ions, SF6(-) negative ions, and electrons. The phase velocity of the ion-acoustic 'fast' mode increases with increasing epsilon, the concentration of negative ions. The wave damping decreases with increasing epsilon, and nearly disappears, for the highest wave frequencies investigated, when epsilon is more than about 0.9. Both results are in agreement with predictions from Vlasov theory.
    Keywords: PLASMA PHYSICS
    Type: Physics of Fluids B (ISSN 0899-8221); 3; 284-287
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  • 2
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    Transition from moving to stationary double layers in a single-ended Q machine (1990)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: Large-amplitude (less than about 100 percent) relaxation oscillations in the plasma potential are known to be generated when the cold endplate of a single-ended Q machine is biased positively. These oscillations are associated with double layers that form near the hot plate (plasma source) and travel toward the endplate at about the ion-acoustic velocity. At the endplate they dissolve and then form again near the hot plate, the entire process repeating itself in a regular manner. By admitting a sufficient amount of neutral gas into the system, the moving double layers were slowed down and eventually stopped. The production of stationary double layers requires an ion source on the high-potential side of the double layers. These ions are provided by ionization of the neutral gas by electrons that are accelerated through the double layer. The dependence of the critical neutral gas pressure required for stationary double-layer formation on endplate voltage, magnetic field strength, and neutral atom mass has been examined. These results are discussed in terms of a simple model of ion production and loss, including ion losses across the magnetic field.
    Keywords: PLASMA PHYSICS
    Type: Physics of Fluids B (ISSN 0899-8221); 2; 1936-194
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  • 3
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    Enhancement of the Accretion of Jupiters Core by a Voluminous Low-Mass Envelope (2013)
    In:  CASI
    Details
    Publication Date: 2019-07-19
    Description: We present calculations of the early stages of the formation of Jupiter via core nucleated accretion and gas capture. The core begins as a seed body of about 350 kilometers in radius and orbits in a swarm of planetesimals whose initial radii range from 15 meters to 100 kilometers. We follow the evolution of the swarm by accounting for growth and fragmentation, viscous and gravitational stirring, and for drag-induced migration and velocity damping. Gas capture by the core substantially enhances the cross-section of the planet for accretion of small planetesimals. The dust opacity within the atmosphere surrounding the planetary core is computed self-consistently, accounting for coagulation and sedimentation of dust particles released in the envelope as passing planetesimals are ablated. The calculation is carried out at an orbital semi-major axis of 5.2 AU and an initial solids' surface density of 10/g/cm^2 at that distance. The results give a core mass of 7 Earth masses and an envelope mass of approximately 0.1 Earth mass after 500,000 years, at which point the envelope growth rate surpasses that of the core. The same calculation without the envelope gives a core mass of only 4 Earth masses.
    Keywords: Astronomy
    Type: ARC-E-DAA-TN10762 , NCTS# 16972-14; Annual Meeting, Division for Planetary Science; Oct 06, 2013 - Oct 11, 2013; Denver, CO; United States
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  • 4
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    DP Capabilities: LWA Current and Future Users Meeting (2011)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: No abstract available
    Keywords: Astronomy
    Type: LWA Current and Future Users Meeting; May 12, 2011 - May 13, 2011; Albuquerque, NM; United States
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  • 5
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    A FPGA-based Fast Transient Search Platform for the Australian Square Kilometre Array Pathfinder (2011)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: No abstract available
    Keywords: Astronomy
    Type: National Radio Science Meeting (USNC-URSI); Jan 05, 2011 - Jan 09, 2011; Boulder, CO; United States
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  • 6
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    Photoevaporation and the Dispersal of Disks (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-20
    Description: Disk disperse in a few million years, before which they must form planets. Photoevaporation and viscosity are mainly responsible for disk dispersal. EUV, FUV and X-rays have all been suggested as photoevaporation agents, disk evolutionary scenarios and predicted mass loss rates in each case differ. Stellar mass and radiation field, disk properties, magnitude of viscosity, and dust evolution all play significant roles in determining the evolution of the disk and its lifetime. Observational diagnostics of photoevaperative flows include [Nell] and perhaps [OI]. These are at present inconclusive and better diagnostics are needed.
    Keywords: Astronomy
    Type: ARC-E-DAA-TN17213 , The Disk in Relation to The Formation of Planets And Their Protoatmospheres; Aug 25, 2014 - Aug 29, 2014; Beijing; China
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  • 7
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    Signal Processing for a Lunar Array: Minimizing Power Consumption (2011)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Motivation for the study is: (1) Lunar Radio Array for low frequency, high redshift Dark Ages/Epoch of Reionization observations (z =6-50, f=30-200 MHz) (2) High precision cosmological measurements of 21 cm H I line fluctuations (3) Probe universe before first star formation and provide information about the Intergalactic Medium and evolution of large scale structures (5) Does the current cosmological model accurately describe the Universe before reionization? Lunar Radio Array is for (1) Radio interferometer based on the far side of the moon (1a) Necessary for precision measurements, (1b) Shielding from earth-based and solar RFI (12) No permanent ionosphere, (2) Minimum collecting area of approximately 1 square km and brightness sensitivity 10 mK (3)Several technologies must be developed before deployment The power needed to process signals from a large array of nonsteerable elements is not prohibitive, even for the Moon, and even in current technology. Two different concepts have been proposed: (1) Dark Ages Radio Interferometer (DALI) (2)( Lunar Array for Radio Cosmology (LARC)
    Keywords: Astronomy
    Type: National Radio Science Meeting; Jan 05, 2011; Boulder, CO; United States
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  • 8
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    The Most Powerful Flaring Activity from the NLSy1 PMN J0948+0022 (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: We report on multifrequency observations performed during 2012 December-2013 August of the first narrow-line Seyfert 1 galaxy detected in gamma rays, PMN J0948+0022 (z equal to 0.5846). A gamma-ray flare was observed by the Large Area Telescope on board Fermi during 2012 December-2013 January, reaching a daily peak flux in the 0.1-100 GeV energy range of (155 plus or minus 31)10(exp 8) ph cm (exp 2) s (exp 1) on 2013 January 1, corresponding to an apparent isotropic luminosity of approximately 1.510)exp 48) erg s(exp 1). The gamma-ray flaring period triggered Swift and VERITAS observations in addition to radio and optical monitoring by OVRO, MOJAVE, and CRTS. A strong flare was observed in optical, UV, and X- rays on 2012 December 30, quasi-simultaneously to the gamma-ray flare, reaching a record flux for this source from optical to gamma rays. VERITAS observations at very high energy (E greater than 100 GeV) during 2013 January 6-17 resulted in an upper limit of F(sub greater than 0.2 TeV) less than 4.0 10(exp 12) ph cm(exp 2) s(exp 1). We compared the spectral energy distribution (SED) of the flaring state in 2013 January with that of an intermediate state observed in 2011. The two SEDs, modelled as synchrotron emission and an external Compton scattering of seed photons from a dust torus, can be modelled by changing both the electron distribution parameters and the magnetic field.
    Keywords: Astronomy
    Type: GSFC-E-DAA-TN24339 , Monthly Notices of the Royal Astronomical Society (ISSN 0035-8711) (e-ISSN 1365-2966); 446; 3; 2456-2467
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  • 9
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    The effect of a magnetic field gradient on anode double layers (1992)
    In:  Other Sources
    Details
    Publication Date: 2019-08-27
    Description: In experiments on anode (ionization) double layers in nonuniform magnetic fields it has been noted that the magnetic field gradient seems to stabilize the double layer position. This effect is further investigated in a Q machine in which the magnetic field geometry could be varied. It is found that the position of the double layers, along the axis of the device, could be controlled by changing the magnetic geometry. This effect is accounted for in a physical model which takes into account the effect of ion reflection by the magnetic mirror force in the region of magnetic field nonuniformity. This model is also able to account for variation of the double layer position when the neutral gas pressure is varied.
    Keywords: PLASMA PHYSICS
    Type: Physica Scripta (ISSN 0031-8949); 45; 395-398
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  • 10
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    On the stability of strong double layers (1992)
    In:  Other Sources
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    Publication Date: 2019-08-27
    Description: For a strong, stationary double layer to be maintained, the electron flux Phi(e) and the ion flux Phi(i) must satisfy the Langmuir condition Phi(e)/Phi(i) = sq rt m(i)/m(e). However, this condition alone does not determine whether or not the double layer will remain in stable equilibrium at a given position. A new stability condition is obtained, and the results of several laboratory double layer experiments are examined using this condition.
    Keywords: PLASMA PHYSICS
    Type: Physica Scripta (ISSN 0031-8949); 45; 391-394
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