3,645 research outputs found
Complex permittivity measurements of lunar samples at microwave and millimeter wavelengths
The relative dielectric constant and loss tangent of lunar sample 14163,164 (fine dust) were determined as a function of density at 9.375, 24, 35, and 60 GHz. In addition, such measurements have also been performed on lunar sample 14310,74 (solid rock) at 9.375 GHz. The loss tangent was found to be frequency independent at these test frequencies and had a value of 0.015 for the lunar dust sample
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Deep brine aquifers in the Palo Duro Basin : regional flow and geochemical constraints
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Forensic Isotope Hydrology as a Support to Complex Water Issues in the Arid Southwest U.S.
Water resources in the arid southwest are frequently the subject of arbitration between competing interests of private and public entities. Although the legal remedies allow for a path forward, the technical nature of the problem often determines the future success of any solution. The term technical used here encompasses: 1) the hydrogeological properties of water quantity, availability, distribution, and 2) the geochemical issues of water quality, transport and migration, contamination, or mixing with lower quality reservoirs. These examples of multi-use issues change over time and could negatively impact another water user or natural resource. One successful approach can be the inclusion of all agencies early in the technical evaluations from data collection to interpretation, and in addition, to include private sector firms with state of the art investigative techniques
S-band omnidirectional antenna for the SERT-C satellite
The program to design an S-band omnidirectional antenna system for the SERT-C spacecraft is discussed. The program involved the tasks of antenna analyses by computer techniques, scale model radiation pattern measurements of a number of antenna systems, full-scale RF measurements, and the recommended design, including detailed drawings. A number of antenna elements were considered: the cavity-backed spiral, quadrifilar helix, and crossed-dipoles were chosen for in-depth studies. The final design consisted of a two-element array of cavity-backed spirals mounted on opposite sides of spacecraft and fed in-phase through a hybrid junction. This antenna system meets the coverage requirement of having a gain of at least minus 10 dBi over 50 percent of a 4 pi steradian sphere with the solar panels in operation. This coverage level is increased if the ground station has the capability to change polarization
Optical Signatures of Quantum Emitters in Suspended Hexagonal Boron Nitride
Hexagonal boron nitride (h-BN) is a tantalizing material for solid-state
quantum engineering. Analogously to three-dimensional wide-bandgap
semiconductors like diamond, h-BN hosts isolated defects exhibiting visible
fluorescence, and the ability to position such quantum emitters within a
two-dimensional material promises breakthrough advances in quantum sensing,
photonics, and other quantum technologies. Critical to such applications,
however, is an understanding of the physics underlying h-BN's quantum emission.
We report the creation and characterization of visible single-photon sources in
suspended, single-crystal, h-BN films. The emitters are bright and stable over
timescales of several months in ambient conditions. With substrate interactions
eliminated, we study the spectral, temporal, and spatial characteristics of the
defects' optical emission, which offer several clues about their electronic and
chemical structure. Analysis of the defects' spectra reveals similarities in
vibronic coupling despite widely-varying fluorescence wavelengths, and a
statistical analysis of their polarized emission patterns indicates a
correlation between the optical dipole orientations of some defects and the
primitive crystallographic axes of the single-crystal h-BN film. These
measurements constrain possible defect models, and, moreover, suggest that
several classes of emitters can exist simultaneously in free-standing h-BN,
whether they be different defects, different charge states of the same defect,
or the result of strong local perturbations
Gravitational waves in preheating
We study the evolution of gravitational waves through the preheating era that
follows inflation. The oscillating inflaton drives parametric resonant growth
of scalar field fluctuations, and although super-Hubble tensor modes are not
strongly amplified, they do carry an imprint of preheating. This is clearly
seen in the Weyl tensor, which provides a covariant description of
gravitational waves.Comment: 8 pages, 8 figures, Revte
A new twist to preheating
Metric perturbations typically strengthen field resonances during preheating.
In contrast we present a model in which the super-Hubble field resonances are
completely {\em suppressed} when metric perturbations are included. The model
is the nonminimal Fakir-Unruh scenario which is exactly solvable in the
long-wavelength limit when metric perturbations are included, but exhibits
exponential growth of super-Hubble modes in their absence. This gravitationally
enhanced integrability is exceptional, both for its rarity and for the power
with which it illustrates the importance of including metric perturbations in
consistent studies of preheating. We conjecture a no-go result - there exists
no {\em single-field} model with growth of cosmologically-relevant metric
perturbations during preheating.Comment: 6 pages, 3 figures, Version to appear in Physical Review
Conceptual design of an orbital propellant transfer experiment. Volume 2: Study results
The OTV configurations, operations and requirements planned for the period from the 1980's to the 1990's were reviewed and a propellant transfer experiment was designed that would support the needs of these advanced OTV operational concepts. An overall integrated propellant management technology plan for all NASA centers was developed. The preliminary cost estimate (for planning purposes only) is 31.8 M is for shuttle user costs
Oscillons in Scalar Field Theories: Applications in Higher Dimensions and Inflation
The basic properties of oscillons -- localized, long-lived, time-dependent
scalar field configurations -- are briefly reviewed, including recent results
demonstrating how their existence depends on the dimensionality of spacetime.
Their role on the dynamics of phase transitions is discussed, and it is shown
that oscillons may greatly accelerate the decay of metastable vacuum states.
This mechanism for vacuum decay -- resonant nucleation -- is then applied to
cosmological inflation. A new inflationary model is proposed which terminates
with fast bubble nucleation.Comment: 11 pages, 4 figures, to appear in Int. J. Mod. Phys.
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