11,934 research outputs found
Progress on the PT-1 Prototype Plasmoid Thruster
The design and construction of a plasmoid thruster prototype is described. This thruster operates by expelling inductively formed plasmoids at high velocities. These plasmoids are field reversed configuration plasmas which are formed by reversing a magnetic flux frozen in an ionized gas inside a theta-pinch coil. The pinch coil is a unique multi-turn, multi-lead design chosen for optimization of inductance and field uniformity. A table-top bread-board demonstrator has been built at MSFC, and will be delivered to Radiance Technologies Inc. for further testing at the Auburn Space Power Institute
A model for gelation with explicit solvent effects: Structure and dynamics
We study a two-component model for gelation consisting of -functional
monomers (the gel) and inert particles (the solvent). After equilibration as a
simple liquid, the gel particles are gradually crosslinked to each other until
the desired number of crosslinks has been attained. At a critical crosslink
density the largest gel cluster percolates and an amorphous solid forms. This
percolation process is different from ordinary lattice or continuum percolation
of a single species in the sense that the critical exponents are new. As the
crosslink density approaches its critical value , the shear viscosity
diverges: with a nonuniversal
concentration-dependent exponent.Comment: 6 pages, 9 figure
Data Assimilation of Satellite Fire Detection in Coupled Atmosphere-Fire Simulation by WRF-SFIRE
Currently available satellite active fire detection products from the VIIRS
and MODIS instruments on polar-orbiting satellites produce detection squares in
arbitrary locations. There is no global fire/no fire map, no detection under
cloud cover, false negatives are common, and the detection squares are much
coarser than the resolution of a fire behavior model. Consequently, current
active fire satellite detection products should be used to improve fire
modeling in a statistical sense only, rather than as a direct input. We
describe a new data assimilation method for active fire detection, based on a
modification of the fire arrival time to simultaneously minimize the difference
from the forecast fire arrival time and maximize the likelihood of the fire
detection data. This method is inspired by contour detection methods used in
computer vision, and it can be cast as a Bayesian inverse problem technique, or
a generalized Tikhonov regularization. After the new fire arrival time on the
whole simulation domain is found, the model can be re-run from a time in the
past using the new fire arrival time to generate the heat fluxes and to spin up
the atmospheric model until the satellite overpass time, when the coupled
simulation continues from the modified state.Comment: 9 pages, VII International Conference on Forest Fire Research,
Coimbra, Portugal, November 17-20, 201
ALMA Multi-line Imaging of the Nearby Starburst Galaxy NGC 253
We present spatially resolved (50 pc) imaging of molecular gas species
in the central kiloparsec of the nearby starburst galaxy NGC 253, based on
observations taken with the Atacama Large Millimeter/submillimeter Array
(ALMA). A total of 50 molecular lines are detected over a 13 GHz bandwidth
imaged in the 3 mm band. Unambiguous identifications are assigned for 27 lines.
Based on the measured high CO/CO isotopic line ratio (350), we
show that CO(1-0) has moderate optical depths. A comparison of the HCN
and HCO with their C-substituted isotopologues shows that the
HCN(1-0) and HCO(1-0) lines have optical depths at least comparable to
CO(1-0). HCN/HCO (and HCN/HNC) line ratios
provide tighter constraints on dense gas properties in this starburst. SiO has
elevated abundances across the nucleus. HNCO has the most distinctive
morphology of all the bright lines, with its global luminosity dominated by the
outer parts of the central region. The dramatic variation seen in the HNCO/SiO
line ratio suggests that some of the chemical signatures of shocked gas are
being erased in the presence of dominating central radiation fields (traced by
CH and CN). High density molecular gas tracers (including HCN, HCO,
and CN) are detected at the base of the molecular outflow. We also detect
hydrogen recombination lines that, like their counterparts,
show compact, centrally peaked morphologies, distinct from the molecular gas
tracers. A number of sulfur based species are mapped (CS, SO, NS, CS,
HCS and CHSH) and have morphologies similar to SiO.Comment: 20 pages, 10 figures, accepted to the Astrophysical Journa
Chandra Detection of X-ray Absorption Associated with a Damped Lyman Alpha System
We have observed three quasars, PKS 1127-145, Q 1331+171 and Q0054+144, with
the ACIS-S aboard the Chandra X-ray Observatory, in order to measure soft X-ray
absorption associated with intervening 21-cm and damped Ly absorbers.
For PKS 1127-145, we detect absorption which, if associated with an intervening
z_{abs}=0.312 absorber, implies a metallicity of 23% solar. If the absorption
is not at z_{abs}=0.312, then the metallicity is still constrained to be less
than 23% solar. The advantage of the X-ray measurement is that the derived
metallicity is insensitive to ionization, inclusion of an atom in a molecule,
or depletion onto grains. The X-ray absorption is mostly due to oxygen, and is
consistent with the oxygen abundance of 30% solar derived from optical nebular
emission lines in a foreground galaxy at the redshift of the absorber.
For Q1331+171 and Q 0054+144, only upper limits were obtained, although the
exposure times were intentionally short, since for these two objects we were
interested primarily in measuring flux levels to plan for future observations.
The imaging results are presented in a companion paper.Comment: 23 pages, 6 figures, accepted for publication in the Astrophysical
Journa
Coil system for plasmoid thruster
A coil system for a plasmoid thruster includes a bias coil, a drive coil and field coils. The bias and drive coils are interleaved with one another as they are helically wound about a conical region. A first field coil defines a first passage at one end of the conical region, and is connected in series with the bias coil. A second field coil defines a second passage at an opposing end of the conical region, and is connected in series with the bias coil
- …