21,642 research outputs found
Identification of the Sequence of Steps Intrinsic to Spheromak Formation
A planar coaxial electrostatic helicity source is used for studying the relaxation process intrinsic to spheromak formation Experimental observations reveal that spheromak formation involves: (1) breakdown and creation of a number of distinct, arched, filamentary, plasma-filled flux loops that span from cathode to anode gas nozzles, (2) merging of these loops to form a central column, (3) jet-like expansion of the central column, (4) kink instability of the central column, (5) conversion of toroidal flux to poloidal flux by the kink instability. Steps 1 and 3 indicate that spheromak formation involves an MHD pumping of plasma from the gas nozzles into the magnetic flux tube linking the nozzles. In order to measure this pumping, the gas puffing system has been modified to permit simultaneous injection of different gas species into the two ends of the flux tube linking the wall. Gated CCD cameras with narrow-band optical filters are used to track the pumped flows
Large density amplification measured on jets ejected from a magnetized plasma gun
Observation of a large density amplification in the collimating plasma jet ejected from a coplanar coaxial plasma gun is reported. The jet velocity is ~30 km s^-1 and the electron density increases from ~10^20 to 10^(22–23) m^-3. In previous spheromak experiments, electron density of the order 10^(19–21) m^-3 had been measured in the flux conserver region, but no density measurement had been reported for the source gun region. The coplanar geometry of our electrodes permits direct observation of the entire plasma dynamics including the source region. Analysis of Stark broadened spectral lines shows that the electron density increases by a factor of 100 as the jet collimates, with a peak density of up to 10^(22–23) m^-3. The observed density amplification is interpreted according to an MHD theory that explains collimation of current-carrying plasma-filled magnetic flux tubes. Issues affecting interpretation of Stark broadened line profiles and the possibility of using the high-density plasma jet for tokamak fuel injection are discussed
Supernova pencil beam survey
Type Ia supernovae (SNe Ia) can be calibrated to be good standard candles at
cosmological distances. We propose a supernova pencil beam survey that could
yield between dozens to hundreds of SNe Ia in redshift bins of 0.1 up to
, which would compliment space based SN searches, and enable the proper
consideration of the systematic uncertainties of SNe Ia as standard candles, in
particular, luminosity evolution and gravitational lensing. We simulate SNe Ia
luminosities by adding weak lensing noise (using empirical fitting formulae)
and scatter in SN Ia absolute magnitudes to standard candles placed at random
redshifts. We show that flux-averaging is powerful in reducing the combined
noise due to gravitational lensing and scatter in SN Ia absolute magnitudes.
The SN number count is not sensitive to matter distribution in the universe; it
can be used to test models of cosmology or to measure the SN rate. The SN
pencil beam survey can yield a wealth of data which should enable accurate
determination of the cosmological parameters and the SN rate, and provide
valuable information on the formation and evolution of galaxies.
The SN pencil beam survey can be accomplished on a dedicated 4 meter
telescope with a square degree field of view. This telescope can be used to
conduct other important observational projects compatible with the SN pencil
beam survey, such as QSOs, Kuiper belt objects, and in particular, weak lensing
measurements of field galaxies, and the search for gamma-ray burst afterglows.Comment: Final version, to appear in ApJ, 531, #2 (March 10, 2000). 22 pages
including 5 figures. Improved presentatio
The Neutral Hydrogen Distribution in Merging Galaxies: Differences between Stellar and Gaseous Tidal Morphologies
We have mapped the neutral atomic gas (HI) in the three disk-disk merger
systems NGC 520, Arp 220, and Arp 299. These systems differ from the majority
of the mergers mapped in HI, in that their stellar and gaseous tidal features
do not coincide. In particular, they exhibit large stellar tidal features with
little if any accompanying neutral gas and large gas-rich tidal features with
little if any accompanying starlight. On smaller scales, there are striking
anti-correlations where the gaseous and stellar tidal features appear to cross.
We explore several possible causes for these differences, including dust
obscuration, ram pressure stripping, and ionization effects. No single
explanation can account for all of the observed differences. The fact that each
of these systems shows evidence for a starburst driven superwind expanding in
the direction of the most striking anti-correlations leads us to suggest that
the superwind is primarily responsible for the observed differences, either by
sweeping the features clear of gas via ram pressure, or by excavating a clear
sightline towards the starburst and allowing UV photons to ionize regions of
the tails.Comment: 16 pages, 5 figures, uses emulateapj.sty. To appear in the March 2000
issue of AJ. Version with full resolution figures is available via
http://www.cv.nrao.edu/~jhibbard/HIdisp/HIdisp.htm
Chaos at the border of criticality
The present paper points out to a novel scenario for formation of chaotic
attractors in a class of models of excitable cell membranes near an
Andronov-Hopf bifurcation (AHB). The mechanism underlying chaotic dynamics
admits a simple and visual description in terms of the families of
one-dimensional first-return maps, which are constructed using the combination
of asymptotic and numerical techniques. The bifurcation structure of the
continuous system (specifically, the proximity to a degenerate AHB) endows the
Poincare map with distinct qualitative features such as unimodality and the
presence of the boundary layer, where the map is strongly expanding. This
structure of the map in turn explains the bifurcation scenarios in the
continuous system including chaotic mixed-mode oscillations near the border
between the regions of sub- and supercritical AHB. The proposed mechanism
yields the statistical properties of the mixed-mode oscillations in this
regime. The statistics predicted by the analysis of the Poincare map and those
observed in the numerical experiments of the continuous system show a very good
agreement.Comment: Chaos: An Interdisciplinary Journal of Nonlinear Science
(tentatively, Sept 2008
Polyfluorene as a model system for space-charge-limited conduction
Ethyl-hexyl substituted polyfluorene (PF) with its high level of molecular
disorder can be described very well by one-carrier space-charge-limited
conduction for a discrete set of trap levels with energy 0.5 eV above
the valence band edge. Sweeping the bias above the trap-filling limit in the
as-is polymer generates a new set of exponential traps, which is clearly seen
in the density of states calculations. The trapped charges in the new set of
traps have very long lifetimes and can be detrapped by photoexcitation. Thermal
cycling the PF film to a crystalline phase prevents creation of additional
traps at higher voltages.Comment: 13 pages, 4 figures. Physical Review B (accepted, 2007
Detection of CO (2-1) and Radio Continuum Emission from the z = 4.4 QSO BRI 1335-0417
We have detected redshifted CO (2-1) emission at 43 GHz and radio continuum
emission at 1.47 and 4.86 GHz from the z = 4.4 QSO BRI 1335-0417 using the Very
Large Array. The CO data imply optically thick emission from warm (>30 K)
molecular gas with a total mass, M(H_2), of 1.5+/-0.3 x10^{11} M_solar, using
the Galactic gas mass-to-CO luminosity conversion factor. We set an upper limit
to the CO source size of 1.1", and a lower limit of 0.23"x(T_ex/50K)^{-1/2},
where T_ex is the gas excitation temperature. We derive an upper limit to the
dynamical mass of 2x10^{10} x sin^{-2} i M_solar, where i is the disk
inclination angle. To reconcile the gas mass with the dynamical mass requires
either a nearly face-on disk (i < 25deg), or a gas mass-to-CO luminosity
conversion factor significantly lower than the Galactic value. The spectral
energy distribution from the radio to the rest-frame infrared of BRI 1335-0417
is consistent with that expected from a nuclear starburst galaxy, with an
implied massive star formation rate of 2300+/-600 M_solar yr^{-1}.Comment: standard AAS LATEX forma
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