1,016 research outputs found
Study of growth parameters for refractory carbide single crystals Quarterly status report no. 6, 1 Jun. - 1 Sep. 1965
Growth parameters for refractory tantalum carbide single crystal
Global axisymmetric Magnetorotational Instability with density gradients
We examine global incompressible axisymmetric perturbations of a
differentially rotating MHD plasma with radial density gradients. It is shown
that the standard magnetorotational instability, (MRI) criterion drawn from the
local dispersion relation is often misleading. If the equilibrium magnetic
field is either purely axial or purely toroidal, the problem reduces to finding
the global radial eigenvalues of an effective potential. The standard Keplerian
profile including the origin is mathematically ill-posed, and thus any solution
will depend strongly on the inner boundary. We find a class of unstable modes
localized by the form of the rotation and density profiles, with reduced
dependence on boundary conditions.Comment: 22 pages, 5 figure
Technology requirements for post-1985 communications satellites
The technical and functional requirements for commercial communication satellites are discussed. The need for providing quality service at an acceptable cost is emphasized. Specialized services are postulated in a needs model which forecasts future demands. This needs model is based upon 322 separately identified needs for long distance communication. It is shown that the 1985 demand for satellite communication service for a domestic region such as the United States, and surrounding sea and air lanes, may require on the order of 100,000 MHz of bandwith. This level of demand can be met by means of the presently allocated bandwidths and developing some key technologies. Suggested improvements include: (1) improving antennas so that high speed switching will be possible; (2) development of solid state transponders for 12 GHz and possibly higher frequencies; (3) development of switched or steered beam antennas with 10 db or higher gain for aircraft; and (4) continued development of improved video channel compression techniques and hardware
The pre-shock gas of SN1006 from HST/ACS observations
We derive the pre-shock density and scale length along the line of sight for
the collisionless shock from a deep HST image that resolves the H alpha
filament in SN1006 and updated model calculations. The very deep ACS
high-resolution image of the Balmer line filament in the northwest (NW)
quadrant shows that 0.25 < n_0 < le$ 0.4 cm-3 and that the scale along the line
of sight is about 2 x 10^{18} cm, while bright features within the filament
correspond to ripples with radii of curvature less than 1/10 that size. The
derived densities are within the broad range of earlier density estimates, and
they agree well with the ionization time scale derived from the Chandra X-ray
spectrum of a region just behind the optical filament. This provides a test for
widely used models of the X-ray emission from SNR shocks. The scale and
amplitude of the ripples are consistent with expectations for a shock
propagating though interstellar gas with ~ 20% density fluctuations on parsec
scales as expected from studies of interstellar turbulence. One bulge in the
filament corresponds to a knot of ejecta overtaking the blast wave, however.
The interaction results from the rapid deceleration of the blast wave as it
encounters an interstellar cloud.Comment: 16 pages, 6 figures, to appear in Ap
Technology requirements for communication satellites in the 1980's
The key technology requirements are defined for meeting the forecasted demands for communication satellite services in the 1985 to 1995 time frame. Evaluation is made of needs for services and technical and functional requirements for providing services. The future growth capabilities of the terrestrial telephone network, cable television, and satellite networks are forecasted. The impact of spacecraft technology and booster performance and costs upon communication satellite costs are analyzed. Systems analysis techniques are used to determine functional requirements and the sensitivities of technology improvements for reducing the costs of meeting requirements. Recommended development plans and funding levels are presented, as well as the possible cost saving for communications satellites in the post 1985 era
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