32,915 research outputs found
High reliability cathode heaters for ion thrusters
A number of space missions were proposed which utilize 30-cm mercury bombardment ion thrusters and also require a large number of thruster restarts. A test program was carried out to determine thermal cycle life of several different cathode heater designs. Plasma/flame sprayed heaters and swaged type heaters were tested. Four of the five plasma/flame sprayed heaters tested failed in a comparatively short time. Four tantalum swaged heaters that were brazed to the tantalum cathode tube were successfully tested and met the goals that were set at the start of the test
Flexible electrical conductors for high-temperature switchgear
Arch-shaped conductors fabricated from flat strips of beryllium oxide dispersion-strengthened copper alloy serve as flexible electrical connectors capable of operating in 1000 deg F temperature range, under vacuum conditions for periods of 10,000 hours or more without failure
System reliability analysis through corona testing
A corona vacuum test facility for nondestructive testing of power system components was built in the Reliability and Quality Engineering Test Laboratories at the NASA Lewis Research Center. The facility was developed to simulate operating temperature and vacuum while monitoring corona discharges with residual gases. The facility is being used to test various high-voltage power system components
Quenching of hadron spectra in media
We determine how the yield of large transverse momentum hadrons is modified
due to induced gluon radiation off a hard parton traversing a QCD medium. The
quenching factor is formally a collinear- and infrared-safe quantity and can be
treated perturbatively. In spite of that, in the region of practical
interest, its value turns out to be extremely sensitive to large distances and
can be used to unravel the properties of dense quark-gluon final states
produced in heavy ion collisions. We also find that the standard modelling of
quenching by shifting in the hard parton cross section by the mean
energy loss is inadequate.Comment: 20 pp, 5 eps figure
Non-Gaussian Correlations in the McLerran-Venugopalan Model
We argue that the statistical weight function W[rho] appearing in the
McLerran-Venugopalan model of a large nucleus is intrinsically non-Gaussian,
even if we neglect quantum corrections. Based on the picture where the nucleus
of radius R consists of a collection of color-neutral nucleons, each of radius
a<<R, we show that to leading order in alpha_s and a/R only the Gaussian part
of W[rho] enters into the final expression for the gluon number density. Thus,
the existing results in the literature which assume a Gaussian weight remain
valid.Comment: 21 pages with 4 figures (revtex
Explanation of 100-fold reduction of spectral shifts for hydrogen on helium films
We show that helium film-mediated hydrogen-hydrogen interactions account for
a two orders of magnitude discrepancy between previous theory and recent
experiments on cold collision shifts in spin-polarized hydrogen adsorbed on a
helium film. These attractive interactions also explain the anomalous
dependence of the cold collision frequency shifts on the He covering of the
film. Our findings suggest that the gas will become mechanically unstable
before reaching the Kosterlitz-Thouless transition unless the experiment is
performed in a drastically different regime, for example with a much different
helium film geometry.Comment: 4+ pages, 1 figure (3 subfigures), revtex
Eddington limited starbursts in the central 10pc of AGN, and the Torus in NGC1068
We present results from a survey of nearby AGN using the near infrared
adaptive optics integral field spectrograph SINFONI. These data enable us to
probe the distribution and kinematics of the gas and stars at spatial
resolutions as small as 0.085arcsec. We find strong evidence for recent but
short lived starbursts residing in very dense nuclear disks. On scales of less
than 10pc these would have reached Eddington-limited luminosities when active,
perhaps accounting for their short duration. In addition, for NGC1068 at a
resolution of 6pc, we present direct observations of molecular gas close around
the AGN which we identify with the obscuring torus.Comment: Conference proceedings to appear in "The Central Engine of Active
Galactic Nuclei", ed. L. C. Ho and J.-M. Wang (San Francisco: ASP
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