1,323 research outputs found
A design study of hydrazine and biowaste resistojets
A generalized modeling program was adapted in BASIC on a personal computer to compare the performance of four types of biowaste resistojets and two types of hydrazine augmenters. Analyzed biowaste design types were: (1) an electrically conductive ceramic heater-exchanger of zirconia; (2) a truss heater of platinum in cross flow; (3) an immersed bicoiled tubular heater-exchanger; and (4) a nonexposed, refractory metal, radiant heater in a central cavity within a heat exchanger case. Concepts 2 and 3 are designed to have an efficient, stainless steel outer pressure case. The hydrazine design types are: (5) an immersed bicoil heater exchanger and (6) a nonexposed radiant heater now with a refractory metal case. The ceramic biowaste resistojet has the highest specific impulse growth potential at 2000 K of 192.5 (CO2) and 269 s (H2O). The bicoil produces the highest augmenter temperature of 1994 K for a 2073 K heater giving 317 s at .73 overall efficiency. Detailed temperature profiles of each of the designs are shown. The scaled layout drawings of each are presented with recommended materials and fabrication methods
Magnetic tight-binding and the iron-chromium enthalpy anomaly
We describe a self consistent magnetic tight-binding theory based in an
expansion of the Hohenberg-Kohn density functional to second order, about a non
spin polarised reference density. We show how a first order expansion about a
density having a trial input magnetic moment leads to the Stoner--Slater rigid
band model. We employ a simple set of tight-binding parameters that accurately
describes electronic structure and energetics, and show these to be
transferable between first row transition metals and their alloys. We make a
number of calculations of the electronic structure of dilute Cr impurities in
Fe which we compare with results using the local spin density approximation.
The rigid band model provides a powerful means for interpreting complex
magnetic configurations in alloys; using this approach we are able to advance a
simple and readily understood explanation for the observed anomaly in the
enthalpy of mixing.Comment: Submitted to Phys Rev
Spin Josephson effect in ferromagnet/ferromagnet tunnel junctions
We consider the tunnel spin current between two ferromagnetic metals from a
perspective similar to the one used in superconductor/superconductor tunnel
junctions. We use fundamental arguments to derive a Josephson-like spin tunnel
current . Here the phases are
associated with the planar contribution to the magnetization,
. The crucial step in our
analysis is the fact that the -component of the spin is canonically
conjugate to the phase of the planar contribution: . This is
analogous to the commutation relation in superconductors, where
is the phase associated to the superconducting order parameter and
is the Cooper pair number operator. We briefly discuss the experimental
consequences of our theoretical analysis.Comment: LaTex, seven pages, no figures; version to appear in Europhys. Lett.;
in order to make room for a more extended microscopic analysis, the
phenomenological discussion contained in v2 was remove
Thermally activated escape rates of uniaxial spin systems with transverse field
Classical escape rates of uniaxial spin systems are characterized by a
prefactor differing from and much smaller than that of the particle problem,
since the maximum of the spin energy is attained everywhere on the line of
constant latitude: theta=const, 0 =< phi =< 2*pi. If a transverse field is
applied, a saddle point of the energy is formed, and high, moderate, and low
damping regimes (similar to those for particles) appear. Here we present the
first analytical and numerical study of crossovers between the uniaxial and
other regimes for spin systems. It is shown that there is one HD-Uniaxial
crossover, whereas at low damping the uniaxial and LD regimes are separated by
two crossovers.Comment: 4 PR pages, 3 figures, final published versio
Russell Lecture: Dark Star Formation and Cooling Instability
Optically thin cooling gas at most temperatures above 30K will make
condensations by pressure pushing material into cool dense regions. This works
without gravity. Cooling condensations will flatten and become
planar/similarity solutions. Most star formation may start from cooling
condensations - with gravity only important in the later stages. The idea that
some of the dark matter could be pristine white dwarfs that condensed slowly on
to planetary sized seeds without firing nuclear reactions is found lacking.
However, recent observations indicate fifty times more halo white dwarfs than
have been previously acknowledged; enough to make the halo fraction observed as
MACHOS.
A cosmological census shows that only 1% of the mass of the Universe is of
known constitution.Comment: 32 Pages, Latex (uses aastex & natbib), 5 eps figures, submitted to
ApJ April 200
Layer dependent band dispersion and correlations using tunable Soft X-ray ARPES
Soft X-ray Angle-Resolved Photoemission Spectroscopy is applied to study
in-plane band dispersions of Nickel as a function of probing depth. Photon
energies between 190 and 780 eV were used to effectively probe up to 3-7
layers. The results show layer dependent band dispersion of the Delta_2
minority-spin band which crosses the Fermi level in 3 or more layers, in
contrast to known top 1-2 layers dispersion obtained using ultra-violet rays.
The layer dependence corresponds to an increased value of exchange splitting
and suggests reduced correlation effects in the bulk compared to the surface.Comment: 7 pages, 3 figures Revised text and figur
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