22,801 research outputs found
Helical Magnetorotational Instability in Magnetized Taylor-Couette Flow
Hollerbach and Rudiger have reported a new type of magnetorotational
instability (MRI) in magnetized Taylor-Couette flow in the presence of combined
axial and azimuthal magnetic fields. The salient advantage of this "helical''
MRI (HMRI) is that marginal instability occurs at arbitrarily low magnetic
Reynolds and Lundquist numbers, suggesting that HMRI might be easier to realize
than standard MRI (axial field only). We confirm their results, calculate HMRI
growth rates, and show that in the resistive limit, HMRI is a weakly
destabilized inertial oscillation propagating in a unique direction along the
axis. But we report other features of HMRI that make it less attractive for
experiments and for resistive astrophysical disks. Growth rates are small and
require large axial currents. More fundamentally, instability of highly
resistive flow is peculiar to infinitely long or periodic cylinders: finite
cylinders with insulating endcaps are shown to be stable in this limit. Also,
keplerian rotation profiles are stable in the resistive limit regardless of
axial boundary conditions. Nevertheless, the addition of toroidal field lowers
thresholds for instability even in finite cylinders.Comment: 16 pages, 2 figures, 1 table, submitted to PR
Correlations and fluctuations of a confined electron gas
The grand potential and the response of a phase-coherent confined noninteracting electron gas depend
sensitively on chemical potential or external parameter . We compute
their autocorrelation as a function of , and temperature. The result
is related to the short-time dynamics of the corresponding classical system,
implying in general the absence of a universal regime. Chaotic, diffusive and
integrable motions are investigated, and illustrated numerically. The
autocorrelation of the persistent current of a disordered mesoscopic ring is
also computed.Comment: 12 pages, 1 figure, to appear in Phys. Rev.
Teenage drinking and interethnic friendships.
This report explores the links between young people’s interethnic friendships and their drinking patterns and behaviours.
Britain is a multicultural society, but little is currently known about if, and how, young people mix with friends from different ethnic backgrounds and the potential impact of this on drinking attitudes and behaviours. Research was undertaken to examine these links using quantitative and qualitative methods among a sample of 14-and 15-year-olds in diverse locations in London and Berkshire.
The report:
• explores the intra- and interethnic mix of young people’s
friendship groups as described by young people in
questionnaires and interviews;
• analyses how drinking patterns vary by ethnicity, religion and gender;
• investigates the links between young people’s background
characteristics, their friendship groups (including the ethnicity of friends) and their reported drinking rates; and
• looks at the implications of the findings, including
recommendations for harm reduction based on education and
peer support programmes
Radiobiological studies with monoenergetic neutrons
The Radiological Research Accelerator Facility (RARAF) has the capability of
producing essentially monoenergetic neutron beams, ranging in energy from 16.4 MeV
down to 220 keV. In addition, two lower energy neutron beams are available which
consist of a wide spectrum of energies and are described as the 110 keV and 60 keV
spectra. Seedlings of Vicia faba have been used to measure the oxygen enhancement
ratio (OER) and the relative biological effectiveness (RBE) of each of these neutron
beams. The OER decreases as the neutron energy is reduced between 15.4 MeV and
220 keV, but does not appear to decrease further for lower energy neutrons. RBE increases
as the neutron energy is reduced from 15.4 AleV to 440 keV; the curve then
goes through a maximum at around 350 keV, and for lower energies the RBE falls again
The Evolution of Cuspy Triaxial Galaxies Harboring Central Black Holes
We use numerical simulations to study the evolution of triaxial elliptical
galaxies with central black holes. In contrast to earlier numerical studies
which used galaxy models with central density ``cores,'' our galaxies have
steep central cusps, like those observed in real ellipticals. As a black hole
grows in these cuspy triaxial galaxies, the inner regions become rounder owing
to chaos induced in the orbit families which populate the model. At larger
radii, however, the models maintain their triaxiality, and orbital analyses
show that centrophilic orbits there resist stochasticity over many dynamical
times. While black hole induced evolution is strong in the inner regions of
these galaxies, and reaches out beyond the nominal ``sphere of influence'' of a
black hole, our simulations do not show evidence for a rapid {\it global}
transformation of the host. The triaxiality of observed elliptical galaxies is
therefore not inconsistent with the presence of supermassive black holes at
their centers.Comment: 15 pages, 7 figures (1 color). Accepted for publication in Ap
Material studies related to lunar surface exploration. Volume 4 - Preliminary studies for the design of engineering probes Final report, 6 Mar. 1967 - 30 Jun. 1968
Preliminary design of engineering probes for studying lunar surface material propertie
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