16,980 research outputs found
Mechanical component screening for scanner
The critical mechanical components of the scan mirror mechanism are described and their evaluation and screening procedures are discussed. A bumper/damper unit is used in the design to reverse motion of the mirror and effect scan and retrace cycles. A wear evaluation was conducted on the bumper impact surfaces that established nylon 6-10 as an acceptable material. The elastomeric dampers were subjected to thermal vacuum tests for condensables and outgassing as well as parametric life tests. The flexure pivots that support the mirror were tested to establish a curve of stress plotted as a function of cycles to failure for rotational operation. The pivots met the life requirement of 150,000,000 cycles at a + or - 2.9 deg amplitude during fatigue testing. Screening procedures were established for dampers and flexure pivots to obtain flight quality components
Braginskii magnetohydrodynamics for arbitrary magnetic topologies: coronal applications
We investigate single-fluid magnetohydrodynamics (MHD) with anisotropic viscosity,
often referred to as Braginskii MHD, with a particular eye to solar coronal applications.
First, we examine the full Braginskii viscous tensor in the single-fluid limit. We pay
particular attention to how the Braginskii tensor behaves as the magnetic field strength
vanishes. The solar corona contains a magnetic field with a complex and evolving
topology, so the viscosity must revert to its isotropic form when the field strength is zero,
e.g. at null points. We highlight that the standard form in which the Braginskii tensor
is written is not suitable for inclusion in simulations as singularities in the individual
terms can develop. Instead, an altered form, where the parallel and perpendicular tensors
are combined, provides the required asymptotic behaviour in the weak-field limit. We
implement this combined form of the tensor into the Lare3D code, which is widely used
for coronal simulations. Since our main focus is the viscous heating of the solar corona,
we drop the drift terms of the Braginskii tensor. In a stressed null point simulation,
we discover that small-scale structures, which develop very close to the null, lead to
anisotropic viscous heating at the null itself (that is, heating due to the anisotropic
terms in the viscosity tensor). The null point simulation we present has a much higher
resolution than many other simulations containing null points so this excess heating is
a practical concern in coronal simulations. To remedy this unwanted heating at the null
point, we develop a model for the viscosity tensor that captures the most important
physics of viscosity in the corona: parallel viscosity for strong field and isotropic viscosity
at null points. We derive a continuum model of viscosity where momentum transport,
described by this viscosity model, has the magnetic field as its preferred orientation.
When the field strength is zero, there is no preferred direction for momentum transport
and viscosity reverts to the standard isotropic form. The most general viscous stress
tensor of a (single-fluid) plasma satisfying these conditions is found. It is shown that
the Braginskii model, without the drift terms, is a specialization of the general model.
Performing the stressed null point simulation with this simplified model of viscosity
reveals very similar heating profiles compared to the full Braginskii model. The new
model, however, does not produce anisotropic heating at the null point, as required.
Since the vast majority of coronal simulations use only isotropic viscosity, we perform the
stressed null point simulation with isotropic viscosity and compare the heating profiles
to those of the anisotropic models. It is shown than the fully isotropic viscosity can
over-estimate the viscous heating by an order of magnitude
Circumplanetary disks around young giant planets: a comparison between core-accretion and disk instability
Circumplanetary disks can be found around forming giant planets, regardless
of whether core accretion or gravitational instability built the planet. We
carried out state-of-the-art hydrodynamical simulations of the circumplanetary
disks for both formation scenarios, using as similar initial conditions as
possible to unveil possible intrinsic differences in the circumplanetary disk
mass and temperature between the two formation mechanisms. We found that the
circumplanetary disks mass linearly scales with the circumstellar disk mass.
Therefore, in an equally massive protoplanetary disk, the circumplanetary disks
formed in the disk instability model can be only a factor of eight more massive
than their core-accretion counterparts. On the other hand, the bulk
circumplanetary disk temperature differs by more than an order of magnitude
between the two cases. The subdisks around planets formed by gravitational
instability have a characteristic temperature below 100 K, while the core
accretion circumplanetary disks are hot, with temperatures even greater than
1000 K when embedded in massive, optically thick protoplanetary disks. We
explain how this difference can be understood as the natural result of the
different formation mechanisms. We argue that the different temperatures should
persist up to the point when a full-fledged gas giant forms via disk
instability, hence our result provides a convenient criteria for observations
to distinguish between the two main formation scenarios by measuring the bulk
temperature in the planet vicinity.Comment: 12 pages, 9 figures, 1 table, accepted for publication at MNRA
The self-reference effect on memory in early childhood
The self-reference effect in memory is the advantage for information encoded about self, relative to other people. The early development of this effect was explored here using a concrete encoding paradigm. Trials comprised presentation of a self- or other-image paired with a concrete object. In Study 1, 4- to 6-year-old children (N = 53) were asked in each trial whether the child pictured would like the object. Recognition memory showed an advantage for self-paired objects. Study 2 (N = 55) replicated this finding in source memory. In Study 3 (N = 56), participants simply indicated object location. Again, recognition and source memory showed an advantage for self-paired items. These findings are discussed with reference to mechanisms that ensure information of potential self-relevance is reliably encoded
An 8.4-GHz dual-maser front-end system for Parkes reimplementation
An 8.4-GHz front-end system consisting of a feedhorn, a waveguide feed assembly, dual masers, and downconverters was reimplemented at Parkes as part of the Parkes Canberra Telemetry Array for the Voyager Neptune encounter. The front-end system was originally assembled by the European Space Agency and installed on the Parkes antenna for the Giotto project. It was also used on a time-sharing basis by the Deep Space Network as part of the Parkes Canberra Telemetry Array to enhance the data return from the Voyager Uranus encounter. At the conclusion of these projects in 1986, part of the system was then shipped to JPL on loan for reimplementation at Parkes for the Voyager Neptune encounter. New design and implementation required to make the system operable at Parkes included new microwave front-end control cabinets, closed-cycle refrigeration monitor system, noise-adding radiometer system, front-end controller assembly, X81 local oscillator multiplier, and refurbishment of the original dual 8.4-GHz traveling-wave masers and waveguide feed system. The front-end system met all requirements during the encounter and was disassembled in October 1989 and returned to JPL
Zero-bias Anomaly of Tunneling into the Edge of a 2D Electron System
We investigate the electron tunneling into the edge of a clean weakly
interacting two-dimensional electron gas. It is shown that the corresponding
differential conductance has a cusp at zero bias, and is characterized
by a universal slope at . This singularity originates from the
electron scattering on the Friedel oscillation caused by the boundary of the
system.Comment: 10 pages, uuencoded compressed Postscript file, to appear in Phys.
Rev. B (Rapid Communications
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