17,006 research outputs found
Preliminary design analysis for the solar optical telescope main mirror actuator
The resolution of the SOT Gregorian telescope was maintained if the conic foci of the elliptical secondary and parabolic primary were made to coincide within plus or minus 38 microns across the prime focus plane and to within 5 microns in focus. An error in coincidence across the focal plane caused all point images to show additional coma with all the comatic tails pointing in the same direction. An error in focus became magnified by the square of the magnification of the secondary and simply increased the diameter of the point source. Offsetting or rastering the sun may be accomplished by swinging the primary in an arc about the point of coincidence of the conic foci so long as the coincidence is kept to within the tolerance stated
Coaxial inverted geometry transistor having buried emitter
The invention relates to an inverted geometry transistor wherein the emitter is buried within the substrate. The transistor can be fabricated as a part of a monolithic integrated circuit and is particularly suited for use in applications where it is desired to employ low actuating voltages. The transistor may employ the same doping levels in the collector and emitter, so these connections can be reversed
Evaluating the Applicability of the Fokker-Planck Equation in Polymer Translocation: A Brownian Dynamics Study
Brownian dynamics (BD) simulations are used to study the translocation
dynamics of a coarse-grained polymer through a cylindrical nanopore. We
consider the case of short polymers, with a polymer length, N, in the range
N=21-61. The rate of translocation is controlled by a tunable friction
coefficient, gamma_{0p}, for monomers inside the nanopore. In the case of
unforced translocation, the mean translocation time scales with polymer length
N as ~ (N-N_p)^alpha, where N_p is the average number of monomers in the
nanopore. The exponent approaches the value alpha=2 when the pore friction is
sufficiently high, in accord with the prediction for the case of the
quasi-static regime where pore friction dominates. In the case of forced
translocation, the polymer chain is stretched and compressed on the cis and
trans sides, respectively, for low gamma_{0p}. However, the chain approaches
conformational quasi-equilibrium for sufficiently large gamma_{0p}. In this
limit the observed scaling of with driving force and chain length
supports the FP prediction that is proportional to N/f_d for sufficiently
strong driving force. Monte Carlo simulations are used to calculate
translocation free energy functions for the system. The free energies are used
with the Fokker-Planck equation to calculate translocation time distributions.
At sufficiently high gamma_{0p}, the predicted distributions are in excellent
agreement with those calculated from the BD simulations. Thus, the FP equation
provides a valid description of translocation dynamics for sufficiently high
pore friction for the range of polymer lengths considered here. Increasing N
will require a corresponding increase in pore friction to maintain the validity
of the FP approach. Outside the regime of low N and high pore friction, the
polymer is out of equilibrium, and the FP approach is not valid.Comment: 13 pages, 11 figure
Observational manifestations of solar magneto-convection -- center-to-limb variation
We present the first center-to-limb G-band images synthesized from high
resolution simulations of solar magneto-convection. Towards the limb the
simulations show "hilly" granulation with dark bands on the far side, bright
granulation walls and striated faculae, similar to observations. At disk center
G-band bright points are flanked by dark lanes. The increased brightness in
magnetic elements is due to their lower density compared with the surrounding
intergranular medium. One thus sees deeper layers where the temperature is
higher. At a given geometric height, the magnetic elements are cooler than the
surrounding medium. In the G-band, the contrast is further increased by the
destruction of CH in the low density magnetic elements. The optical depth unity
surface is very corrugated. Bright granules have their continuum optical depth
unity 80 km above the mean surface, the magnetic elements 200-300 km below. The
horizontal temperature gradient is especially large next to flux
concentrations. When viewed at an angle, the deep magnetic elements optical
surface is hidden by the granules and the bright points are no longer visible,
except where the "magnetic valleys" are aligned with the line of sight. Towards
the limb, the low density in the strong magnetic elements causes unit
line-of-sight optical depth to occur deeper in the granule walls behind than
for rays not going through magnetic elements and variations in the field
strength produce a striated appearance in the bright granule walls.Comment: To appear in ApJL. 6 pages 4 figure
On the Use of Group Theoretical and Graphical Techniques toward the Solution of the General N-body Problem
Group theoretic and graphical techniques are used to derive the N-body wave
function for a system of identical bosons with general interactions through
first-order in a perturbation approach. This method is based on the maximal
symmetry present at lowest order in a perturbation series in inverse spatial
dimensions. The symmetric structure at lowest order has a point group
isomorphic with the S_N group, the symmetric group of N particles, and the
resulting perturbation expansion of the Hamiltonian is order-by-order invariant
under the permutations of the S_N group. This invariance under S_N imposes
severe symmetry requirements on the tensor blocks needed at each order in the
perturbation series. We show here that these blocks can be decomposed into a
basis of binary tensors invariant under S_N. This basis is small (25 terms at
first order in the wave function), independent of N, and is derived using
graphical techniques. This checks the N^6 scaling of these terms at first order
by effectively separating the N scaling problem away from the rest of the
physics. The transformation of each binary tensor to the final normal
coordinate basis requires the derivation of Clebsch-Gordon coefficients of S_N
for arbitrary N. This has been accomplished using the group theory of the
symmetric group. This achievement results in an analytic solution for the wave
function, exact through first order, that scales as N^0, effectively
circumventing intensive numerical work. This solution can be systematically
improved with further analytic work by going to yet higher orders in the
perturbation series.Comment: This paper was submitted to the Journal of Mathematical physics, and
is under revie
Analysis of defect structure in silicon. Characterization of SEMIX material. Silicon sheet growth development for the large area silicon sheet task of the low-cost solar array project
Statistically significant quantitative structural imperfection measurements were made on samples from ubiquitous crystalline process (UCP) Ingot 5848 - 13C. Important correlation was obtained between defect densities, cell efficiency, and diffusion length. Grain boundary substructure displayed a strong influence on the conversion efficiency of solar cells from Semix material. Quantitative microscopy measurements gave statistically significant information compared to other microanalytical techniques. A surface preparation technique to obtain proper contrast of structural defects suitable for quantimet quantitative image analyzer (QTM) analysis was perfected and is used routinely. The relationships between hole mobility and grain boundary density was determined. Mobility was measured using the van der Pauw technique, and grain boundary density was measured using quantitative microscopy technique. Mobility was found to decrease with increasing grain boundary density
Limestone As a buffering Agent in High-Concentrate Diets
In recent years, there has been evidence indicating that limestone in excess of normal requirement levels for calcium has beneficial effects for ruminants. Research at this station has shown that limestone supplementation is effective in preventing phosphatic urinary calculi formation in ruminants when fed at higher levels than normally recommended. Work at other stations has shown positive effects on weight gain from limestone added to highconcentrate diets, which was attributed to buffering action within the digestive tract. In short-term finishing periods, buffer supplementation may help ruminants adapt to high-grain diets if roughage has been their major feedstuff in the past. Graded levels of limestone were fed in this experiment to determine beneficial effects in protecting lambs from the effects of rumen acidosis during an extremely short adaptation period and to determine the optimum level of supplementation needed to improve performance during the longer term finishing period
Sodium Bentonite and Sodium Bicarbonate in High-Concentrate Beef Cattle Diets
Sodium bentonite and sodium bicarbonate alter conditions in the rumen and may offer animals protection from some of the harmful effects of rumen acidosis. The potential benefits that may be derived from use of these materials have become increasingly important as backgrounding periods are extended and finishing periods are shortened. The experiment reported herein were designed to investigate the effectiveness of these materials, individually or in combination, in the adaptation of beef cattle to high-concentrate diets
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