19,891 research outputs found
Emulating a flexible space structure: Modeling
Control Dynamics, in conjunction with Marshall Space Flight Center, has participated in the modeling and testing of Flexible Space Structures. Through the series of configurations tested and the many techniques used for collecting, analyzing, and modeling the data, many valuable insights have been gained and important lessons learned. This paper discusses the background of the Large Space Structure program, Control Dynamics' involvement in testing and modeling of the configurations (especially the Active Control Technique Evaluation for Spacecraft (ACES) configuration), the results from these two processes, and insights gained from this work
Affective bias and current, past and future adolescent depression: A familial high risk study.
Affective bias is a common feature of depressive disorder. However, a lack of longitudinal studies means that the temporal relationship between affective bias and depression is not well understood. One group where studies of affective bias may be particularly warranted is the adolescent offspring of depressed parents, given observations of high rates of depression and a severe and impairing course of disorder in this group
Decoherence Effects in Reactive Scattering
Decoherence effects on quantum and classical dynamics in reactive scattering
are examined using a Caldeira-Leggett type model. Through a study of dynamics
of the collinear H+H2 reaction and the transmission over simple one-dimensional
barrier potentials, we show that decoherence leads to improved agreement
between quantum and classical reaction and transmission probabilities,
primarily by increasing the energy dispersion in a well defined way. Increased
potential nonlinearity is seen to require larger decoherence in order to attain
comparable quantum-classical agreement.Comment: 25 pages, 6 figures, to be published in J. Chem. Phy
Stochastic modelling of intermittent scrape-off layer plasma fluctuations
Single-point measurements of fluctuations in the scrape-off layer of
magnetized plasmas are generally found to be dominated by large-amplitude
bursts which are associated with radial motion of blob-like structures. A
stochastic model for these fluctuations is presented, with the plasma density
given by a random sequence of bursts with a fixed wave form. Under very general
conditions, this model predicts a parabolic relation between the skewness and
kurtosis moments of the plasma fluctuations. In the case of exponentially
distributed burst amplitudes and waiting times, the probability density
function for the fluctuation amplitudes is shown to be a Gamma distribution
with the scale parameter given by the average burst amplitude and the shape
parameter given by the ratio of the burst duration and waiting times.Comment: 11 pages, 1 figur
Placing Confidence Limits on Polarization Measurements
The determination of the true source polarization given a set of measurements
is complicated by the requirement that the polarization always be positive.
This positive bias also hinders construction of upper limits, uncertainties,
and confidence regions, especially at low signal-to-noise levels. We generate
the likelihood function for linear polarization measurements and use it to
create confidence regions and upper limits. This is accomplished by integrating
the likelihood function over the true polarization (parameter space), rather
than the measured polarization (data space). These regions are valid for both
low and high signal-to-noise measurements.Comment: 8 pages, 3 figures, 1 table, submitted to PAS
Strong Coupling Expansions for Antiferromagnetic Heisenberg S=1/2 Ladders
The properties of antiferromagnetic Heisenberg ladders with
2, 3, and 4 chains are expanded in the ratio of the intra- and interchain
coupling constants. A simple mapping procedure is introduced to relate the 4
and 2-chain ladders which holds down to moderate values of the expansion
parameters. A second order calculation of the spin gap to the lowest triplet
excitation in the 2- and 4-chain ladders is found to be quite accurate even at
the isotropic point where the couplings are equal. Similar expansions and
mapping procedures are presented for the 3-chain ladders which are in the same
universality class as single chains.Comment: 10 physical pages, uuencoded compressed PostScript file including 12
figures, ETH-TH/942
Probabilistic state preparation of a single molecular ion by projection measurement
We show how to prepare a single molecular ion in a specific internal quantum
state in a situation where the molecule is trapped and sympathetically cooled
by an atomic ion and where its internal degrees of freedom are initially in
thermal equilibrium with the surroundings. The scheme is based on conditional
creation of correlation between the internal state of the molecule and the
translational state of the collective motion of the two ions, followed by a
projection measurement of this collective mode by atomic ion shelving
techniques. State preparation in a large number of internal states is possible.Comment: 4 pages, 2 figures, 2 table
Coherent dynamics of photoinduced nucleation processes
We study the dynamics of initial nucleation processes of photoinduced
structural change of molecular crystals. In order to describe the nonadiabatic
transition in each molecule, we employ a model of localized electrons coupled
with a fully quantized phonon mode, and the time-dependent Schr\"odinger
equation for the model is numerically solved. We found a minimal model to
describe the nucleation induced by injection of an excited state of a single
molecule in which multiple types of intermolecular interactions are required.
In this model coherently driven molecular distortion plays an important role in
the successive conversion of electronic states which leads to photoinduced
cooperative phenomena.Comment: 14 pages, 5 figure
Coherent Control of Ultracold Collisions with Chirped Light: Direction Matters
We demonstrate the ability to coherently control ultracold atomic Rb
collisions using frequency-chirped light on the nanosecond time scale. For
certain center frequencies of the chirp, the rate of inelastic trap-loss
collisions induced by negatively chirped light is dramatically suppressed
compared to the case of a positive chirp. We attribute this to a fundamental
asymmetry in the system: an excited wavepacket always moves inward on the
attractive molecular potential. For a positive chirp, the resonance condition
moves outward in time, while for a negative chirp, it moves inward, in the same
direction as the excited wavepacket; this allows multiple interactions between
the wavepacket and the light, enabling the wavepacket to be returned coherently
to the ground state. Classical and quantum calculations support this
interpretation
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