434 research outputs found
Direct measurement of the electron density of extended femtosecond laser pulse-induced filaments
We present direct time- and space- resolved measurements of the electron
density of femtosecond laser pulse-induced plasma filaments. The dominant
nonlinearity responsible for extended atmospheric filaments is shown to be
field-induced rotation of air molecules.Comment: 12 pages, 5 figure
Heating of Micro-protrusions in Accelerating Structures
The thermal and field emission of electrons from protrusions on metal
surfaces is a possible limiting factor on the performance and operation of
high-gradient room temperature accelerator structures. We present here the
results of extensive numerical simulations of electrical and thermal behavior
of protrusions. We unify the thermal and field emission in the same numerical
framework, describe bounds for the emission current and geometric enhancement,
then we calculate the Nottingham and Joule heating terms and solve the heat
equation to characterize the thermal evolution of emitters under RF electric
field. Our findings suggest that, heating is entirely due to the Nottingham
effect, that thermal runaway scenarios are not likely, and that high RF
frequency causes smaller swings in temperature and cooler tips. We build a
phenomenological model to account for the effect of space charge and show that
space charge eliminates the possibility of tip melting, although near melting
temperatures reached.Comment: 8 pages, 10 figure
Limitations in Predicting Radiation-Induced Pharmaceutical Instability during Long-Duration Spaceflight
As human spaceflight seeks to expand beyond low-Earth orbit, NASA and its
international partners face numerous challenges related to ensuring the safety
of their astronauts, including the need to provide a safe and effective
pharmacy for long-duration spaceflight. Historical missions have relied upon
frequent resupply of onboard pharmaceuticals; as a result, there has been
little study into the effects of long-term exposure of pharmaceuticals to the
space environment. Of particular concern are the long-term effects of space
radiation on drug stability, especially as missions venture away from the
protective proximity of the Earth. Here we highlight the risk of space
radiation to pharmaceuticals during exploration spaceflight, identifying the
limitations of current understanding. We further seek to identify ways in which
these limitations could be addressed through dedicated research efforts aimed
towards the rapid development of an effective pharmacy for future spaceflight
endeavors.Comment: in press, Nature Microgravit
Dynamics and Pattern Formation in Large Systems of Spatially-Coupled Oscillators with Finite Response Times
We consider systems of many spatially distributed phase oscillators that
interact with their neighbors. Each oscillator is allowed to have a different
natural frequency, as well as a different response time to the signals it
receives from other oscillators in its neighborhood. Using the ansatz of Ott
and Antonsen (Ref. \cite{OA1}) and adopting a strategy similar to that employed
in the recent work of Laing (Ref. \cite{Laing2}), we reduce the microscopic
dynamics of these systems to a macroscopic partial-differential-equation
description. Using this macroscopic formulation, we numerically find that
finite oscillator response time leads to interesting spatio-temporal dynamical
behaviors including propagating fronts, spots, target patterns, chimerae,
spiral waves, etc., and we study interactions and evolutionary behaviors of
these spatio-temporal patterns
Inducing Barbero-Immirzi Connections along SU(2)-reductions of Bundles on Spacetime
We shall present here a general apt technique to induce connections along
bundle reductions which is different from the standard restriction. This
clarifies and generalizes the standard procedure to define Barbero-Immirzi (BI)
connection, though on spacetime. The standard spacial BI connection used in LQG
is then obtained by its spacetime version by standard restriction. The general
prescription to define such a reduced connection is interesting from a
mathematical viewpoint and it allows a general and direct control on
transformation laws of the induced object. Moreover, unlike what happens by
using standard restriction, we shall show that once a bundle reduction is
given, then any connection induces a reduced connection with no constraint on
the original holonomy as it happens when connections are simply restricted.Comment: 6 pages, some comments adde
Echoes and revival echoes in systems of anharmonically confined atoms
We study echoes and what we call 'revival echoes' for a collection of atoms
that are described by a single quantum wavefunction and are confined in a
weakly anharmonic trap. The echoes and revival echoes are induced by applying
two, successive temporally localized potential perturbations to the confining
potential, one at time , and a smaller one at time . Pulse-like
responses in the expectation value of position are predicted at $t
\approx n\tau$ ($n=2,3,...$) and are particularly evident at $t \approx 2\tau$.
A novel result of our study is the finding of 'revival echoes'. Revivals (but
not echoes) occur even if the second perturbation is absent. In particular, in
the absence of the second perturbation, the response to the first perturbation
dies away, but then reassembles, producing a response at revival times $mT_x$
($m=1,2,...$). Including the second perturbation at $t=\tau$, we find
temporally localized responses, revival echoes, both before and after $t\approx
mT_x$, e.g., at $t\approx m T_x-n \tau$ (pre-revival echoes) and at $t\approx
mT_x+n\tau$, (post-revival echoes), where $m$ and $n$ are $1,2,...$ . Depending
on the form of the perturbations, the 'principal' revival echoes at $t \approx
T_x \pm \tau$ can be much larger than the echo at $t \approx 2\tau$. We develop
a perturbative model for these phenomena, and compare its predictions to the
numerical solutions of the time-dependent Schr\"odinger Equation. The scaling
of the size of the various echoes and revival echoes as a function of the
symmetry and size of the perturbations applied at $t=0$ and $t=\tau$ is
investigated. We also study the presence of revivals and revival echoes in
higher moments of position, , , and the effect of atom-atom
interactions on these phenomena.Comment: 33 pages, 13 figures, corrected typos and added reference
Scalar Decay in Chaotic Mixing
I review the local theory of mixing, which focuses on infinitesimal blobs of
scalar being advected and stretched by a random velocity field. An advantage of
this theory is that it provides elegant analytical results. A disadvantage is
that it is highly idealised. Nevertheless, it provides insight into the
mechanism of chaotic mixing and the effect of random fluctuations on the rate
of decay of the concentration field of a passive scalar.Comment: 35 pages, 15 figures. Springer-Verlag conference style svmult.cls
(included). Published in "Transport in Geophysical Flows: Ten Years After,"
Proceedings of the Grand Combin Summer School, 14-24 June 2004, Valle
d'Aosta, Italy. Fixed some typo
Exact Results for the Kuramoto Model with a Bimodal Frequency Distribution
We analyze a large system of globally coupled phase oscillators whose natural
frequencies are bimodally distributed. The dynamics of this system has been the
subject of long-standing interest. In 1984 Kuramoto proposed several
conjectures about its behavior; ten years later, Crawford obtained the first
analytical results by means of a local center manifold calculation.
Nevertheless, many questions have remained open, especially about the
possibility of global bifurcations. Here we derive the system's complete
stability diagram for the special case where the bimodal distribution consists
of two equally weighted Lorentzians. Using an ansatz recently discovered by Ott
and Antonsen, we show that in this case the infinite-dimensional problem
reduces exactly to a flow in four dimensions. Depending on the parameters and
initial conditions, the long-term dynamics evolves to one of three states:
incoherence, where all the oscillators are desynchronized; partial synchrony,
where a macroscopic group of phase-locked oscillators coexists with a sea of
desynchronized ones; and a standing wave state, where two counter-rotating
groups of phase-locked oscillators emerge. Analytical results are presented for
the bifurcation boundaries between these states. Similar results are also
obtained for the case in which the bimodal distribution is given by the sum of
two Gaussians.Comment: 28 pages, 7 figures; submitted to Phys. Rev. E Added comment
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