843 research outputs found
Demonstrating Universal Scaling in Quench Dynamics of a Yukawa One-Component Plasma
The Yukawa one-component plasma (OCP) is a paradigm model for describing
plasmas that contain one component of interest and one or more other components
that can be treated as a neutralizing, screening background. In appropriately
scaled units, interactions are characterized entirely by a screening parameter,
. As a result, systems of similar show the same dynamics,
regardless of the underlying parameters (e.g., density and temperature). We
demonstrate this behavior using ultracold neutral plasmas (UNP) created by
photoionizing a cold ( mK) gas. The ions in UNP systems are well
described by the Yukawa model, with the electrons providing the screening.
Creation of the plasma through photoionization can be thought of as a rapid
quench from to a final value set by the electron
density and temperature. We demonstrate experimentally that the post-quench
dynamics are universal in over a factor of 30 in density and an order
of magnitude in temperature. Results are compared with molecular dynamics
simulations. We also demonstrate that features of the post-quench kinetic
energy evolution, such as disorder-induced heating and kinetic-energy
oscillations, can be used to determine the plasma density and the electron
temperature.Comment: 10 pages, 12 figures, to be submitted to Physical Review
The fluid dynamics of swimming by jumping in copepods
Author Posting. © The Authors, 2010. This is the author's version of the work. It is posted here by permission of Royal Society for personal use, not for redistribution. The definitive version was published in Journal of the Royal Society Interface 8 (2011): 1090-1103, doi:10.1098/rsif.2010.0481.Copepods swim either continuously by vibrating their feeding appendages or erratically
by repeatedly beating their swimming legs resulting in a series of small jumps. The two
swimming modes generate different hydrodynamic disturbances and therefore expose the
swimmers differently to rheotactic predators. We developed an impulsive stresslet model to
quantify the jump-imposed flow disturbance. The predicted flow consists of two counterrotating
viscous vortex rings of similar intensity, one in the wake and one around the body of
the copepod. We showed that the entire jumping flow is spatially limited and temporally
ephemeral owing to jump-impulsiveness and viscous decay. In contrast, continuous steady
swimming generates two well-extended long-lasting momentum jets both in front of and
behind the swimmer, as suggested by the well-known steady stresslet model. Based on the
observed jump-swimming kinematics of a small copepod Oithona davisae, we further
showed that jump-swimming produces a hydrodynamic disturbance with much smaller
spatial extension and shorter temporal duration than that produced by a same-size copepod
cruising steadily at the same average translating velocity. Hence, small copepods in jumpswimming
are much less detectable by rheotactic predators. The present impulsive stresslet
model improves a previously published impulsive Stokeslet model that applies only to the
wake vortex.This work was supported by National Science Foundation grants NSF OCE-0352284 &
IOS-0718506 and an award from WHOI’s Ocean Life Institute to H.J and by grants from the
Danish Research Council for independent research and the Niels Bohr Foundation to T.K
Ion temperature evolution in an ultracold neutral plasma
We study the long-time evolution of the ion temperature in an expanding ultracold neutral plasma using spatially resolved, laser-induced-fluorescence spectroscopy. Adiabatic cooling reduces the ion temperature by an order of magnitude during the plasma expansion, to temperatures as low as 0.2 K. Cooling is limited by heat exchange between ions and the much hotter electrons. We also present evidence for an additional heating mechanism and discuss possible sources. Data are described by a model of the plasma evolution, including the effects of ion-electron heat exchange. We show that for appropriate initial conditions, the degree of Coulomb coupling of ions in the plasma increases during expansion
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Efficacy of Beauveria bassiana for Control of Lygus Bugs in Alfalfa Seed Fields
Ion holes in the hydrodynamic regime in ultracold neutral plasmas
We describe the creation of localized density perturbations, or ion holes, in an ultracold neutral
plasma in the hydrodynamic regime, and show that the holes propagate at the local ion acoustic wave
speed. We also observe the process of hole splitting, which results from the formation of a density
depletion initially at rest in the plasma. One-dimensional, two-fluid hydrodynamic simulations
describe the results well. Measurements of the ion velocity distribution also show the effects of the
ion hole and confirm the hydrodynamic conditions in the plasma
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Sublethal Effects of Beauveria bassiana on Lygus hesperus Feeding and Oviposition
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Efficacy of Beauveria bassiana for Control of Lygus Bugs in Alfalfa Seed Fields
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Experimental Evaluation of the Radiation Protection Afforded by Typical Oak Ridge Homes Against Distributed Sources
The protection afforded against simulated fall-out radiation has been evaluated for several typical homes in the Oak Ridge area. Nine houses were chosen to represent a variety of construction materials, topographical conditions and sizes; they included three types of Oak Ridge Cemesto houses, one concrete-block house with a basement fall-out shelter, and two wood-frame houses. The protection factor (ratio of open-field exposure dose rate to exposure dose rate in the house) in all these houses ranged from 2 to 5 on the main floor and from 5 to 30 in the basements, except in the fall-out shelter, where the protection factor was greater than 100. The analysis showed that sloping lots. common to Oak Ridge, do not appreciably affect the protection factor for the main floor. Owing to the generally increased exposure of the basement walls on such lots, the protection factors in the basements were typically lower than in similar basements built on level lots. (auth
Azimuthal clumping instabilities in a ZZ-pinch wire array
A simple model is constructed to evaluate the temporal evolution of azimuthal clumping instabilities in a cylindrical array of current-carrying wires. An analytic scaling law is derived, which shows that randomly seeded perturbations evolve at the rate of the fastest unstable mode, almost from the start. This instability is entirely analogous to the Jeans instability in a self-gravitating disk, where the mutual attraction of gravity is replaced by the mutual attraction among the current-carrying wires.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87765/2/052701_1.pd
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