1,207 research outputs found
Dynamics of charge-displacement channeling in intense laser-plasma interactions
The dynamics of transient electric fields generated by the interaction of
high intensity laser pulses with underdense plasmas has been studied
experimentally with the proton projection imaging technique. The formation of a
charged channel, the propagation of its front edge and the late electric field
evolution have been characterised with high temporal and spatial resolution.
Particle-in-cell simulations and an electrostatic, ponderomotive model
reproduce the experimental features and trace them back to the ponderomotive
expulsion of electrons and the subsequent ion acceleration.Comment: 5 figures, accepted for publication in New Journal of Physic
On the Hydrodynamic Interaction of Shock Waves with Interstellar Clouds. II. The Effect of Smooth Cloud Boundaries on Cloud Destruction and Cloud Turbulence
The effect of smooth cloud boundaries on the interaction of steady planar
shock waves with interstellar clouds is studied using a high-resolution local
AMR technique with a second-order accurate axisymmetric Godunov hydrodynamic
scheme. A 3D calculation is also done to confirm the results of the 2D ones. We
consider an initially spherical cloud whose density distribution is flat near
the cloud center and has a power-law profile in the cloud envelope. When an
incident shock is transmitted into a smooth cloud, velocity gradients in the
cloud envelope steepen the smooth density profile at the upstream side,
resulting in a sharp density jump having an arc-like shape. Such a ``slip
surface'' forms immediately when a shock strikes a cloud with a sharp boundary.
For smoother boundaries, the formation of slip surface and therefore the onset
of hydrodynamic instabilities are delayed. Since the slip surface is subject to
the Kelvin-Helmholtz and Rayleigh-Taylor instabilities, the shocked cloud is
eventually destroyed in cloud crushing times. After complete cloud
destruction, small blobs formed by fragmentation due to hydrodynamic
instabilities have significant velocity dispersions of the order of 0.1 ,
where is the shock velocity in the ambient medium. This suggests that
turbulent motions generated by shock-cloud interaction are directly associated
with cloud destruction. The interaction of a shock with a cold HI cloud should
lead to the production of a spray of small HI shreds, which could be related to
the small cold clouds recently observed by Stanimirovic & Heiles (2005). The
linewidth-size relation obtained from our 3D simulation is found to be
time-dependent. A possibility for gravitational instability triggered by shock
compression is also discussed.Comment: 62 pages, 16 figures, submitted to Ap
Bias in exponential and power function fits due to noise: Comment on Myung, Kim, and Pitt
Nonlinear Aeroelastic Framework Based on Vortex-Lattice Method and Corotational Shell Finite Element
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/97077/1/AIAA2012-1976.pd
Is Thermal Instability Significant in Turbulent Galactic Gas?
We investigate numerically the role of thermal instability (TI) as a
generator of density structures in the interstellar medium (ISM), both by
itself and in the context of a globally turbulent medium. Simulations of the
instability alone show that the condenstion process which forms a dense phase
(``clouds'') is highly dynamical, and that the boundaries of the clouds are
accretion shocks, rather than static density discontinuities. The density
histograms (PDFs) of these runs exhibit either bimodal shapes or a single peak
at low densities plus a slope change at high densities. Final static situations
may be established, but the equilibrium is very fragile: small density
fluctuations in the warm phase require large variations in the density of the
cold phase, probably inducing shocks into the clouds. This result suggests that
such configurations are highly unlikely. Simulations including turbulent
forcing show that large- scale forcing is incapable of erasing the signature of
the TI in the density PDFs, but small-scale, stellar-like forcing causes
erasure of the signature of the instability. However, these simulations do not
reach stationary regimes, TI driving an ever-increasing star formation rate.
Simulations including magnetic fields, self-gravity and the Coriolis force show
no significant difference between the PDFs of stable and unstable cases, and
reach stationary regimes, suggesting that the combination of the stellar
forcing and the extra effective pressure provided by the magnetic field and the
Coriolis force overwhelm TI as a density-structure generator in the ISM. We
emphasize that a multi-modal temperature PDF is not necessarily an indication
of a multi-phase medium, which must contain clearly distinct thermal
equilibrium phases.Comment: 18 pages, 11 figures. Submitted to Ap
Utilizing ARC EMCS Seedling Cassettes as Highly Versatile Miniature Growth Chambers for Model Organism Experiments
The aim of our ground testing was to demonstrate the capability of safely putting specific model organisms into dehydrated stasis, and to later rehydrate and successfully grow them inside flight proven ARC EMCS seedling cassettes. The ARC EMCS seedling cassettes were originally developed to support seedling growth during space flight. The seeds are attached to a solid substrate, launched dry, and then rehydrated in a small volume of media on orbit to initiate the experiment. We hypothesized that the same seedling cassettes should be capable of acting as culture chambers for a wide range of organisms with minimal or no modification. The ability to safely preserve live organisms in a dehydrated state allows for on orbit experiments to be conducted at the best time for crew operations and more importantly provides a tightly controlled physiologically relevant growth experiment with specific environmental parameters. Thus, we performed a series of ground tests that involved growing the organisms, preparing them for dehydration on gridded Polyether Sulfone (PES) membranes, dry storage at ambient temperatures for varying periods of time, followed by rehydration. Inside the culture cassettes, the PES membranes were mounted above blotters containing dehydrated growth media. These were mounted on stainless steel bases and sealed with plastic covers that have permeable membrane covered ports for gas exchange. The results showed we were able to demonstrate acceptable normal growth of C.elegans (nematodes), E.coli (bacteria), S.cerevisiae (yeast), Polytrichum (moss) spores and protonemata, C.thalictroides (fern), D.discoideum (amoeba), and H.dujardini (tardigrades). All organisms showed acceptable growth and rehydration in both petri dishes and culture cassettes initially, and after various time lengths of dehydration. At the end of on orbit ISS European Modular Cultivation System experiments the cassettes could be frozen at ultra-low temperatures, refrigerated, or chemically preserved before being returned to Earth for analyses. Our results suggest that with protocol modifications and future verification testing we can utilize the versatile EMCS to conduct tightly controlled experiments inside our culture cassettes for a wide variety of organisms. These physiological experiments would be designed to answer questions at the molecular level about the specific stress responses of space flight
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