1,086 research outputs found
Phase dependence of Thomson scattering in an ultraintense laser field
The Thomson scattering spectra of an electron by an ultraintense laser field are computed. It is found that the electron orbit, and therefore its nonlinear Thomson scattering spectra, depend critically on the amplitude of the ultraintense laser field and on the phase at which the electron sees the laser electric field. Contrary to some customary notions, the Thomson scattering spectra, in general, do not occur at integer multiples of the laser frequency and the maximum frequency is proportional to the first instead of the third power of the electric field strength for the case of an ultraintense laser. The implications of these findings are discussed. © 2002 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69907/2/PHPAEN-9-10-4325-1.pd
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
The Spectral Types of White Dwarfs in Messier 4
We present the spectra of 24 white dwarfs in the direction of the globular
cluster Messier 4 obtained with the Keck/LRIS and Gemini/GMOS spectrographs.
Determining the spectral types of the stars in this sample, we find 24 type DA
and 0 type DB (i.e., atmospheres dominated by hydrogen and helium
respectively). Assuming the ratio of DA/DB observed in the field with effective
temperature between 15,000 - 25,000 K, i.e., 4.2:1, holds for the cluster
environment, the chance of finding no DBs in our sample due simply to
statistical fluctuations is only 6 X 10^(-3). The spectral types of the ~100
white dwarfs previously identified in open clusters indicate that DB formation
is strongly suppressed in that environment. Furthermore, all the ~10 white
dwarfs previously identified in other globular clusters are exclusively type
DA. In the context of these two facts, this finding suggests that DB formation
is suppressed in the cluster environment in general. Though no satisfactory
explanation for this phenomenon exists, we discuss several possibilities.Comment: Accepted for Publication in Astrophys. J. 11 pages including 4
figures and 2 tables (journal format
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
The Quasar SDSS J105041.35+345631.3: Black Hole Recoil or Extreme Double-Peaked Emitter?
The quasar SDSS J105041.35+345631.3 (z = 0.272) has broad emission lines
blueshifted by 3500 km/s relative to the narrow lines and the host galaxy. Such
an object may be a candidate for a recoiling supermassive black hole, binary
black hole, a superposition of two objects, or an unusual geometry for the
broad emission-line region. The absence of narrow lines at the broad line
redshift argues against superposition. New Keck spectra of J1050+3546 place
tight constraints on the binary model. The combination of large velocity shift
and symmetrical H-beta profile, as well as aspects of the narrow line spectrum,
make J1050+3546 an interesting candidate for black hole recoil. Other aspects
of the spectrum, however, suggest that the object is most likely an extreme
case of a ``double-peaked emitter.'' We discuss possible observational tests to
determine the true nature of this exceptional object.Comment: 5 pages, 2 figures, LaTeX; substantial revision
Imaging the evolution of an ultracold strontium Rydberg gas
Clouds of ultracold strontium 5s48s1S0 or 5s47d1D2 Rydberg atoms are created by two-photon excitation of laser-cooled 5s21S0 atoms. The spontaneous evolution of the cloud of low orbital angular momentum (low-â„“) Rydberg states towards an ultracold neutral plasma is observed by imaging resonant light scattered from core ions, a technique that provides both spatial and temporal resolution. Evolution is observed to be faster for the S states, which display isotropic attractive interactions, than for the D states, which exhibit anisotropic, principally repulsive interactions. Immersion of the atoms in a dilute ultracold neutral plasma speeds up the evolution and allows the number of Rydberg atoms initially created to be determined
Turbulent Friction in Rough Pipes and the Energy Spectrum of the Phenomenological Theory
The classical experiments on turbulent friction in rough pipes were performed
by J. Nikuradse in the 1930's. Seventy years later, they continue to defy
theory. Here we model Nikuradse's experiments using the phenomenological theory
of Kolmog\'orov, a theory that is widely thought to be applicable only to
highly idealized flows. Our results include both the empirical scalings of
Blasius and Strickler, and are otherwise in minute qualitative agreement with
the experiments; they suggest that the phenomenological theory may be relevant
to other flows of practical interest; and they unveil the existence of close
ties between two milestones of experimental and theoretical turbulence.Comment: Accepted for publication in PRL; 4 pages, 4 figures; revised versio
Caterpillar structures in single-wire Z-pinch experiments
A series of experiments have been performed on single-wire Z pinches (1–2 kA, 20 kV, pulse length 500 ns; Al, Ag, W, or Cu wire of diameter 7.5–50 μm, length 2.5 cm). Excimer laser absorption photographs show expansion of metallic plasmas on a time scale of order 100 ns. The edge of this plasma plume begins to develop structures resembling a caterpillar only after the current pulse reaches its peak value. The growth of these caterpillar structures is shown to be consistent with the Rayleigh–Taylor instability of the decelerating plasma plume front at the later stage of the current pulse. © 2003 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71205/2/APPLAB-83-24-4915-1.pd
Resilience in Virginia: Outlook 2021
VCPC\u27s October webinar included an update on Virginia’s participation in the Regional Greenhouse Gas Initiative and the implementation of Executive Order 24; remarks from the Speaker of the House of Delegates, Eileen Filler -Corn, and a legislative panel discussed resilience issues for the upcoming 2021 Virginia General Assembly session. VCPC was honored to host Governor Ralph Northam for Closing Remarks
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