6,339 research outputs found
Radiation From Particles Moving in Small-Scale Magnetic Fields Created in Solid-Density Laser-Plasma Laboratory Experiments
Plasmas created by high-intensity lasers are often subject to the formation
of kinetic-streaming instabilities, such as the Weibel instability, which lead
to the spontaneous generation of high-amplitude, tangled magnetic fields. These
fields typically exist on small spatial scales, i.e. "sub-Larmor scales".
Radiation from charged particles moving through small-scale electromagnetic
(EM) turbulence has spectral characteristics distinct from both synchrotron and
cyclotron radiation, and it carries valuable information on the statistical
properties of the EM field structure and evolution. Consequently, this
radiation from laser-produced plasmas may offer insight into the underlying
electromagnetic turbulence. Here we investigate the prospects for, and
demonstrate the feasibility of, such direct radiative diagnostics for mildly
relativistic, solid-density laser plasmas produced in lab experiments.Comment: 18 pages, 2 figures, (This version corrects numerous issues.
Electron impact excitation of N IV: calculations with the DARC code and a comparison with ICFT results
There have been discussions in the recent literature regarding the accuracy
of the available electron impact excitation rates (equivalently effective
collision strengths ) for transitions in Be-like ions. In the present
paper we demonstrate, once again, that earlier results for are
indeed overestimated (by up to four orders of magnitude), for over 40\% of
transitions and over a wide range of temperatures. To do this we have performed
two sets of calculations for N~IV, with two different model sizes consisting of
166 and 238 fine-structure energy levels. As in our previous work, for the
determination of atomic structure the GRASP (General-purpose Relativistic
Atomic Structure Package) is adopted and for the scattering calculations (the
standard and parallelised versions of) the Dirac Atomic R-matrix Code ({\sc
darc}) are employed. Calculations for collision strengths and effective
collision strengths have been performed over a wide range of energy (up to
45~Ryd) and temperature (up to 2.010~K), useful for applications in
a variety of plasmas. Corresponding results for energy levels, lifetimes and
A-values for all E1, E2, M1 and M2 transitions among 238 levels of N~IV are
also reported.Comment: This paper with 5 Figs. and 8 Tables will appear in MNRAS (2016
The fate of cannibalized fundamental-plane ellipticals
Evolution and disruption of galaxies orbiting in the gravitational field of a
larger cluster galaxy are driven by three coupled mechanisms: 1) the heating
due to its time dependent motion in the primary; 2) mass loss due to the tidal
strain field; and 3) orbital decay. Previous work demonstrated that tidal
heating is effective well inside the impulse approximation limit. Not only does
the overall energy increase over previous predictions, but the work is done
deep inside the secondary galaxy, e.g. at or inside the half mass radius in
most cases. Here, these ideas applied to cannibalization of elliptical galaxies
with fundamental-plane parameters. In summary, satellites which can fall to the
center of a cluster giant by dynamical friction are evaporated by internal
heating by the time they reach the center. This suggests that true
merger-produced multiple nuclei giants should be rare. Specifically,
secondaries with mass ratios as small as 1\% on any initial orbit evaporate and
those on eccentric orbits with mass ratios as small as 0.1\% evolve
significantly and nearly evaporate in a galaxian age. Captured satellites with
mass ratios smaller than roughly 1\% have insufficient time to decay to the
center. After many accretion events, the model predicts that the merged system
has a profile similar to that of the original primary with a weak increase in
concentration.Comment: 19 pages, 10 Postscript figures, uses aaspp4.sty. Submitted to
Astrophysical Journa
An assessment of Fe XX - Fe XXII emission lines in SDO/EVE data as diagnostics for high density solar flare plasmas using EUVE stellar observations
The Extreme Ultraviolet Variability Experiment (EVE) on the Solar Dynamics
Observatory obtains extreme-ultraviolet (EUV) spectra of the full-disk Sun at a
spectral resolution of ~1 A and cadence of 10 s. Such a spectral resolution
would normally be considered to be too low for the reliable determination of
electron density (N_e) sensitive emission line intensity ratios, due to
blending. However, previous work has shown that a limited number of Fe XXI
features in the 90-60 A wavelength region of EVE do provide useful
N_e-diagnostics at relatively low flare densities (N_e ~ 10^11-10^12 cm^-3).
Here we investigate if additional highly ionised Fe line ratios in the EVE
90-160 A range may be reliably employed as N_e-diagnostics. In particular, the
potential for such diagnostics to provide density estimates for high N_e
(~10^13 cm^-3) flare plasmas is assessed. Our study employs EVE spectra for
X-class flares, combined with observations of highly active late-type stars
from the Extreme Ultraviolet Explorer (EUVE) satellite plus experimental data
for well-diagnosed tokamak plasmas, both of which are similar in wavelength
coverage and spectral resolution to those from EVE. Several ratios are
identified in EVE data which yield consistent values of electron density,
including Fe XX 113.35/121.85 and Fe XXII 114.41/135.79, with confidence in
their reliability as N_e-diagnostics provided by the EUVE and tokamak results.
These ratios also allow the determination of density in solar flare plasmas up
to values of ~10^13 cm^-3.Comment: 7 pages, 3 figures, 2 tables, MNRAS in pres
Extreme ultraviolet emission lines of Ni XII in laboratory and solar spectra
Wavelengths for emission lines arising from 3s23p5-3s3p6 and 3s23p5-3s23p43d transitions in Ni XII have been measured in extreme ultraviolet spectra of the Joint European Torus(JET) tokamak. The 3s23p5 2P1/2-3s23p4(3P)3d 2D3/2 line is found to lie at 152.90 ± 0.02 A, a significant improvement over the previous experimental determination of 152.95 ± 0.5 A. This new wavelength is in good agreement with a solar identification at 152.84 ± 0.06 A, confirming the presence of this line in the solar spectrum. The Ni XII feature at 152.15 A may be a result only of the 3s23p5 2P3/2-3s23p4(3P)3d 2D5/2 transition, rather than a blend of this line with 3s23p5 2P3/2-3s23p (3P)3d 2P1/2, as previously suggested. Unidentified emission
lines at 295.32 and 317.61 A in solar flare spectra from the Skylab mission are tentatively identified as the 3s23p5 2P3/2-3s3p6 2S1/2 and 3s23p5 2P1/2-3s3p6 2S1/2 transitions in Ni XII, which have laboratory wavelengths of 295.33 and 317.50 A, respectively. Additional support for these identifications is provided by the line intensity ratio for the solar features, which shows good agreement between theory and observation
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