5,403 research outputs found
Spatiotemporal evolution of runaway electrons from synchrotron images in Alcator C-Mod
In the Alcator C-Mod tokamak, relativistic runaway electron (RE) generation
can occur during the flattop current phase of low density, diverted plasma
discharges. Due to the high toroidal magnetic field (B = 5.4 T), RE synchrotron
radiation is measured by a wide-view camera in the visible wavelength range
(~400-900 nm). In this paper, a statistical analysis of over one thousand
camera images is performed to investigate the plasma conditions under which
synchrotron emission is observed in C-Mod. In addition, the spatiotemporal
evolution of REs during one particular discharge is explored in detail via a
thorough analysis of the distortion-corrected synchrotron images. To accurately
predict RE energies, the kinetic solver CODE [Landreman et al 2014 Comput.
Phys. Commun. 185 847-855] is used to evolve the electron momentum-space
distribution at six locations throughout the plasma: the magnetic axis and flux
surfaces q = 1, 4/3, 3/2, 2, and 3. These results, along with the
experimentally-measured magnetic topology and camera geometry, are input into
the synthetic diagnostic SOFT [Hoppe et al 2018 Nucl. Fusion 58 026032] to
simulate synchrotron emission and detection. Interesting spatial structure near
the surface q = 2 is found to coincide with the onset of a locked mode and
increased MHD activity. Furthermore, the RE density profile evolution is fit by
comparing experimental to synthetic images, providing important insight into RE
spatiotemporal dynamics
Interpretation of runaway electron synchrotron and bremsstrahlung images
The crescent spot shape observed in DIII-D runaway electron synchrotron
radiation images is shown to result from the high degree of anisotropy in the
emitted radiation, the finite spectral range of the camera and the distribution
of runaways. The finite spectral camera range is found to be particularly
important, as the radiation from the high-field side can be stronger by a
factor than the radiation from the low-field side in DIII-D. By
combining a kinetic model of the runaway dynamics with a synthetic synchrotron
diagnostic we see that physical processes not described by the kinetic model
(such as radial transport) are likely to be limiting the energy of the
runaways. We show that a population of runaways with lower dominant energies
and larger pitch-angles than those predicted by the kinetic model provide a
better match to the synchrotron measurements. Using a new synthetic
bremsstrahlung diagnostic we also simulate the view of the Gamma Ray Imager
(GRI) diagnostic used at DIII-D to resolve the spatial distribution of
runaway-generated bremsstrahlung.Comment: 21 pages, 11 figure
Some Effects of Color on Perception of Relative Motion at Night
Past research by the present authors (1951) has demonstrated that increasing the visibility of a vehicle at night decreases the time and difficulty for perception of a speed differential between two vehicles traveling in the same direction. Three of the criteria given by Luckiesh (1944), size, contrast and over-all illumination, were used as basic factors for varying the visibility. The contrast between the vehicle and background was varied by using materials with different reflection characteristics on the rear panel. The materials used reflected white light, but the availability of colored reflecting materials has made necessary this pilot study for the evaluation of the use of such materials for increasing the visibility of a vehicle at night. The data herein reported are part of a study on both color and pattern but time and space permit only presentation of the data on color here
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Extreme H isotopic anomalies in chondritic organic matter
Extract from introduction: We have conducted ionprobe (IMS6f and NanoSIMS) imaging studies of various samples for H, D, C, 14N and 15N. These will ultimately be correlated with micro-analytic techniques such as FIB/TEM or STXM/XANES. We analyzed matrix fragments from Bells (CM2), Al Rais (CR2) and Tagish Lake (unique) [2], high purity insoluble organic matter (IOM) [3] extracted from EET92042 (“EET”, CR2), Bells, Murchison (CM2), Allende (CV3), Krymka (LL3.1) and, for comparison, 3 IDPs
Influence of peptidylarginine deiminase type 4 genotype and shared epitope on clinical characteristics and autoantibody profile of rheumatoid arthritis.
Background: Recent evidence suggests that distinction
of subsets of rheumatoid arthritis (RA) depending on anticyclic
citrullinated peptide antibody (anti-CCP) status may
be helpful in distinguishing distinct aetiopathologies and in
predicting the course of disease. HLA-DRB1 shared
epitope (SE) and peptidylarginine deiminase type 4
(PADI4) genotype, both of which have been implicated in
anti-CCP generation, are assumed to be associated with
RA.
Objectives: To elucidate whether PADI4 affects the
clinical characteristics of RA, and whether it would
modulate the effect of anti-CCPs on clinical course. The
combined effect of SE and PADI4 on autoantibody profile
was also analysed.
Methods: 373 patients with RA were studied. SE,
padi4_94C.T, rheumatoid factor, anti-CCPs and antinuclear
antibodies (ANAs) were determined. Disease
severity was characterised by cumulative therapy
intensity classified into ordinal categories (CTI-1 to CTI-3)
and by Steinbrocker score.
Results: CTI was significantly associated with disease
duration, erosive disease, disease activity score (DAS) 28
and anti-CCPs. The association of anti-CCPs with CTI was
considerably influenced by padi4_94C.T genotype (C/C:
ORadj=0.93, padj=0.92; C/T: ORadj=2.92,
padj=0.093; T/T: ORadj=15.3, padj=0.002). Carriage of
padi4_94T exhibited a significant trend towards higher
Steinbrocker scores in univariate and multivariate
analyses. An association of padi4_94C.T with ANAs
was observed, with noteworthy differences depending on
SE status (SE2: ORadj=6.20, padj,0.04; SE+:
ORadj=0.36, padj=0.02) and significant heterogeneity
between the two SE strata (p=0.006).
Conclusions: PADI4 genotype in combination with anti-
CCPs and SE modulates clinical and serological characteristics
of RA
The solar abundance problem and eMSTOs in clusters
We study the impact of accretion from protoplanetary discs on stellar
evolution of AFG-type stars. We use a simplified disc model computed using the
Two-Pop-Py code that contains the growth and drift of dust particles in the
protoplanetary disc. It is used to model the accretion scenarios for a range of
physical conditions of protoplanetary discs. Two limiting cases are combined
with the evolution of stellar convective envelopes computed using the Garstec
stellar evolution code. We find that the accretion of metal-poor (gas) or
metal-rich (dust) material has a significant impact on the chemical composition
of the stellar convective envelope. As a consequence, the evolutionary track of
the star diverts from the standard scenario predicted by canonical stellar
evolution models, which assume a constant and homogeneous chemical composition
after the assembly of the star has finished. In the case of the Sun, we find a
modest impact on the solar chemical composition. Accretion of metal-poor
material indeed reduces the overall metallicity of the solar atmosphere, and it
is consistent, within the uncertainty, with the solar Z reported by Caffau et
al. (2011), but our model is not consistent with the measurement by Asplund et
al. (2009). Another effect is the change of the position of the star in the
colour-magnitude diagram. We compare our predictions to a set of open clusters
from the Gaia DR2 and show that it is possible to produce a scatter close to
the turn-off of young clusters that could contribute to explain the observed
scatter in CMDs. Detailed measurements of metallicities and abundances in the
nearby open clusters will provide a stringent observational test of our
proposed scenario.Comment: 10 pages, 7 figures, 1 table. Accepted for publication in A&
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Were Presolar Grains Destroyed by the Nebular Process Responsible for the Volatile Element Fractionation?
We present SiC abundances from a number of CM and CR chondrites using NanoSIMS raster ion imaging of acid residues. We find higher SiC abundances for CRs than previously estimated based on noble gases
Hamiltonian dynamics of extended objects
We consider a relativistic extended object described by a reparametrization
invariant local action that depends on the extrinsic curvature of the
worldvolume swept out by the object as it evolves. We provide a Hamiltonian
formulation of the dynamics of such higher derivative models which is motivated
by the ADM formulation of general relativity. The canonical momenta are
identified by looking at boundary behavior under small deformations of the
action; the relationship between the momentum conjugate to the embedding
functions and the conserved momentum density is established. The canonical
Hamiltonian is constructed explicitly; the constraints on the phase space, both
primary and secondary, are identified and the role they play in the theory
described. The multipliers implementing the primary constraints are identified
in terms of the ADM lapse and shift variables and Hamilton's equations shown to
be consistent with the Euler-Lagrange equations.Comment: 24 pages, late
Applications of Abundance Data and Requirements for Cosmochemical Modeling
Understanding the evolution of the universe from Big Bang to its present state requires an understanding of the evolution of the abundances of the elements and isotopes in galaxies, stars, the interstellar medium, the Sun and the heliosphere, planets and meteorites. Processes that change the state of the universe include Big Bang nucleosynthesis, star formation and stellar nucleosynthesis, galactic chemical evolution, propagation of cosmic rays, spallation, ionization and particle transport of interstellar material, formation of the solar system, solar wind emission and its fractionation (FIP/FIT effect), mixing processes in stellar interiors, condensation of material and subsequent geochemical fractionation. Here, we attempt to compile some major issues in cosmochemistry that can be addressed with a better knowledge of the respective element or isotope abundances. Present and future missions such as Genesis, Stardust, Interstellar Pathfinder, and Interstellar Probe, improvements of remote sensing instrumentation and experiments on extraterrestrial material such as meteorites, presolar grains, and lunar or returned planetary or cometary samples will result in an improved database of elemental and isotopic abundances. This includes the primordial abundances of D, ^3He, ^4He, and ^7Li, abundances of the heavier elements in stars and galaxies, the composition of the interstellar medium, solar wind and comets as well as the (highly) volatile elements in the solar system such as helium, nitrogen, oxygen or xenon
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