573 research outputs found
A phase-shift-periodic parallel boundary condition for low-magnetic-shear scenarios
We formulate a generalized periodic boundary condition as a limit of the
standard twist-and-shift parallel boundary condition that is suitable for
simulations of plasmas with low magnetic shear. This is done by applying a
phase shift in the binormal direction when crossing the parallel boundary.
While this phase shift can be set to zero without loss of generality in the
local flux-tube limit when employing the twist-and-shift boundary condition, we
show that this is not the most general case when employing periodic parallel
boundaries, and may not even be the most desirable. A non-zero phase shift can
be used to avoid the convective cells that plague simulations of the
three-dimensional Hasegawa-Wakatani system, and is shown to have measurable
effects in periodic low-magnetic-shear gyrokinetic simulations. We propose a
numerical program where a sampling of periodic simulations at random
pseudo-irrational flux surfaces are used to determine physical observables in a
statistical sense. This approach can serve as an alternative to applying the
twist-and-shift boundary condition to low-magnetic-shear scenarios which, while
more straightforward, can be computationally demanding.Comment: 4 figures, 1 tabl
Electronic band gap reduction and intense luminescence in Co and Mn ion-implanted SiO
Cobalt and manganese ions are implanted into SiO over a wide range of
concentrations. For low concentrations, the Co atoms occupy interstitial
locations, coordinated with oxygen, while metallic Co clusters form at higher
implantation concentrations. For all concentrations studied here, Mn ions
remain in interstitial locations and do not cluster. Using resonant x-ray
emission spectroscopy and Anderson impurity model calculations, we determine
the strength of the covalent interaction between the interstitial ions and the
SiO valence band, finding it comparable to Mn and Co monoxides. Further, we
find an increasing reduction in the SiO electronic band gap for increasing
implantation concentration, due primarily to the introduction of Mn- and
Co-derived conduction band states. We also observe a strong increase in a band
of x-ray stimulated luminescence at 2.75 eV after implantation, attributed to
oxygen deficient centers formed during implantation.Comment: 8 pages, 6 figure
Glacial geomorphology of the northern Kivalliq region, Nunavut, Canada, with an emphasis on meltwater drainage systems
This paper presents a glacial geomorphological map of glacial lineations, ribbed terrain, moraines, meltwater channels (subglacial and ice-marginal/proglacial), eskers, glaciofluvial deposits, ice-contact outwash fans and deltas and abandoned shorelines on the bed of the former Laurentide Ice Sheet in northern Canada. Mapping was compiled from satellite imagery and digital elevation data and landforms were digitised directly into a Geographical Information System. The map reveals a complex glacial history characterised by multiple ice-flow events, including fast-flowing ice-streams. Moraines record a series of pauses or re-advances during overall SE retreat towards the Keewatin Ice Divide. The distribution of subglacial meltwater landforms indicates that several distinctive scales and modes of drainage system operated beneath the retreating ice sheet. This includes a large (>100 km) integrated network of meltwater channels, eskers, ice-contact outwash fans and deltas and glaciofluvial deposits that originates at the northern edge of Aberdeen Lake. The map comprises zone 66 of the Canadian National Topographic System, which encompasses an area of 160,000 km2. It is presented at a scale of 1:500,000 and is designed to be printed at A0 size
Fluctuation dynamo in a weakly collisional plasma
The turbulent amplification of cosmic magnetic fields depends upon the
material properties of the host plasma. In many hot, dilute astrophysical
systems, such as the intracluster medium (ICM) of galaxy clusters, the rarity
of particle--particle collisions allows departures from local thermodynamic
equilibrium. These departures exert anisotropic viscous stresses on the plasma
motions that inhibit their ability to stretch magnetic-field lines. We present
a numerical study of the fluctuation dynamo in a weakly collisional plasma
using magnetohydrodynamic (MHD) equations endowed with a field-parallel viscous
(Braginskii) stress. When the stress is limited to values consistent with a
pressure anisotropy regulated by firehose and mirror instabilities, the
Braginskii-MHD dynamo largely resembles its MHD counterpart. If instead the
parallel viscous stress is left unabated -- a situation relevant to recent
kinetic simulations of the fluctuation dynamo and to the early stages of the
dynamo in a magnetized ICM -- the dynamo changes its character, amplifying the
magnetic field while exhibiting many characteristics of the saturated state of
the large-Prandtl-number () MHD dynamo. We construct an
analytic model for the Braginskii-MHD dynamo in this regime, which successfully
matches magnetic-energy spectra. A prediction of this model, confirmed by our
simulations, is that a Braginskii-MHD plasma without pressure-anisotropy
limiters will not support a dynamo if the ratio of perpendicular and parallel
viscosities is too small. This ratio reflects the relative allowed rates of
field-line stretching and mixing, the latter of which promotes resistive
dissipation of the magnetic field. In all cases that do exhibit a dynamo, the
generated magnetic field is organized into folds that persist into the
saturated state and bias the chaotic flow to acquire a scale-dependent spectral
anisotropy.Comment: 62 pages, 25 figures, 1 table, accepted for publication to Journal of
Plasma Physic
Seismological structure of the 1.8 Ga Trans-Hudson Orogen of North America
Precambrian tectonic processes are debated: what was the nature and scale of orogenic events on the younger, hotter, and more ductile Earth? Northern Hudson Bay records the Paleoproterozoic collision between the Western Churchill and Superior plates—the ∼1.8 Ga Trans-Hudson Orogeny (THO)—and is an ideal locality to study Precambrian tectonic structure. Integrated field, geochronological, and thermobarometric studies suggest that the THO was comparable to the present-day Himalayan-Karakoram-Tibet Orogen (HKTO). However, detailed understanding of the deep crustal architecture of the THO, and how it compares to that of the evolving HKTO, is lacking. The joint inversion of receiver functions and surface wave data provides new Moho depth estimates and shear velocity models for the crust and uppermost mantle of the THO. Most of the Archean crust is relatively thin (∼39 km) and structurally simple, with a sharp Moho; upper-crustal wave speed variations are attributed to postformation events. However, the Quebec-Baffin segment of the THO has a deeper Moho (∼45 km) and a more complex crustal structure. Observations show some similarity to recent models, computed using the same methods, of the HKTO crust. Based on Moho character, present-day crustal thickness, and metamorphic grade, we support the view that southern Baffin Island experienced thickening during the THO of a similar magnitude and width to present-day Tibet. Fast seismic velocities at >10 km below southern Baffin Island may be the result of partial eclogitization of the lower crust during the THO, as is currently thought to be happening in Tibet
Turbulent transport of impurities in 3D devices
A large diffusive turbulent contribution to the radial impurity transport in
Wendelstein 7-X (W7-X) plasmas has been experimentally inferred during the
first campaigns and numerically confirmed by means of gyrokinetic simulations
with the code stella. In general, the absence of strong impurity accumulation
during the initial W7-X campaigns is attributed to this diffusive term. In the
present work the diffusive contribution is also calculated in other stellarator
plasmas. In particular, the diffusion (D) and convection (V) coefficients of
carbon and iron impurities produced by ion-temperature-gradient (ITG)
turbulence are obtained for W7-X, LHD, TJ-II and NCSX. The results show that,
although the size of D and V can differ across the four devices, inward
convection is found for all of them. For W7-X, TJ-II and NCSX the two
coefficients are comparable and the turbulent peaking factor is surprisingly
similar. In LHD, appreciably weaker diffusive and convective impurity transport
and significantly larger turbulent peaking factor are predicted. All this
suggests that ITG turbulence, although not strongly, would lead to negative
impurity density gradients in stellarators. Then, considering mixed ITG/Trapped
Electron Mode (TEM) turbulence for the specific case of W7-X, it has been
quantitatively assessed to what degree pellet fueled reduced turbulence
scenarios feature reduced turbulent transport of impurities as well. The
results for trace iron impurities show that, although their turbulent transport
is not entirely suppressed, a significant reduction of V and a stronger
decrease of D are found. Although the diffusion is still above neoclassical
levels, the neoclassical convection would gain under such conditions a greater
specific weight on the dynamics of impurities in comparison with standard ECRH
scenarios.Comment: 16 pages, 8 figure
IODP workshop: developing scientific drilling proposals for the Argentina Passive Volcanic Continental Margin (APVCM) – basin evolution, deep biosphere, hydrates, sediment dynamics and ocean evolution
The Argentine margin contains important sedimentological, paleontological and chemical records of regional and local tectonic evolution, sea level, climate evolution and ocean circulation since the opening of the South Atlantic in the Late Jurassic–Early Cretaceous as well as the present-day results of post-depositional chemical and biological alteration. Despite its important location, which underlies the exchange of southern- and northern-sourced water masses, the Argentine margin has not been investigated in detail using scientific drilling techniques, perhaps because the margin has the reputation of being erosional. However, a number of papers published since 2009 have reported new high-resolution and/or multichannel seismic surveys, often combined with multi-beam bathymetric data, which show the common occurrence of layered sediments and prominent sediment drifts on the Argentine and adjacent Uruguayan margins. There has also been significant progress in studying the climatic records in surficial and near-surface sediments recovered in sediment cores from the Argentine margin.
Encouraged by these recent results, our 3.5-day IODP (International Ocean Discovery Program) workshop in Buenos Aires (8–11 September 2015) focused on opportunities for scientific drilling on the Atlantic margin of Argentina, which lies beneath a key portion of the global ocean conveyor belt of thermohaline circulation. Significant opportunities exist to study the tectonic evolution, paleoceanography and stratigraphy, sedimentology, and biosphere and geochemistry of this margin.Facultad de Ciencias Naturales y Muse
Sismoestratigrafía y evolución cenozoica de un sector de las terrazas Nágera y Perito Moreno, Margen Continental Patagónico.
El Margen Continental Patagónico (MCP) es, desde un punto de vista geológico, la porción del Margen Continental Argentino Pasivo Volcánico (MCAPV) ubicada al sur de la Zona de la Fractura Colorado. Esta zona del margen se encuentra influenciada desde el límite Eoceno-Oligoceno, por la circulación termohalina de las corrientes de agua profundas generadas en el sector antártico. La erosión y depositación producida por estas corrientes dio origen a la formación de cuatro extensas terrazas contorníticas que cubren una gran parte del MCP. El objetivo de este trabajo es aportar nuevos conocimientos sobre las dos terrazas más occi- dentales, la Terraza Nágera (TN, ~600 a 1000 m de profundidad) y la Terra- za Perito Moreno (TPM, ~1000 a 1500 m de profundidad). En estas terrazas la sismoestratigrafía y los procesos asociados a su evolución no han sido estudiados en forma exhaustiva. Por tal motivo, los trabajos realizados en la zona se refieren a las edades de las unidades sísmicas en forma especulativa o proponen una sola unidad denominada “post-rift indefinido”. Tras el análisis integrado de un perfil sísmico de mediana resolución y gran pene- tración, y perfiles sísmicos de alta resolución y baja penetración, se han definido una serie de unidades sísmicas, las cuales fueron correlacionadas con esquemas sismoestratigráficos propuestos previamente para el MCAPV, e información bioes- tratigráfica publicada. De este modo se presenta en este trabajo un esquema sismoestratigráfico novedoso para la parte más boreal de la TN y la TPM, el cual se acopla de forma armoniosa a los modelos sismoestratigráficos propuestos para el resto del MCAPV.
A partir de las unidades sísmicas definidas se proponen cuatro etapas evolutivas para el Cenozoico de la zona de estudio:
1) Desde el Paleoceno al Eoceno-Oligoceno, se caracteriza por una subsidencia principalmente termal y una sedimentación hemipelágica y gravitacional con poca o nula actividad de corrientes oceánicas.
2) Desde el Eoceno-Oligoceno al Mioceno tardío, se caracteriza por la presencia de un ambiente marino somero, con clinoformas que llegan desde la plataforma externa hasta el talud medio y es interpretada como una progradación deltaica asociada a un estadío de nivel de mar bajo.
3) Desde el Mioceno tardío al Plioceno, caracterizada por las primeras evidencias de acción de corrientes oceánicas, seguido de una superficie erosiva y formación de la TN y la TPM.
4) Desde el Plioceno a la actualidad, se caracteriza por alta actividad de las corrientes oceánicas en el talud superior y medio, intensa interacción entre corrientes oceánicas y el lecho marino y desarrollo de depósitos contorníticos
Personal preferences for Personalised Trials among patients with chronic diseases: an empirical Bayesian analysis of a conjoint survey.
OBJECTIVE: To describe individual patient preferences for Personalised Trials and to identify factors and conditions associated with patient preferences.
DESIGN: Each participant was presented with 18 conjoint questions via an online survey. Each question provided two choices of Personalised Trials that were defined by up to eight attributes, including treatment types, clinician involvement, study logistics and trial burden on a patient.
SETTING: Online survey of adults with at least two common chronic conditions in the USA.
PARTICIPANTS: A nationally representative sample of 501 individuals were recruited from the Chronic Illness Panel by Harris Poll Online. Participants were recruited from several sources, including emails, social media and telephone recruitment of the target population.
MAIN OUTCOME MEASURES: The choice of Personalised Trial design that the participant preferred with each conjoint question.
RESULTS: There was large variability in participants\u27 preferences for the design of Personalised Trials. On average, they preferred certain attributes, such as a short time commitment and no cost. Notably, a population-level analysis correctly predicted 62% of the conjoint responses. An empirical Bayesian analysis of the conjoint data, which supported the estimation of individual-level preferences, improved the accuracy to 86%. Based on estimates of individual-level preferences, patients with chronic pain preferred a long study duration (p≤0.001). Asthma patients were less averse to participation burden in terms of data-collection frequency than patients with other conditions (p=0.002). Patients with hypertension were more cost-sensitive (p\u3c0.001).
CONCLUSION: These analyses provide a framework for elucidating individual-level preferences when implementing novel patient-centred interventions. The data showed that patient preference in Personalised Trials is highly variable, suggesting that individual differences must be accounted for when marketing Personalised Trials. These results have implications for advancing precise interventions in Personalised Trials by indicating when rigorous scientific principles, such as frequent monitoring, is feasible in a substantial subset of patients
Murmuration
Murmuration is an outcome of the EU Culture funded EMDL project (EMDL.eu), exhibited at Society for Arts and Technology’s (SAT) Satosphere. The artistic research initiative brought together an international interdisciplinary group to explore the fulldome environment as a platform for creative innovation. Murmuration consolidated research into networked navigation devices for virtual spaces, swarm algorithms for audience participation, 3D scanning (MRI volumetric modelling) and 3D sonification for game engines in performance
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