568 research outputs found
On the relevance of uncorrelated Lorentzian pulses for the interpretation of turbulence in the edge of magnetically confined toroidal plasmas
Recently, it has been proposed that the turbulent fluctuations measured in a
linear plasma device could be described as a superposition of uncorrelated
Lorentzian pulses with a narrow distribution of durations, which would provide
an explanation for the reported quasi-exponential power spectra. Here, we study
the applicability of this proposal to edge fluctuations in toroidal magnetic
confinement fusion plasmas. For the purpose of this analysis, we introduce a
novel wavelet-based pulse detection technique that offers important advantages
over existing techniques. It allows extracting the properties of individual
pulses from the experimental time series, and quantifying the distribution of
pulse duration and energy, as well as temporal correlations.
We apply the wavelet technique to edge turbulent fluctuation data from the
W7-AS stellarator and the JET tokamak, and find that the pulses detected in the
data do not have a narrow distribution of durations and are not uncorrelated.
Instead, the distributions are of the power law type, exhibiting temporal
correlations over scales much longer than the typical pulse duration. These
results suggest that turbulence in open and closed field line systems may be
distinct and cast doubt on the proposed ubiquity of exponential power spectra
in this context.Comment: 10 pages, 4 figure
Bounds on the mass and abundance of dark compact objects and black holes in dwarf spheroidal galaxy halos
We establish new dynamical constraints on the mass and abundance of compact
objects in the halo of dwarf spheroidal galaxies. In order to preserve
kinematically cold the second peak of the Ursa Minor dwarf spheroidal (UMi
dSph) against gravitational scattering, we place upper limits on the density of
compact objects as a function of their assumed mass. The mass of the dark
matter constituents cannot be larger than 1000 solar masses at a halo density
in UMi's core of 0.35 solar masses/pc^3. This constraint rules out a scenario
in which dark halo cores are formed by two-body relaxation processes. Our
bounds on the fraction of dark matter in compact objects with masses >3000
solar masses improve those based on dynamical arguments in the Galactic halo.
In particular, objects with masses solar masses can comprise no
more than a halo mass fraction . Better determinations of the
velocity dispersion of old overdense regions in dSphs may result in more
stringent constraints on the mass of halo objects. For illustration, if the
preliminary value of 0.5 km/s for the secondary peak of UMi is confirmed,
compact objects with masses above solar masses could be excluded
from comprising all its dark matter halo.Comment: 6 pages, 2 figures, accepted for publication in ApJ Letter
Low-Cost Prototype to Automate the 3D Digitization of Pieces: An Application Example and Comparison
This work is aimed at describing the design of a mechanical and programmable 3D capturing system to be used by either 3D scanner or DSLR camera through photogrammetry. Both methods are widely used in diverse areas, from engineering, architecture or archaeology, up to the field of medicine; but they also entail certain disadvantages, such as the high costs of certain equipment, such as scanners with some precision, and the need to resort to specialized operatives, among others. The purpose of this design is to create a robust, precise and cost-effective system that improves the limitations of the present equipment on the market, such as robotic arms or rotary tables. For this reason, a preliminary study has been conducted to analyse the needs of improvement, later, we have focused on the 3D design and prototyping. For its construction, there have been used the FDM additive technology and structural components that are easy to find in the market. With regards to electronic components, basic electronics and Arduino-based 3D printers firmware have been selected. For system testing, the capture equipment consists of a Spider Artec 3D Scanner and a Nikon 5100 SLR Camera. Finally, 3D models have been developed by comparing the 3D meshes obtained by the two methods, obtaining satisfactory results
Asymptotics of Eigenvalues and Eigenfunctions for the Laplace Operator in a Domain with Oscillating Boundary. Multiple Eigenvalue Case
We study the asymptotic behavior of the solutions of a spectral problem for
the Laplacian in a domain with rapidly oscillating boundary. We consider the
case where the eigenvalue of the limit problem is multiple. We construct the
leading terms of the asymptotic expansions for the eigenelements and verify the
asymptotics
Indirect three-dimensional printing: A method for fabricating polyurethane-urea based cardiac scaffolds.
Biomaterial scaffolds are a key part of cardiac tissue engineering therapies. Our group has recently synthesised a novel polycaprolactone based polyurethane-urea copolymer that shows improved mechanical properties compared to its previously published counterparts. The aim of this study was to explore whether indirect 3D printing could provide a means to fabricate this novel, biodegradable polymer into a scaffold suitable for cardiac tissue engineering. Indirect 3D printing was carried out through printing water dissolvable poly(vinyl alcohol) porogens in three different sizes based on a wood-stack model, into which a polyurethane-urea solution was pressure injected. The porogens were removed, leading to soft polyurethane-urea scaffolds with regular tubular pores. The scaffolds were characterised for their compressive and tensile mechanical behaviour; and their degradation was monitored for 12 months under simulated physiological conditions. Their compatibility with cardiac myocytes and performance in novel cardiac engineering-related techniques, such as aggregate seeding and bi-directional perfusion, was also assessed. The scaffolds were found to have mechanical properties similar to cardiac tissue, and good biocompatibility with cardiac myocytes. Furthermore, the incorporated cells preserved their phenotype with no signs of de-differentiation. The constructs worked well in perfusion experiments, showing enhanced seeding efficiency. This article is protected by copyright. All rights reserved
Qualitative interpretation of galaxy spectra
We describe a simple step-by-step guide to qualitative interpretation of
galaxy spectra. Rather than an alternative to existing automated tools, it is
put forward as an instrument for quick-look analysis, and for gaining physical
insight when interpreting the outputs provided by automated tools. Though the
recipe is of general application, it was developed for understanding the nature
of the Automatic Spectroscopic K-means based (ASK) template spectra. They
resulted from the classification of all the galaxy spectra in the Sloan Digital
Sky Survey data release 7 (SDSS-DR7), thus being a comprehensive representation
of the galaxy spectra in the local universe. Using the recipe, we give a
description of the properties of the gas and the stars that characterize the
ASK classes, from those corresponding to passively evolving galaxies, to HII
galaxies undergoing a galaxy-wide starburst. The qualitative analysis is found
to be in excellent agreement with quantitative analyses of the same spectra. A
number of byproducts follow from the analysis. There is a tight correlation
between the age of the stellar population and the metallicity of the gas, which
is stronger than the correlations between galaxy mass and stellar age, and
galaxy mass and gas metallicity. The galaxy spectra are known to follow a
1-dimensional sequence, and we identify the luminosity-weighted mean stellar
age as the affine parameter that describes the sequence. All ASK classes happen
to have a significant fraction of old stars, although spectrum-wise they are
outshined by the youngest populations. Old stars are metal rich or metal poor
depending on whether they reside in passive galaxies or in star-forming
galaxies.Comment: Simple step-by-step guide to interpreting galaxy spectra. Accepted
for publication in ApJ. 17 pages with 21 figure
Biomarkers and inorganic proxies in the paleoenvironmental reconstruction of mires: The importance of landscape in Las Conchas (Asturias, Northern Spain)
We determined the lipid distributions (n-alkanes, n-alkan-2-ones, n-alkanoic acids), total organic carbon (TOC), total nitrogen (TN), Ca/Mg and ash content in Las Conchas mire, a 3.2 m deep bryophyte-dominated mire in Northern Spain covering 8000 cal yr BP. Bog conditions developed in the bottom 20 cm of the profile, and good preservation of organic matter (OM) was inferred from n-alkanoic acid distribution, with the exception of the uppermost 20 cm (last ca. 200 yr). Microbial synthesis of long chain saturated fatty acids from primary OM likely produced a dominance of short chain n-alkanoic acids with a bimodal distribution, as well as the lack of correspondence between the n-alkane and n-alkanoic acid profiles in the upper 20 cm. This was accompanied by an increase in ash content, a decrease in TOC and variation in n-alkane ratios, thereby suggesting significant changes in the mire, namely drainage and transformation to a meadow, in the last ca. 200 yr. The distribution of n-alkan-2-ones indicated an increase in bacterial source from the bottom of the record to 94 cm, whereas their distribution in the upper part could be attributed mainly to plant input and/or the microbial oxidation of n-alkanes. The different n-alkane proxies showed variations, which we interpreted in terms of changes in vegetation (Sphagnum vs. non-Sphagnum dominated phases) during the last 8000 cal yr BP. C23 was the most abundant homolog throughout most of the record, thereby suggesting dominant humid conditions alternating with short drier phases. However, such humid conditions were not linked to paleoclimatic variation but rather to geomorphological characteristics: Las Conchas mire, at the base of the Cuera Range, receives continuous runoff—even during drier periods—which is not necessarily accompanied by additional mineral input to peat, producing the development of Sphagnum moss typical of waterlogged ecotopes and damp habitats. Thus, although geochemical proxies indicated an ombrotrophic regime in the mire, geomorphological characteristics may make a considerable contribution to environmental conditions
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