186 research outputs found
Optical emission spectra of high current and high voltage generated arcs representing lightning
A recently developed spectrographic technique was used for the comparison of observed optical emission spectra from self-breakdown in air of high current and high voltage impulse arcs which are often used to represent the respective components of a natural lightning strike for a variety of lightning studies. These components are separated as it is practically impossible to simultaneously generate both in a single arc within the laboratory. The high current aspect is typically related to the amount of damage a lightning arc does to an impacted object and can be used to characterize conductivity properties, whereas the high voltage aspect is typically related to the attachment mechanism of lightning to an object when creating a path to ground and can be used to characterize insulation properties. The spectra of a range of high current arcs up to 100 kA and high voltage arcs up to 120 kV across a 14 mm air-gap between a pair of hemispherical tungsten electrodes were investigated and compared with the natural lightning reference spectrum reconstructed from the literature. All three spectra were found to present similar trends, but the high voltage arc had notably lower blackbody radiation, indicating a less energetic and cooler arc. Further, whereas atomic lines from oxygen and nitrogen were clearly present, an atomic line from argon was not prominent, indicating that the bulk energy delivered into the air was in the range of 1400–1520 kJ/mol. The high current arc, much like natural lightning, delivered a bulk energy greater than 1520 kJ/mol
Analysis of light emission and Schlieren from short gap high voltage streamers representing lightning impulses
Light emissions and Schlieren structures were simultaneously observed from streamers produced by
tens of kilovolts 1.2/50 μs impulses, representing the high voltage component of lightning, applied
across a 4 cm air gap between a variety of electrode geometries and a ground plane in an unconfned
environment. The results demonstrated that the light emissions and Schlieren structures coincide
along the same streamer flaments but on diferent timescales; the light existing only during the
microsecond timeframe impulse whereas the Schlieren continued to develop into the millisecond
timeframe, moving towards the centre of the air gap whilst difusing into the surrounding air within
100 ms. If an electrical breakdown did occur, the Schlieren structures outside the arc remained visible.
Streamer formation theory for high voltage impulses is subsequently refned to include the observed
Schlieren mechanism
Comment on 'A first map of tropical Africa's above-ground biomass derived from satellite imagery'
Copyright Institute of Physics © 2011We present a critical evaluation of the above-ground biomass (AGB) map of Africa published in this journal by Baccini et al (2008 Environ. Res. Lett. 3 045011). We first test their map against an independent dataset of 1154 scientific inventory plots from 16 African countries, and find only weak correspondence between our field plots and the AGB value given for the surrounding 1 km pixel by Baccini et al. Separating our field data using a continental landcover classification suggests that the Baccini et al map underestimates the AGB of forests and woodlands, while overestimating the AGB of savannas and grasslands. Secondly, we compare their map to 216 000 × 0.25 ha spaceborne LiDAR footprints. A comparison between Lorey's height (basal-area-weighted average height) derived from the LiDAR data for 1 km pixels containing at least five LiDAR footprints again does not support the hypothesis that the Baccini et al map is accurate, and suggests that it significantly underestimates the AGB of higher AGB areas. We conclude that this is due to the unsuitability of some of the field data used by Baccini et al to create their map, and overfitting in their model, resulting in low accuracies outside the small areas from which their field data are drawn
Laryngeal transplantation in minipigs: vascular, myologic and functional outcomes
There is no effective way of replacing all the functions of the larynx in those requiring laryngectomy. Regenerative medicine offers promise, but cannot presently deliver implants with functioning neuromuscular units. A single well-documented laryngeal transplant in man was a qualified success, but more information is required before clinical trials may be proposed. We studied the early response of the larynx to laryngeal transplantation between 17 pairs of NIH minipigs full matched at the MHC2 locus. Following iterative technical improvements, pigs had good swallowing and a patent airway at 1Â week. No significant changes in mucosal blood flux were observed compared with pre-operative measurements. Changes in muscle morphology and fibre phenotype were observed in transplant muscles retrieved after 7Â days: the levels of fast and slow myosin heavy chain (MyHC) protein were reduced and embryonic MyHC was up regulated consistent with denervation induced atrophy. At 1Â week laryngeal transplantation can result in good swallowing, and is not associated with clinical evidence of ischemia-reperfusion injury in MHC-matched pigs
Evidence For The Production Of Slow Antiprotonic Hydrogen In Vacuum
We present evidence showing how antiprotonic hydrogen, the quasistable
antiproton-proton (pbar-p) bound system, has been synthesized following the
interaction of antiprotons with the hydrogen molecular ion (H2+) in a nested
Penning trap environment. From a careful analysis of the spatial distributions
of antiproton annihilation events, evidence is presented for antiprotonic
hydrogen production with sub-eV kinetic energies in states around n=70, and
with low angular momenta. The slow antiprotonic hydrogen may be studied using
laser spectroscopic techniques.Comment: 5 pages with 4 figures. Published as Phys. Rev. Letters 97, 153401
(2006), in slightly different for
Mechanical Forces Due to Lightning Strikes to Aircraft A Pseudo-Stereo DIC Technique for Measuring Full-Field Displacement
Abstract. One of the major considerations currently affecting the design of composite aircraft structures is the damage resulting from lightning attachment. Full-field measurements of the displacement of materials under lightning attachment would provide a greater understanding of the forces induced by the high current waveform. Furthermore the understanding of the forces involved would allow for the validation of finite element models to simulate the effects of lightning attachment, therefore aiding in the design of solutions to reduce damage to aircraft structures. The study aimed to develop a pseudo-stereo high speed digital image correlation technique in order to obtain full-field information during lightning attachment based on a 100kA initial strike over a 500μs duration, the most severe waveform experienced. The technique that was developed gave full-field measurements for a 550x550x2mm 6082-T6 aluminium panel under a 100kA lightning attachment. Two correlation measurements were recorded at 3000 and 5000 frames per second. The displacement results are comparative with the theory of a cylindrical pressure expansion arising from the acoustic shockwave on attachment to the material. Further developments to this system could allow for more reliable results and higher frame rates which can be used to develop finite element simulations based on measured physical data
ATHENA -- First Production of Cold Antihydrogen and Beyond
Atomic systems of antiparticles are the laboratories of choice for tests of
CPT symmetry with antimatter. The ATHENA experiment was the first to report the
production of copious amounts of cold antihydrogen in 2002. This article
reviews some of the insights that have since been gained concerning the
antihydrogen production process as well as the external and internal properties
of the produced anti-atoms. Furthermore, the implications of those results on
future prospects of symmetry tests with antimatter are discussed.Comment: Proc. of the Third Meeting on CPT and Lorentz Symmetry, Bloomington
(Indiana), USA, August 2004, edited by V. A. Kostelecky (World Scientific,
Singapore). 10 pages, 5 figures, 1 table. Author affiliations cor
Cold-Antimatter Physics
The CPT theorem and the Weak Equivalence Principle are foundational
principles on which the standard description of the fundamental interactions is
based. The validity of such basic principles should be tested using the largest
possible sample of physical systems. Cold neutral antimatter (low-energy
antihydrogen atoms) could be a tool for testing the CPT symmetry with high
precision and for a direct measurement of the gravitational acceleration of
antimatter. After several years of experimental efforts, the production of
low-energy antihydrogen through the recombination of antiprotons and positrons
is a well-established experimental reality. An overview of the ATHENA
experiment at CERN will be given and the main experimental results on
antihydrogen formation will be reviewed.Comment: Proceedings of the XLIII International Meeting on Nuclear Physics,
Bormio (Italy), March 13-20 (2005). 10 pages, 4 figures, 1 tabl
Structure of the Vacuum in Nuclear Matter - A Nonperturbative Approach
We compute the vacuum polarisation correction to the binding energy of
nuclear matter in the Walecka model using a nonperturbative approach. We first
study such a contribution as arising from a ground state structure with
baryon-antibaryon condensates. This yields the same results as obtained through
the relativistic Hartree approximation of summing tadpole diagrams for the
baryon propagator. Such a vacuum is then generalized to include quantum effects
from meson fields through scalar-meson condensates. The method is applied to
study properties of nuclear matter and leads to a softer equation of state
giving a lower value of the incompressibility than would be reached without
quantum effects. The density dependent effective sigma mass is also calculated
including such vacuum polarisation effects.Comment: 26 pages including 5 eps files, uses revtex style; PACS number:
21.65.+f,21.30.+
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