Puncture discharges in surface dielectrics as contaminant sources in spacecraft environments

Spacecraft in geosynchronous orbits are known to become charged to large negative potentials during the local midnight region of the satellite orbit. Such discharges have been studied by the electron beam irradiation of dielectric samples in a vacuum environment. In addition to static measurements and photographic examination of the puncture discharges in Teflon samples, the transient characteristics of the electrical discharges are determined from oscillographs of voltage and current and by charged particle measurements employing a biased Faraday cup and a retarding potential analyzer. Using these latter techniques, studies of angular and energy distributions of charged particles have indicated an initial burst of high energy electrons (5 x 10 to the 13th power per discharge at energies greater than 300 eV) followed by a less intense burst of lower energy negative particles. Positive ions are emitted from the discharge site in an initial high velocity burst followed by a lower velocity burst tentatively identified as carbon

Optimal traps in graphene

We transform the two-dimensional Dirac-Weyl equation, which governs the charge carriers in graphene, into a non-linear first-order differential equation for scattering phase shift, using the so-called variable phase method. This allows us to utilize the Levinson Theorem to find zero-energy bound states created electrostatically in realistic structures. These confined states are formed at critical potential strengths, which leads to us posit the use of `optimal traps' to combat the chiral tunneling found in graphene, which could be explored experimentally with an artificial network of point charges held above the graphene layer. We also discuss scattering on these states and find the zero angular momentum states create a dominant peak in scattering cross-section as energy tends towards the Dirac point energy, suggesting a dominant contribution to resistivity.Comment: 11 pages, 5 figure

Laboratory simulation of irradiation-induced dielectric breakdown in spacecraft charging

The discharging of dielectric samples irradiated by a beam of monoenergetic electrons is investigated. The development of a model, or models, which describe the discharge phenomena occuring on the irradiated dielectric targets is discussed. The electrical discharge characteristics of irradiated dielectric samples are discussed and the electrical discharge paths along dielectric surfaces and within the dielectric material are determined. The origin and destination of the surface emitted particles is examined and the charge and energy balance in the system is evaluated

The Chemical and Ionization Conditions in Weak Mg II Absorbers

We present an analysis of the chemical and ionization conditions in a sample of 100 weak Mg II absorbers identified in the VLT/UVES archive of quasar spectra. Using a host of low ionization lines associated with each absorber in this sample, and on the basis of ionization models, we infer that the metallicity in a significant fraction of weak Mg II clouds is constrained to values of solar or higher, if they are sub-Lyman limit systems. Based on the observed constraints, we present a physical picture in which weak Mg II absorbers are predominantly tracing two different astrophysical processes/structures. A significant population of weak Mg II clouds, those in which N(Fe II) is much less than N(Mg II), identified at both low (z ~ 1) and high (z ~ 2) redshift, are potentially tracing gas in the extended halos of galaxies, analogous to the Galactic high velocity clouds. These absorbers might correspond to alpha-enhanced interstellar gas expelled from star-forming galaxies, in correlated supernova events. On the other hand, N(FeII) approximately equal to N(Mg II) clouds, which are prevalent only at lower redshifts (z < 1.5), must be tracing Type Ia enriched gas in small, high metallicity pockets in dwarf galaxies, tidal debris, or other intergalactic structures.Comment: 35 pages (including tables & figures). Accepted for publication in ApJ. A high resolution version of the paper is available at "http://www.astro.wisc.edu/~anand/weakMgII.pdf

Ultrasonic Inspection of Graphite-Epoxy Solid Rocket Motor Canisters

Thick filament-wound composite materials are particularly attractive for use in solid rocket motor structures. However, these materials are difficult to inspect because of the scattering losses associated with multiple fiber layers. Damage caused by either low or high velocity impact which results in matrix cracking, delaminations and broken fibers cannot be tolerated because of the possibility of catastrophic system failure. American Research Corporation of Virginia has performed a 2-year Phase II Small Business Innovation Research contract to develop an ultrasonic inspection system for graphite/epoxy rocket motor canisters [1]. This paper details the experimental apparatus and testing of rocket motor canisters, presents testing results and discusses observations and detection thresholds

Mg II Absorber Number Density at z~0.05: Implications for Omega_DLA Evolution

An unbiased sample of 147 quasar/AGN spectra, obtained with the FOS/HST, has been searched for intervening MgII absorbers over the redshift range 0<z<0.15. The total redshift path searched is 18.8, with the survey being 80% complete to a 5-sigma rest-frame equivalent width, W_r(2796), of 0.6 Ang. Main results of this work are: [1] Four systems were found, with a mean redshift of =0.06, yielding a redshift number density dN/dz=0.22(+0.12)(-0.09) for absorbers with W_r(2796)>0.6 Ang. This is consistent with the value expected if these systems do not evolve from higher redshifts (z=2.2). [2] No systems with W_r(2796)<0.6 Ang were found. It is a 2-sigma result to have a null detection of smaller W_r(2796) systems. If this implies a turnover in the low W_r(2796) region of the equivalent width distribution at z~0, then there is at least a 25% reduction in the average galaxy gas cross section from z<0.2 galaxies. [3] These systems have strong FeII absorption and are good candidates for damped Ly-alpha absorbers DLAs (see Rao & Turnshek 2000, ApJS, 130, 1). This translates to a redshift number density of dN/dz=0.08(+0.09)(-0.05) for DLAs at z~0. In tandem with the data analyzed by Rao & Turnshek, these results indicate that the redshift number density of DLAs does not evolve from z~4 to z~0. If the HI mass function does not evolve from z~0.5 to z~0, then the cosmological HI mass density is also deduced to not evolve from z~4 to z~0. These z~0 results for MgII absorption-selected DLAs are at odds with those based upon 21-cm emission from HI galaxies by a factor of five to six.Comment: 23 pages, 7 Figures, accepted to ApJ. Replaced version includes additional figures and tables and substantial modifications to the tex

Recreation reduces tick density through fine-scale risk effects on deer space-use

Altered interactions between pathogens, their hosts and vectors have potential consequences for human disease risk. Notably, tick-borne pathogens, many of which are associated with growing deer abundance, show global increasing prevalence and pose increasing challenges for disease prevention. Human activities can largely affect the patterns of deer space-use and can therefore be potential management tools to alleviate human-wildlife conflicts. Here, we tested how deer space-use patterns are influenced by human recreational activities, and how this in turn affects the spatial distribution of the sheep tick (Ixodes ricinus), a relevant disease vector of zoonoses such as Lyme borrelioses. We compared deer dropping and questing tick density on transects near (20 m) and further away from(100 m) forest trails that were either frequently used (open for recreation) or infrequently used (closed for recreation, but used by park managers). In contrast to infrequently used trails, deer dropping density was 31% lower near (20 m) than further away from (100 m) frequently used trails. Similarly, ticks were 62% less abundant near (20 m) frequently used trails compared to further away from (100 m) these trails, while this decline in tick numbers was only 14% near infrequently used trails. The avoidance by deer of areas close to human-used trails was thus associated with a similar reduction in questing tick density near these trails. As tick abundance generally correlates to pathogen prevalence, the use of trails for recreation may reduce tick-borne disease risk for humans on and near these trails. Our study reveals an unexplored effect of human activities on ecosystems and how this knowledge could be potentially used to mitigate zoonotic disease risk

Adiabatic Approximation for weakly open systems

We generalize the adiabatic approximation to the case of open quantum systems, in the joint limit of slow change and weak open system disturbances. We show that the approximation is ``physically reasonable'' as under wide conditions it leads to a completely positive evolution, if the original master equation can be written on a time-dependent Lindblad form. We demonstrate the approximation for a non-Abelian holonomic implementation of the Hadamard gate, disturbed by a decoherence process. We compare the resulting approximate evolution with numerical simulations of the exact equation.Comment: New material added, references added and updated, journal reference adde

Revisiting the need for a literature search narrative: A brief methodological note

This is the author accepted manuscript. The final version is available from Wiley via the DOI inthis recordIn this method note, we question if the primary search strategy in a systematic review should be accompanied by a search narrative. A search narrative could offer a conceptual and contextual report on the search strategy, which we suggest might benefit the peer review of literature searches and increase engagement with, and discussion of, the literature search strategy from review stakeholders, topic experts, and lay users of research. Search narratives would also increase the transparency of decision-making in literature searching.Centre for Outcomes Research and Effectiveness (CORE)National Institute for Health Research (NIHR)Cochrane Common Mental Disorders (CCMD) Grou

Back gating of a two-dimensional hole gas in a SiGe quantum well

A device comprising a low-resistivity, n-type, Si substrate as a back gate to a p-type (boron), remote-doped, SiGe quantum well has been fabricated and characterized. Reverse and forward voltage biasing of the gate with respect to the two-dimensional hole gas in the quantum well allows the density of holes to be varied from 8 × 1011 cm–2 down to a measurement-limited value of 4 × 1011 cm–2. This device is used to demonstrate the evolution with decreasing carrier density of a re-entrant insulator state between the integer quantum Hall effect states with filling factors 1 and 3