8,324 research outputs found
Archaeology of active galaxies across the electromagnetic spectrum
Analytical and numerical galaxy-formation models indicate that active
galactic nuclei (AGNs) likely play a prominent role in the formation and
evolution of galaxies. However, quantifying this effect requires knowledge of
how the nuclear activity proceeds throughout the life of a galaxy, whether it
alternates with periods of quiescence and, if so, on what timescales these
cycles occur. This topic has attracted growing interest, but making progress
remains a challenging task. For optical and radio AGNs, a variety of techniques
are used to perform a kind of "archaeology" that traces the signatures of past
nuclear activity. Here we summarize recent findings regarding the lifecycle of
an AGN from optical and radio observations. The limited picture we have so far
suggests that these cycles can range from long periods of 10^7-10^8 yr to
shorter periods of 10^4-10^5 yr, even reaching extreme events on timescales of
just a few years. Together with simulations, observational results regarding
the multiple cycles of AGN activity help to create a complete picture of the
AGN lifecycle.Comment: Invited Review for Nature Astronomy, Aug 28 issue. 10 pages, 8
figure
Science-based restoration monitoring of coastal habitats, Volume Two: Tools for monitoring coastal habitats
Healthy coastal habitats are not only important ecologically; they also support healthy coastal communities and improve the quality of people’s lives. Despite their many benefits and values, coastal habitats have been systematically modified, degraded, and destroyed throughout the United States and its protectorates beginning with European colonization in the 1600’s (Dahl 1990). As a result, many coastal habitats around the United States are in desperate need of restoration. The monitoring of restoration projects, the focus of this document, is necessary to ensure that restoration efforts are successful, to further the science, and to increase the efficiency of future restoration efforts
Field methods for rodent studies in Asia and the Indo-Pacific
Farm Management,
Measurement techniques and instruments suitable for life-prediction testing of photovoltaic arrays
Array failure modes, relevant materials property changes, and primary degradation mechanisms are discussed as a prerequisite to identifying suitable measurement techniques and instruments. Candidate techniques and instruments are identified on the basis of extensive reviews of published and unpublished information. These methods are organized in six measurement categories - chemical, electrical, optical, thermal, mechanical, and other physicals. Using specified evaluation criteria, the most promising techniques and instruments for use in life prediction tests of arrays were selected
Biological Control of Weeds: Theory and Practical Application
Crop Production/Industries,
Investigation of FACTS devices to improve power quality in distribution networks
Flexible AC transmission system (FACTS) technologies are power electronic solutions
that improve power transmission through enhanced power transfer volume and stability,
and resolve quality and reliability issues in distribution networks carrying sensitive
equipment and non-linear loads. The use of FACTS in distribution systems is still in
its infancy. Voltages and power ratings in distribution networks are at a level where
realistic FACTS devices can be deployed. Efficient power converters and therefore loss
minimisation are crucial prerequisites for deployment of FACTS devices.
This thesis investigates high power semiconductor device losses in detail. Analytical
closed form equations are developed for conduction loss in power devices as a function
of device ratings and operating conditions. These formulae have been shown to predict
losses very accurately, in line with manufacturer data. The developed formulae enable
circuit designers to quickly estimate circuit losses and determine the sensitivity of those
losses to device voltage and current ratings, and thus select the optimal semiconductor
device for a specific application.
It is shown that in the case of majority carrier devices (such as power MOSFETs), the
conduction power loss (at rated current) increases linearly in relation to the varying rated
current (at constant blocking voltage), but is a square root of the variable blocking voltage
when rated current is fixed. For minority carrier devices (such as a pin diode or IGBT),
a similar relationship is observed for varying current, however where the blocking voltage
is altered, power losses are derived as a square root with an offset (from the origin).
Finally, this thesis conducts a power loss-oriented evaluation of cascade type multilevel
converters suited to reactive power compensation in 11kV and 33kV systems. The cascade
cell converter is constructed from a series arrangement of cell modules. Two prospective
structures of cascade type converters were compared as a case study: the traditional type
which uses equal-sized cells in its chain, and a second with a ternary relationship between
its dc-link voltages. Modelling (at 81 and 27 levels) was carried out under steady state
conditions, with simplified models based on the switching function and using standard
circuit simulators. A detailed survey of non punch through (NPT) and punch through
(PT) IGBTs was completed for the purpose of designing the two cascaded converters.
Results show that conduction losses are dominant in both types of converters in NPT
and PT IGBTs for 11kV and 33kV systems. The equal-sized converter is only likely to
be useful in one case (27-levels in the 33kV system). The ternary-sequence converter
produces lower losses in all other cases, and this is especially noticeable for the 81-level
converter operating in an 11kV network
Advanced Manned Launch System (AMLS) study
To assure national leadership in space operations and exploration in the future, NASA must be able to provide cost effective and operationally efficient space transportation. Several NASA studies and the joint NASA/DoD Space Transportation Architecture Studies (STAS) have shown the need for a multi-vehicle space transportation system with designs driven by enhanced operations and low costs. NASA is currently studying an advanced manned launch system (AMLS) approach to transport crew and cargo to the Space Station Freedom. Several single and multiple stage systems from air-breathing to all-rocket concepts are being examined in a series of studies potential replacements for the Space Shuttle launch system in the 2000-2010 time frame. Rockwell International Corporation, under contract to the NASA Langley Research Center, has analyzed a two-stage all-rocket concept to determine whether this class of vehicles is appropriate for the AMLS function. The results of the pre-phase A study are discussed
The COMPTEL instrumental line background
The instrumental line background of the Compton telescope COMPTEL onboard the
Compton Gamma-Ray Observatory is due to the activation and/or decay of many
isotopes. The major components of this background can be attributed to eight
individual isotopes, namely 2D, 22Na, 24Na, 28Al, 40K, 52Mn, 57Ni, and 208Tl.
The identification of instrumental lines with specific isotopes is based on the
line energies as well as on the variation of the event rate with time,
cosmic-ray intensity, and deposited radiation dose during passages through the
South-Atlantic Anomaly. The characteristic variation of the event rate due to a
specific isotope depends on its life-time, orbital parameters such as the
altitude of the satellite above Earth, and the solar cycle. A detailed
understanding of the background contributions from instrumental lines is
crucial at MeV energies for measuring the cosmic diffuse gamma-ray background
and for observing gamma-ray line emission in the interstellar medium or from
supernovae and their remnants. Procedures to determine the event rate from each
background isotope are described, and their average activity in spacecraft
materials over the first seven years of the mission is estimated.Comment: accepted for publication in A&A, 22 pages, 21 figure
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