1,797 research outputs found
Dielectrics for long term space exposure and spacecraft charging: A briefing
Charging of dielectrics is a bulk, not a surface property. Radiation driven charge stops within the bulk and is not quickly conducted to the surface. Very large electric fields develop in the bulk due to this stopped charge. At space radiation levels, it typically requires hours or days for the internal electric fields to reach steady state. The resulting electric fields are large enough to produce electrical failure within the insulator. This type failure is thought to produce nearly all electric discharge anomalies. Radiation also induces bond breakage, creates reactive radicals, displaces atoms and, in general, severely changes the chemistry of the solid state material. Electric fields can alter this process by reacting with charged species, driving them through the solid. Irradiated polymers often lose as much as a percent of their mass, or more, at exposures typical in space. Very different aging or contaminant emission can be induced by the stopped charge electric fields. These radiation effects are detailed
Results of literature search on dielectric properties and electron interaction phenomena related to spacecraft charging
The objective of the literature search was to determine the required material properties and electron interaction parameters needed for modeling charge buildup and breakdown in insulators. A brief overview of the results of the literature search is given. A partial list of the references covered is included in a bibliography. Although inorganic insulators were also considered in the search, coverage is limited to the organics, primarily Kapton and Teflon
Right of a Surviving Partner to Purchase a Deceased Partner\u27s Interest Under the Uniform Partnership Act
This discussion is intended to demonstrate that, under the act, the likelihood of fraud should no longer be so controlling a factor as to require invariably a liquidation sale of partnership assets when a court of equity has within its supervisory powers the ability to protect fully all of the parties involved when a partnership is dissolved by death
Application of DR4 and BM100 Biodegradability tests to treated and untreated organic wastes
The aerobic DR4 and anaerobic BM100 biodegradability tests are currently
applied in England and Wales for monitoring the reduction in biological municipal waste
(BMW) achieved by mechanical biological treatment (MBT) plants (Environment Agency
2005). The protocol is applied only when outputs are landfilled and is based on estimating the
reduction in potential biogas production between the MBT input, municipal solid waste (MSW),
and all of the landfilled outputs, using the BM100 test. As this is a long term 100 day test the
more rapid 4 day DR4 test may also be applied as this has been shown to correlate with the
BM100 test. We have now applied the DR4 and BM100 tests to 132 organic waste samples
including untreated and treated BMW and specific organic wastes. The results indicate that the
correlation between the DR4 and BM100 tests has proved valid for mixed MSW derived BMW
wastes. However when both tests are applied to specific organic wastes such as turkey feathers,
cardboard packaging waste and pizza food wastes the correlation between the tests is less strong.
It is concluded that the use of the DR4 and BM100 test correlation is valid for its designed
application (monitoring MBT processes treating MSW derived mixed BMW), but that caution
should be exercised when applying both tests to specific single component organic wastes
Characterisation of untreated and treated biodegradable wastes
As part of a Defra sponsored project (WRT220), approximately 40 biodegradable
wastes were characterised according to biodegradability (DR4 and BM100), total PTE content,
C:N ratio and biochemical composition. Two leaching tests were employed; upflow percolation
test and a one step LS10 test; eluates were analysed for TOC, pH, electrical conductivity, PTEs
and a range of cations and anions. This paper contains a limited set of data for a selection of
untreated and treated waste types representing four waste treatment processes (composting,
MBT, MHT, anaerobic digestion). The DR4 and BM100 tests were found to be appropriate for a
wide range of waste types but where possible they should be used in conjunction with other
related tests. Longer-term MBT composting processes appeared to produce compost material
with reduced ammonium concentrations and extractability of some PTEs. Carbon content
(carbon analyzer - LECO) could be estimated as C = LOI/1.9 which is a routine operation. N
LECO values were approximately 12% greater than the equivalent N Kjeldahl values
Radiation-induced insulator discharge pulses in the CRRES internal discharge monitor satellite experiment
The Internal Discharge Monitor (IDM) was designed to observe electrical pulses from common electrical insulators in space service. The sixteen insulator samples included twelve planar printed circuit boards and four cables. The samples were fully enclosed, mutually isolated, and space radiation penetrated 0.02 cm of aluminum before striking the samples. Pulsing began on the seventh orbit, the maximum pulse rate occurred on the seventeenth orbit when 13 pulses occurred, and the pulses slowly diminished to about one per 3 orbits six months later. After 8 months, the radiation belts abruptly increased and the pulse rates attained a new high. These pulse rates were in agreement with laboratory experience on shorter time scales. Several of the samples never pulsed. If the pulses were not confined within IDM, the physical processes could spread to become a full spacecraft anomaly. The IDM results indicate the rate at which small insulator pulses occur. Small pulses are the seeds of larger satellite electrical anomalies. The pulse rates are compared with space radiation intensities, L shell location, and spectral distributions from the radiation spectrometers on the Combined Release and Radiation Effects Satellite
Charge Storage Measuremens of Resistivity for Dielectric Samples from the CRRES Internal Discharge Monitor
Experimentally Derived Resistivity for Dielectric Samples From the CRRES Internal Discharge Monitor
Resistivity values were experimentally determined using charge storage methods for six samples remaining from the construction of the Internal Discharge Monitor (IDM) flown on the Combined Release and Radiation Effects Satellite (CRRES). Three tests were performed over a period of four to five weeks each in a vacuum of ~5×10-6 torr with an average temperature of ~25 ºC to simulate a space environment. Samples tested included FR4, PTFE, and alumina with copper electrodes attached to one or more of the sample surfaces. FR4 circuit board material was found to have a dark current resistivity of ~1×1018 Ω-cm and a moderately high polarization current. Fiber filled PTFE exhibited little polarization current and a dark current resistivity of ~3×1020 Ω-cm. Alumina had a measured dark current resistivity of ~3·1017 Ω-cm, with a very large and more rapid polarization. Experimentally determined resistivity values were two to three orders of magnitude more than found using standard ASTM test methods. The one minute wait time suggested for the standard ASTM tests is much shorter than the measured polarization current decay times for each sample indicating that the primary currents used to determine ASTM resistivity are caused by the polarization of molecules in the applied electric field rather than charge transport through the bulk of the dielectric. Testing over much longer periods of time in vacuum is required to allow this polarization current to decay away and to allow the observation of charged particles transport through a dielectric material. Application of a simple physics-based model allows separation of the polarization current and dark current components from long duration measurements of resistivity over day- to month-long time scales. Model parameters are directly related to the magnitude of charge transfer and storage and the rate of charge transport
Matroid and Knapsack Center Problems
In the classic -center problem, we are given a metric graph, and the
objective is to open nodes as centers such that the maximum distance from
any vertex to its closest center is minimized. In this paper, we consider two
important generalizations of -center, the matroid center problem and the
knapsack center problem. Both problems are motivated by recent content
distribution network applications. Our contributions can be summarized as
follows:
1. We consider the matroid center problem in which the centers are required
to form an independent set of a given matroid. We show this problem is NP-hard
even on a line. We present a 3-approximation algorithm for the problem on
general metrics. We also consider the outlier version of the problem where a
given number of vertices can be excluded as the outliers from the solution. We
present a 7-approximation for the outlier version.
2. We consider the (multi-)knapsack center problem in which the centers are
required to satisfy one (or more) knapsack constraint(s). It is known that the
knapsack center problem with a single knapsack constraint admits a
3-approximation. However, when there are at least two knapsack constraints, we
show this problem is not approximable at all. To complement the hardness
result, we present a polynomial time algorithm that gives a 3-approximate
solution such that one knapsack constraint is satisfied and the others may be
violated by at most a factor of . We also obtain a 3-approximation
for the outlier version that may violate the knapsack constraint by
.Comment: A preliminary version of this paper is accepted to IPCO 201
Selection from read-only memory with limited workspace
Given an unordered array of elements drawn from a totally ordered set and
an integer in the range from to , in the classic selection problem
the task is to find the -th smallest element in the array. We study the
complexity of this problem in the space-restricted random-access model: The
input array is stored on read-only memory, and the algorithm has access to a
limited amount of workspace. We prove that the linear-time prune-and-search
algorithm---presented in most textbooks on algorithms---can be modified to use
bits instead of words of extra space. Prior to our
work, the best known algorithm by Frederickson could perform the task with
bits of extra space in time. Our result separates
the space-restricted random-access model and the multi-pass streaming model,
since we can surpass the lower bound known for the latter
model. We also generalize our algorithm for the case when the size of the
workspace is bits, where . The running time
of our generalized algorithm is ,
slightly improving over the
bound of Frederickson's algorithm. To obtain the improvements mentioned above,
we developed a new data structure, called the wavelet stack, that we use for
repeated pruning. We expect the wavelet stack to be a useful tool in other
applications as well.Comment: 16 pages, 1 figure, Preliminary version appeared in COCOON-201
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