2,162 research outputs found
Simulation Studies of the NLC with Improved Ground Motion Models
The performance of various systems of the Next Linear Collider (NLC) have
been studied in terms of ground motion using recently developed models. In
particular, the performance of the beam delivery system is discussed. Plans to
evaluate the operation of the main linac beam-based alignment and feedback
systems are also outlined.Comment: Submitted to XX International Linac Conferenc
Beam-based Feedback Simulations for the NLC Linac
Extensive beam-based feedback systems are planned as an integral part of the
Next Linear Collider (NLC) control system. Wakefield effects are a significant
influence on the feedback design, imposing both architectural and algorithmic
constraints. Studies are in progress to assure the optimal selection of devices
and to refine and confirm the algorithms for the system design. We show the
results of initial simulations, along with evaluations of system response for
various conditions of ground motion and other operational disturbances.Comment: 3 pages. Linac2000 conferenc
On the optimal measurement of conversion gain in the presence of dark noise
Working from a model of Gaussian pixel noise, we present and unify over
twenty-five years of developments in the statistical analysis of the photon
transfer conversion gain measurement. We then study a two-sample estimator of
the conversion gain that accounts for the general case of non-negligible dark
noise. The moments of this estimator are ill-defined (their integral
representations diverge) and so we propose a method for assigning
pseudomoments, which are shown to agree with actual sample moments under mild
conditions. A definition of optimal sample size pairs for this two-sample
estimator is proposed and used to find approximate optimal sample size pairs
that allow experimenters to achieve a predetermined measurement uncertainty
with as little data as possible. The conditions under which these
approximations hold are also discussed. Design and control of experiment
procedures are developed and used to optimally estimate a per-pixel conversion
gain map of a real image sensor. Experimental results show excellent agreement
with theoretical predictions and are backed up with Monte Carlo simulation. The
per-pixel conversion gain estimates are then applied in a demonstration of
per-pixel read noise estimation of the same image sensor. The results of this
work open the door to a comprehensive pixel-level adaptation of the photon
transfer method.Comment: 16 pages, 5 figure
Feedback Systems for Linear Colliders
Feedback systems are essential for stable operation of a linear collider,
providing a cost-effective method for relaxing tight tolerances. In the
Stanford Linear Collider (SLC), feedback controls beam parameters such as
trajectory, energy, and intensity throughout the accelerator. A novel dithering
optimization system which adjusts final focus parameters to maximize luminosity
contributed to achieving record performance in the 1997-98 run. Performance
limitations of the steering feedback have been investigated, and improvements
have been made. For the Next Linear Collider (NLC), extensive feedback systems
are planned as an intregal part of the design. Feedback requiremetns for JLC
(the Japanese Linear Collider) are essentially identical to NLC; some of the
TESLA requirements are similar but there are significant differences. For NLC,
algorithms which incorporate improvements upon the SLC implementation are being
prototyped. Specialized systems for the damping rings, rf and interaction point
will operate at high bandwidth and fast response. To correct for the motion of
individual bunches within a train, both feedforward and feedback systems are
planned. SLC experience has shown that feedback systems are an invaluable
operational tool for decoupling systems, allowing precision tuning, and
providing pulse-to-pulse diagnostics. Feedback systems for the NLC will
incorporate the key SLC features and the benefits of advancing technologies.Comment: Invited talk presented at IEEE Particle Accelerator Conference
(PAC99
Tree-based Coarsening and Partitioning of Complex Networks
Many applications produce massive complex networks whose analysis would
benefit from parallel processing. Parallel algorithms, in turn, often require a
suitable network partition. For solving optimization tasks such as graph
partitioning on large networks, multilevel methods are preferred in practice.
Yet, complex networks pose challenges to established multilevel algorithms, in
particular to their coarsening phase.
One way to specify a (recursive) coarsening of a graph is to rate its edges
and then contract the edges as prioritized by the rating. In this paper we (i)
define weights for the edges of a network that express the edges' importance
for connectivity, (ii) compute a minimum weight spanning tree with
respect to these weights, and (iii) rate the network edges based on the
conductance values of 's fundamental cuts. To this end, we also (iv)
develop the first optimal linear-time algorithm to compute the conductance
values of \emph{all} fundamental cuts of a given spanning tree. We integrate
the new edge rating into a leading multilevel graph partitioner and equip the
latter with a new greedy postprocessing for optimizing the maximum
communication volume (MCV). Experiments on bipartitioning frequently used
benchmark networks show that the postprocessing already reduces MCV by 11.3%.
Our new edge rating further reduces MCV by 10.3% compared to the previously
best rating with the postprocessing in place for both ratings. In total, with a
modest increase in running time, our new approach reduces the MCV of complex
network partitions by 20.4%
Low temperature magnetic hysteresis in Mn acetate single crystals
Precise magnetic hysteresis measurements of small single crystals of
Mn acetate of spin 10 have been conducted down to 0.4 K using a high
sensitivity Hall magnetometer. At higher temperature (>1.6K) step-like changes
in magnetization are observed at regularly spaced magnetic field intervals, as
previously reported. However, on lowering the temperature the steps in
magnetization shift to higher magnetic fields, initially gradually. These
results are consistent with the presence of a second order uniaxial magnetic
anisotropy, first observed by EPR spectroscopy, and thermally assisted
tunnelling with tunnelling relaxation occurring from levels of progressively
lower energy as the temperature is reduced. At lower temperature an abrupt
shift in step positions is found. We suggest that this shift may be the first
evidence of an abrupt, or first-order, transition between thermally assisted
and pure quantum tunnelling, suggested by recent theory.Comment: 8 pages, 4 figures, submitted to Europhys. Let
Functional responses of methanogenic archaea to syntrophic growth.
Methanococcus maripaludis grown syntrophically with Desulfovibrio vulgaris was compared with M. maripaludis monocultures grown under hydrogen limitation using transcriptional, proteomic and metabolite analyses. These measurements indicate a decrease in transcript abundance for energy-consuming biosynthetic functions in syntrophically grown M. maripaludis, with an increase in transcript abundance for genes involved in the energy-generating central pathway for methanogenesis. Compared with growth in monoculture under hydrogen limitation, the response of paralogous genes, such as those coding for hydrogenases, often diverged, with transcripts of one variant increasing in relative abundance, whereas the other was little changed or significantly decreased in abundance. A common theme was an apparent increase in transcripts for functions using H(2) directly as reductant, versus those using the reduced deazaflavin (coenzyme F(420)). The greater importance of direct reduction by H(2) was supported by improved syntrophic growth of a deletion mutant in an F(420)-dependent dehydrogenase of M. maripaludis. These data suggest that paralogous genes enable the methanogen to adapt to changing substrate availability, sustaining it under environmental conditions that are often near the thermodynamic threshold for growth. Additionally, the discovery of interspecies alanine transfer adds another metabolic dimension to this environmentally relevant mutualism
Stressed backbone and elasticity of random central-force systems
We use a new algorithm to find the stress-carrying backbone of ``generic''
site-diluted triangular lattices of up to 10^6 sites. Generic lattices can be
made by randomly displacing the sites of a regular lattice. The percolation
threshold is Pc=0.6975 +/- 0.0003, the correlation length exponent \nu =1.16
+/- 0.03 and the fractal dimension of the backbone Db=1.78 +/- 0.02. The number
of ``critical bonds'' (if you remove them rigidity is lost) on the backbone
scales as L^{x}, with x=0.85 +/- 0.05. The Young's modulus is also calculated.Comment: 5 pages, 5 figures, uses epsfi
Treatment outcomes among HIV-positive orphaned and non-orphaned children on antiretroviral therapy in Johannesburg, South Africa
Background. Limited research investigating treatment outcomes for HIV-positive orphans compared with non-orphans has shown mixed results, with several studies indicating that HIV-positive orphans are at greater risk of delayed access to HIV care and poor antiretroviral therapy (ART) adherence, while other data suggest that ART outcomes of orphans can be similar to those of non-orphans. Understanding the impact of orphan status on short-term ART outcomes could improve targeted intervention strategies, and subsequent long-term treatment and developmental outcomes, for HIV-positive infants, children and adolescents.Objectives. To evaluate the relationship between orphan status and ART outcomes among HIV-positive infants, children and adolescents initiating ART at two large public sector HIV clinics in Johannesburg, South Africa.Methods. This was a retrospective cohort study of HIV-positive children aged <18 years initiating standard first-line ART between June 2004 and May 2013. Using propensity scores, orphans and non-orphans were matched for age, sex, World Health Organization stage and ART regimen. The effect of orphanhood on attrition from care (all-cause mortality and loss to follow-up) was evaluated using Cox proportional hazards regression analysis, and its effect on having a detectable viral load (≥400 copies/mL) at 12 months on ART using binomial regression analysis with modified Poisson distribution.Results. A total of 251 (29.4%) orphans (maternal, paternal or both) and 603 (70.6%) non-orphans were included at ART initiation. Following multiple imputation for missing data and propensity score matching, 222 orphans and 222 non-orphans were included. Orphans had a median age of 8.0 years (interquartile range (IQR) 4.9 - 10.7) and non-orphans 7.4 years (IQR 4.2 - 10.2). A total of 12 (5.4%) orphans and 33 (14.9%) non-orphans experienced attrition from care during the first 12 months on ART (adjusted hazard ratio 0.32, 95% confidence interval (CI) 0.17 - 0.63). Among those alive and in care, with a viral load at 12 months on ART, 18.0% of orphans (33/183) and 14.8% of non-orphans (24/162) had a detectable viral load (adjusted risk ratio 1.15, 95% CI 1.04 - 1.28).Conclusions. Orphans were less likely than non-orphans to experience attrition, but among those in care at 12 months, orphans were more likely to have detectable viral loads. Lower attrition among orphans may be due to their being in institutional or foster care, ensuring that they make their visits; however, their higher rates of non-suppression may result from lack of psychosocial support or stigma resulting in struggles to adhere. Additional research investigating age-specific outcomes will be important to elucidate these effects further
Discovery of novel reductive elimination pathway for 10-hydroxywarfarin
Coumadin (R/S-warfarin) anticoagulant therapy is highly efficacious in preventing the formation of blood clots; however, significant inter-individual variations in response risks over or under dosing resulting in adverse bleeding events or ineffective therapy, respectively. Levels of pharmacologically active forms of the drug and metabolites depend on a diversity of metabolic pathways. Cytochromes P450 play a major role in oxidizing R- and S-warfarin to 6-, 7-, 8-, 10-, and 4\u27-hydroxywarfarin, and warfarin alcohols form through a minor metabolic pathway involving reduction at the C11 position. We hypothesized that due to structural similarities with warfarin, hydroxywarfarins undergo reduction, possibly impacting their pharmacological activity and elimination. We modeled reduction reactions and carried out experimental steady-state reactions with human liver cytosol for conversion o
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