545 research outputs found
ANOMALOUS PERTURBATIVE TRANSPORT IN TOKAMAKS DUE TO DRIFT-WAVE TURBULENCE
A new method for calculating the anomalous transport in tokamak plasmas is presented. The renormalized nonlinear plasma response function is derived using the direct-interaction approximation (DIA). A complete calculation for the case of electrostatic drift-wave turbulence is presented. Explicit expressions for all coefficients of the anomalous transport matrix relating particle and heat fluxes to density and temperature gradients in the plasma are obtained. The anomalous transport matrix calculated using the DIA does not have the Onsager symmetry. As an example of application, the parameters of the Texas Experimental Tokamak (TEXT) [Nucl. Technol. Fusion 1, 479 (1981)] are used to evaluate all transport coefficients numerically, as well as the spectrum modulation. The relation between the theoretical results and the experimental data is discussed. Although this paper focuses on electron transport for simplicity, the method can also be used to calculate anomalous transport due to ion instabilities, such as the ion-temperature-gradient instability
Streaming complexity of CSPs with randomly ordered constraints
We initiate a study of the streaming complexity of constraint satisfaction
problems (CSPs) when the constraints arrive in a random order. We show that
there exists a CSP, namely , for which random ordering
makes a provable difference. Whereas a approximation of
requires space with adversarial ordering,
we show that with random ordering of constraints there exists a
-approximation algorithm that only needs space. We also give
new algorithms for in variants of the adversarial ordering
setting. Specifically, we give a two-pass space
-approximation algorithm for general graphs and a single-pass
space -approximation algorithm for bounded degree
graphs.
On the negative side, we prove that CSPs where the satisfying assignments of
the constraints support a one-wise independent distribution require
-space for any non-trivial approximation, even when the
constraints are randomly ordered. This was previously known only for
adversarially ordered constraints. Extending the results to randomly ordered
constraints requires switching the hard instances from a union of random
matchings to simple Erd\"os-Renyi random (hyper)graphs and extending tools that
can perform Fourier analysis on such instances.
The only CSP to have been considered previously with random ordering is
where the ordering is not known to change the
approximability. Specifically it is known to be as hard to approximate with
random ordering as with adversarial ordering, for space
algorithms. Our results show a richer variety of possibilities and motivate
further study of CSPs with randomly ordered constraints
Streaming beyond sketching for Maximum Directed Cut
We give an -space single-pass -approximation
streaming algorithm for estimating the maximum directed cut size
() in a directed graph on vertices. This improves over
an -space approximation algorithm due to Chou,
Golovnev, Velusamy (FOCS 2020), which was known to be optimal for
-space algorithms.
is a special case of a constraint satisfaction problem
(CSP). In this broader context, our work gives the first CSP for which
algorithms with space can provably outperform
-space algorithms on general instances. Previously, this was shown
in the restricted case of bounded-degree graphs in a previous work of the
authors (SODA 2023). Prior to that work, the only algorithms for any CSP were
based on generalizations of the -space algorithm for
, and were in particular so-called "sketching" algorithms.
In this work, we demonstrate that more sophisticated streaming algorithms can
outperform these algorithms even on general instances.
Our algorithm constructs a "snapshot" of the graph and then applies a result
of Feige and Jozeph (Algorithmica, 2015) to approximately estimate the
value from this snapshot. Constructing this snapshot is
easy for bounded-degree graphs and the main contribution of our work is to
construct this snapshot in the general setting. This involves some delicate
sampling methods as well as a host of "continuity" results on the
behaviour in graphs.Comment: 57 pages, 2 figure
The vanishing ideal of a finite set of points with multiplicity structures
Given a finite set of arbitrarily distributed points in affine space with
arbitrary multiplicity structures, we present an algorithm to compute the
reduced Groebner basis of the vanishing ideal under the lexicographic ordering.
Our method discloses the essential geometric connection between the relative
position of the points with multiplicity structures and the quotient basis of
the vanishing ideal, so we will explicitly know the set of leading terms of
elements of I. We split the problem into several smaller ones which can be
solved by induction over variables and then use our new algorithm for
intersection of ideals to compute the result of the original problem. The new
algorithm for intersection of ideals is mainly based on the Extended Euclidean
Algorithm.Comment: 12 pages,12 figures,ASCM 201
Small-scale-field Dynamo
Generation of magnetic field energy, without mean field generation, is
studied. Isotropic mirror-symmetric turbulence of a conducting fluid amplifies
the energy of small-scale magnetic perturbations if the magnetic Reynolds
number is high, and the dimensionality of space d satisfies 2.103 < d <8.765.
The result does not depend on the model of turbulence, incompressibility and
isotropy being the only requirements.Comment: 11 pages Plain TeX, no figures, Accepted by Phys. Rev. Let
Development of ASMBS Research Agenda for Bariatric Surgery Using the Delphi Methodology
Background
While the number of research publications related to bariatric surgery have increased remarkably in the past decade, research efforts remain uncoordinated and have limited focus, and numerous important questions remain unanswered.
Objective
To generate a research agenda in bariatric surgery.
Setting
National survey.
Methods
The membership of the American Society of Metabolic and Bariatric Surgery (ASMBS) was asked to submit research questions needed to advance the field of bariatric surgery. An expert panel grouped and collated submitted questions and redistributed them back to the membership to rate their importance on a 5-point Likert scale using a 3-round modified Delphi methodology. The top research questions were determined based on provided rankings.
Results
Two hundred ninety-two research questions were initially submitted that were collapsed to 59 unique questions. The ratings for the top 40 questions ranged from 2.67–4.33 (overall mean, 3.46). The highest-ranked questions centered on the mechanisms of effectiveness of bariatric surgery for weight loss and diabetes resolution, the underlying etiology of weight recidivism, and predictors of success.
Conclusions
A research agenda for bariatric surgery was developed using the Delphi methodology. This research agenda may enhance the ability of investigators and funding organizations, including the ASMBS, to focus attention to areas most likely to advance the field, and by editors and reviewers to assess the merit and relevance of scientific contributions
Formation and Primary Heating of The Solar Corona - Theory and Simulation
An integrated Magneto-Fluid model, that accords full treatment to the
Velocity fields associated with the directed plasma motion, is developed to
investigate the dynamics of coronal structures. It is suggested that the
interaction of the fluid and the magnetic aspects of plasma may be a crucial
element in creating so much diversity in the solar atmosphere. It is shown that
the structures which comprise the solar corona can be created by particle
(plasma) flows observed near the Sun's surface - the primary heating of these
structures is caused by the viscous dissipation of the flow kinetic energy.Comment: 46 pages including 7 pages of figures, Submitted to Phys.Plasma
Utilizing field collected insects for next generation sequencing: effects of sampling, storage and DNA extraction methods
DNA sequencing technologies continue to advance the biological sciences, expanding
opportunities for genomic studies of non‐model organisms for basic and applied
questions. Despite these opportunities, many next generation sequencing protocols
have been developed assuming a substantial quantity of high molecular weight DNA
(>100 ng), which can be difficult to obtain for many study systems. In particular, the
ability to sequence field‐collected specimens that exhibit varying levels of DNA degradation
remains largely unexplored. In this study we investigate the influence of
five traditional insect capture and curation methods on Double‐Digest Restriction
Enzyme Associated DNA (ddRAD) sequencing success for three wild bee species.
We sequenced a total of 105 specimens (between 7–13 specimens per species and
treatment). We additionally investigated how different DNA quality metrics (including
pre‐sequence concentration and contamination) predicted downstream sequencing
success, and also compared two DNA extraction methods. We report successful
library preparation for all specimens, with all treatments and extraction methods producing
enough highly reliable loci for population genetic analyses. Although results
varied between species, we found that specimens collected by net sampling directly
into 100% EtOH, or by passive trapping followed by 100% EtOH storage before pinning
tended to produce higher quality ddRAD assemblies, likely as a result of rapid
specimen desiccation. Surprisingly, we found that specimens preserved in propylene
glycol during field sampling exhibited lower‐quality assemblies. We provide recommendations
for each treatment, extraction method, and DNA quality assessment,
and further encourage researchers to consider utilizing a wider variety of specimens
for genomic analysesinfo:eu-repo/semantics/publishedVersio
Index
The interest in relativistic beam-plasma instabilities has been greatly rejuvenated over the past two decades by novel concepts in laboratory and space plasmas. Recent advances in this long-standing field are here reviewed from both theoretical and numerical points of view. The primary focus is on the two-dimensional spectrum of unstable electromagnetic waves growing within relativistic, unmagnetized, and uniform electron beam-plasma systems. Although the goal is to provide a unified picture of all instability classes at play, emphasis is put on the potentially dominant waves propagating obliquely to the beam direction, which have received little attention over the years. First, the basic derivation of the general dielectric function of a kinetic relativistic plasma is recalled. Next, an overview of two-dimensional unstable spectra associated with various beam-plasma distribution functions is given. Both cold-fluid and kinetic linear theory results are reported, the latter being based on waterbag and Maxwell–Jüttner model distributions. The main properties of the competing modes (developing parallel, transverse, and oblique to the beam) are given, and their respective region of dominance in the system parameter space is explained. Later sections address particle-in-cell numerical simulations and the nonlinear evolution of multidimensional beam-plasma systems. The elementary structures generated by the various instability classes are first discussed in the case of reduced-geometry systems. Validation of linear theory is then illustrated in detail for large-scale systems, as is the multistaged character of the nonlinear phase. Finally, a collection of closely related beam-plasma problems involving additional physical effects is presented, and worthwhile directions of future research are outlined.Original Publication: Antoine Bret, Laurent Gremillet and Mark Eric Dieckmann, Multidimensional electron beam-plasma instabilities in the relativistic regime, 2010, Physics of Plasmas, (17), 12, 120501-1-120501-36. http://dx.doi.org/10.1063/1.3514586 Copyright: American Institute of Physics http://www.aip.org/</p
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