3,741 research outputs found
Final Report for DoE Grant DE-FG02-06ER54878, Laboratory Studies of Reconnection in Magnetically Confined Plasmas
The study of the collisionless magnetic reconnection constituted the primary work carried out under this grant. The investigations utilized two magnetic configurations with distinct boundary conditions. Both configurations were based upon the Versatile Toroidal Facility (VTF). The first configuration is characterized by open boundary conditions where the magnetic field lines interface directly with the vacuum vessel walls. The reconnection dynamics for this configuration has been methodically characterized and it has been shown that kinetic effects related to trapped electron trajectories are responsible for the high rates of reconnection observed. This type of reconnection has not been investigated before. Nevertheless, the results are directly relevant to observations by the Wind spacecraft of fast reconnection deep in the Earth magnetotail. The second configuration was developed to be specifically relevant to numerical simulations of magnetic reconnection, allowing the magnetic field-lines to be contained inside the device. The configuration is compatible with the presence of large current sheets in the reconnection region and reconnection is observed in fast powerful bursts. These reconnection events facilitate the first experimental investigations of the physics governing the spontaneous onset of fast reconnection. In this Report we review the general motivation of this work, the experimental set-up, and the main physics results
Calculating the chiral condensate diagrammatically at strong coupling
We calculate the chiral condensate of QCD at infinite coupling as a function
of the number of fundamental fermion flavours using a lattice diagrammatic
approach inspired by recent work of Tomboulis, and other work from the 80's. We
outline the approach where the diagrams are formed by combining a truncated
number of sub-diagram types in all possible ways. Our results show evidence of
convergence and agreement with simulation results at small Nf. However,
contrary to recent simulation results, we do not observe a transition at a
critical value of Nf. We further present preliminary results for the chiral
condensate of QCD with symmetric or adjoint representation fermions at infinite
coupling as a function of Nf for Nc = 3. In general, there are sources of error
in this approach associated with miscounting of overlapping diagrams, and
over-counting of diagrams due to symmetries. These are further elaborated upon
in a longer paper.Comment: presented at the 32nd International Symposium on Lattice Field Theory
(Lattice 2014), 23-28 June 2014, New York, NY, US
Big Universe, Big Data: Machine Learning and Image Analysis for Astronomy
Astrophysics and cosmology are rich with data. The advent of wide-area
digital cameras on large aperture telescopes has led to ever more ambitious
surveys of the sky. Data volumes of entire surveys a decade ago can now be
acquired in a single night and real-time analysis is often desired. Thus,
modern astronomy requires big data know-how, in particular it demands highly
efficient machine learning and image analysis algorithms. But scalability is
not the only challenge: Astronomy applications touch several current machine
learning research questions, such as learning from biased data and dealing with
label and measurement noise. We argue that this makes astronomy a great domain
for computer science research, as it pushes the boundaries of data analysis. In
the following, we will present this exciting application area for data
scientists. We will focus on exemplary results, discuss main challenges, and
highlight some recent methodological advancements in machine learning and image
analysis triggered by astronomical applications
Lévy driven moving averages and semimartingales
AbstractThe aim of the present paper is to study the semimartingale property of continuous time moving averages driven by Lévy processes. We provide necessary and sufficient conditions on the kernel for the moving average to be a semimartingale in the natural filtration of the Lévy process, and when this is the case we also provide a useful representation. Assuming that the driving Lévy process is of unbounded variation, we show that the moving average is a semimartingale if and only if the kernel is absolutely continuous with a density satisfying an integrability condition
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