37,858 research outputs found
Spectra of magnetic perturbations triggered by pellets in JET plasmas
Aiming at investigating edge localised mode (ELM) pacing for future application on ITER, experiments have been conducted on JET injecting pellets in different plasma configurations, including high confinement regimes with type-I and type-III ELMs, low confinement regimes and Ohmically heated plasmas. The magnetic perturbations spectra and the toroidal mode number, n, of triggered events are compared with those of spontaneous ELMs using a wavelet analysis to provide good time resolution of short-lived coherent modes. It is found that—in all these configurations—triggered events have a coherent mode structure, indicating that pellets can trigger an MHD event basically in every background plasma. Two components have been found in the magnetic perturbations induced by pellets, with distinct frequencies and toroidal mode numbers. In high confinement regimes triggered events have similarities with spontaneous ELMs: both are seen to start from low toroidal mode numbers, then the maximum measured n increases up to about 10 within 0.3 ms before the ELM burst
Event Reconstruction in the PHENIX Central Arm Spectrometers
The central arm spectrometers for the PHENIX experiment at the Relativistic
Heavy Ion Collider have been designed for the optimization of particle
identification in relativistic heavy ion collisions. The spectrometers present
a challenging environment for event reconstruction due to a very high track
multiplicity in a complicated, focusing, magnetic field. In order to meet this
challenge, nine distinct detector types are integrated for charged particle
tracking, momentum reconstruction, and particle identification. The techniques
which have been developed for the task of event reconstruction are described.Comment: Accepted for publication in Nucl. Instrum. A. 34 pages, 23 figure
The ArgoNeuT Detector in the NuMI Low-Energy beam line at Fermilab
The ArgoNeuT liquid argon time projection chamber has collected thousands of
neutrino and antineutrino events during an extended run period in the NuMI
beam-line at Fermilab. This paper focuses on the main aspects of the detector
layout and related technical features, including the cryogenic equipment, time
projection chamber, read-out electronics, and off-line data treatment. The
detector commissioning phase, physics run, and first neutrino event displays
are also reported. The characterization of the main working parameters of the
detector during data-taking, the ionization electron drift velocity and
lifetime in liquid argon, as obtained from through-going muon data complete the
present report.Comment: 43 pages, 27 figures, 5 tables - update referenc
Space weather challenges of the polar cap ionosphere
This paper presents research on polar cap ionosphere space weather phenomena
conducted during the European Cooperation in Science and Technology (COST)
action ES0803 from 2008 to 2012. The main part of the work has been directed
toward the study of plasma instabilities and scintillations in association with
cusp flow channels and polar cap electron density structures/patches,which is
considered as critical knowledge in order to develop forecast models for
scintillations in the polar cap. We have approached this problem by
multi-instrument techniques that comprise the EISCAT Svalbard Radar, SuperDARN
radars, in-situ rocket, and GPS scintillation measurements. The Discussion
section aims to unify the bits and pieces of highly specialized information
from several papers into a generalized picture. The cusp ionosphere appears as
a hot region in GPS scintillation climatology maps. Our results are consistent
with the existing view that scintillations in the cusp and the polar cap
ionosphere are mainly due to multi-scale structures generated by instability
processes associated with the cross-polar transport of polar cap patches. We
have demonstrated that the SuperDARN convection model can be used to track
these patches backward and forward in time. Hence, once a patch has been
detected in the cusp inflow region, SuperDARN can be used to forecast its
destination in the future. However, the high-density gradient of polar cap
patches is not the only prerequisite for high-latitude scintillations.
Unprecedented high resolution rocket measurements reveal that the cusp
ionosphere is associated with filamentary precipitation giving rise to
kilometer scale gradients onto which the gradient drift instability can operate
very efficiently... (continued
Segregating Event Streams and Noise with a Markov Renewal Process Model
DS and MP are supported by EPSRC Leadership Fellowship EP/G007144/1
Measuring, Predicting and Visualizing Short-Term Change in Word Representation and Usage in VKontakte Social Network
Language in social media is extremely dynamic: new words emerge, trend and
disappear, while the meaning of existing words can fluctuate over time. Such
dynamics are especially notable during a period of crisis. This work addresses
several important tasks of measuring, visualizing and predicting short term
text representation shift, i.e. the change in a word's contextual semantics,
and contrasting such shift with surface level word dynamics, or concept drift,
observed in social media streams. Unlike previous approaches on learning word
representations from text, we study the relationship between short-term concept
drift and representation shift on a large social media corpus - VKontakte posts
in Russian collected during the Russia-Ukraine crisis in 2014-2015. Our novel
contributions include quantitative and qualitative approaches to (1) measure
short-term representation shift and contrast it with surface level concept
drift; (2) build predictive models to forecast short-term shifts in meaning
from previous meaning as well as from concept drift; and (3) visualize
short-term representation shift for example keywords to demonstrate the
practical use of our approach to discover and track meaning of newly emerging
terms in social media. We show that short-term representation shift can be
accurately predicted up to several weeks in advance. Our unique approach to
modeling and visualizing word representation shifts in social media can be used
to explore and characterize specific aspects of the streaming corpus during
crisis events and potentially improve other downstream classification tasks
including real-time event detection
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