4,704 research outputs found
Evidence cross-validation and Bayesian inference of MAST plasma equilibria
In this paper, current profiles for plasma discharges on the Mega-Ampere
Spherical Tokamak (MAST) are directly calculated from pickup coil, flux loop
and Motional-Stark Effect (MSE) observations via methods based in the
statistical theory of Bayesian analysis. By representing toroidal plasma
current as a series of axisymmetric current beams with rectangular
cross-section and inferring the current for each one of these beams,
flux-surface geometry and q-profiles are subsequently calculated by elementary
application of Biot-Savart's law. The use of this plasma model in the context
of Bayesian analysis was pioneered by Svensson and Werner on the Joint-European
Tokamak (JET) [J. Svensson and A. Werner. Current tomography for axisymmetric
plasmas. Plasma Physics and Controlled Fusion, 50(8):085002, 2008]. In
this framework, linear forward models are used to generate diagnostic
predictions, and the probability distribution for the currents in the
collection of plasma beams was subsequently calculated directly via application
of Bayes' formula. In this work, we introduce a new diagnostic technique to
identify and remove outlier observations associated with diagnostics falling
out of calibration or suffering from an unidentified malfunction. These
modifications enable good agreement between Bayesian inference of the last
closed flux-surface (LCFS) with other corroborating data, such as such as that
from force balance considerations using EFIT++ [L. Appel et al., Proc. 33rd EPS
Conf., Rome, Italy, 2006]. In addition, this analysis also yields errors on the
plasma current profile and flux-surface geometry, as well as directly
predicting the Shafranov shift of the plasma core.This work was jointly funded by the Australian Government
through International Science Linkages Grant No.
CG130047, the Australian National University, the United
Kingdom Engineering and Physical Sciences Research
Council under Grant No. EP/G003955, and by the European
Communities under the contract of Association between EURATOM and CCFE
Using Bayesian Analysis and Gaussian Processes to Infer Electron Temperature and Density Profiles on the MAST Experiment
A unified, Bayesian inference of midplane electron temperature and density
profiles using both Thompson scattering (TS) and interferometric data is
presented. Beyond the Bayesian nature of the analysis, novel features of the
inference are the use of a Gaussian process prior to infer a mollification
length-scale of inferred profiles and the use of Gauss-Laguerre quadratures to
directly calculate the depolarisation term associated with the TS forward
model. Results are presented from an application of the method to data from the
high resolution TS system on the Mega-Ampere Spherical Tokamak, along with a
comparison to profiles coming from the standard analysis carried out on that
system.This work was jointly funded by the Australian Government
through International Science Linkages Grant
No. CG130047, the Australian National University, the
RCUK Energy Programme under Grant No. EP/I501045, and
the European Communities under the contract of Association
between EURATOM and CCFE
On the Regularity of Optimal Transportation Potentials on Round Spheres
In this paper the regularity of optimal transportation potentials defined on
round spheres is investigated. Specifically, this research generalises the
calculations done by Loeper, where he showed that the strong (A3) condition of
Trudinger and Wang is satisfied on the round sphere, when the cost-function is
the geodesic distance squared. In order to generalise Loeper's calculation to a
broader class of cost-functions, the (A3) condition is reformulated via a
stereographic projection that maps charts of the sphere into Euclidean space.
This reformulation subsequently allows one to verify the (A3) condition for any
case where the cost-fuction of the associated optimal transportation problem
can be expressed as a function of the geodesic distance between points on a
round sphere. With this, several examples of such cost-functions are then
analysed to see whether or not they satisfy this (A3) condition.Comment: 24 pages, 4 figure
FACT -- The G-APD revolution in Cherenkov astronomy
Since two years, the FACT telescope is operating on the Canary Island of La
Palma. Apart from its purpose to serve as a monitoring facility for the
brightest TeV blazars, it was built as a major step to establish solid state
photon counters as detectors in Cherenkov astronomy. The camera of the First
G-APD Cherenkov Telesope comprises 1440 Geiger-mode avalanche photo diodes
(G-APD), equipped with solid light guides to increase the effective light
collection area of each sensor. Since no sense-line is available, a special
challenge is to keep the applied voltage stable although the current drawn by
the G-APD depends on the flux of night-sky background photons significantly
varying with ambient light conditions. Methods have been developed to keep the
temperature and voltage dependent response of the G-APDs stable during
operation. As a cross-check, dark count spectra with high statistics have been
taken under different environmental conditions. In this presentation, the
project, the developed methods and the experience from two years of operation
of the first G-APD based camera in Cherenkov astronomy under changing
environmental conditions will be presented.Comment: Proceedings of the Nuclear Science Symposium and Medical Imaging
Conference (IEEE-NSS/MIC), 201
FACT - The First G-APD Cherenkov Telescope: Status and Results
The First G-APD Cherenkov telescope (FACT) is the first telescope using
silicon photon detectors (G-APD aka. SiPM). It is built on the mount of the
HEGRA CT3 telescope, still located at the Observatorio del Roque de los
Muchachos, and it is successfully in operation since Oct. 2011. The use of
Silicon devices promises a higher photon detection efficiency, more robustness
and higher precision than photo-multiplier tubes. The FACT collaboration is
investigating with which precision these devices can be operated on the
long-term. Currently, the telescope is successfully operated from remote and
robotic operation is under development. During the past months of operation,
the foreseen monitoring program of the brightest known TeV blazars has been
carried out, and first physics results have been obtained including a strong
flare of Mrk501. An instantaneous flare alert system is already in a testing
phase. This presentation will give an overview of the project and summarize its
goals, status and first results
A Proposal for a Three Detector Short-Baseline Neutrino Oscillation Program in the Fermilab Booster Neutrino Beam
A Short-Baseline Neutrino (SBN) physics program of three LAr-TPC detectors
located along the Booster Neutrino Beam (BNB) at Fermilab is presented. This
new SBN Program will deliver a rich and compelling physics opportunity,
including the ability to resolve a class of experimental anomalies in neutrino
physics and to perform the most sensitive search to date for sterile neutrinos
at the eV mass-scale through both appearance and disappearance oscillation
channels. Using data sets of 6.6e20 protons on target (P.O.T.) in the LAr1-ND
and ICARUS T600 detectors plus 13.2e20 P.O.T. in the MicroBooNE detector, we
estimate that a search for muon neutrino to electron neutrino appearance can be
performed with ~5 sigma sensitivity for the LSND allowed (99% C.L.) parameter
region. In this proposal for the SBN Program, we describe the physics analysis,
the conceptual design of the LAr1-ND detector, the design and refurbishment of
the T600 detector, the necessary infrastructure required to execute the
program, and a possible reconfiguration of the BNB target and horn system to
improve its performance for oscillation searches.Comment: 209 pages, 129 figure
Search for Charginos with a Small Mass Difference with the Lightest Supersymmetric Particle at \sqrt{s} = 189 GeV
A search for charginos nearly mass-degenerate with the lightest
supersymmetric particle is performed using the 176 pb^-1 of data collected at
189 GeV in 1998 with the L3 detector. Mass differences between the chargino and
the lightest supersymmetric particle below 4 GeV are considered. The presence
of a high transverse momentum photon is required to single out the signal from
the photon-photon interaction background. No evidence for charginos is found
and upper limits on the cross section for chargino pair production are set. For
the first time, in the case of heavy scalar leptons, chargino mass limits are
obtained for any \tilde{\chi}^{+-}_1 - \tilde{\chi}^0_1 mass difference
- …