146 research outputs found
Computation of milestones for decision support during system restoration
System restoration involves status assessment, optimization of generation capability, and load pickup. The optimization problem needs to take complex constraints into consideration, and therefore, it is not practical to formulate the problem as one single optimization problem. The other critical consideration for the development of decision support tools is its generality, i.e., the tools should be portable from a system to another with minimal customization. This paper reports a practical methodology for construction of system restoration strategies. The strategy adopted by each power system differs, depending on system characteristics and policies. A new method based on the concept of generic restoration milestones (GRMs) is proposed. A specific restoration strategy can be synthesized by a combination of GRMs based on the actual system conditions. The decision support tool is expected to reduce the restoration time, thereby improving system reliability. The proposed decision support tool has been validated with cases based on a simplified Western Electricity Coordinating Council (WECC) 200-Bus system and Hawaiian Electric Company system. © 2011 IEEE.published_or_final_versio
An Electron-Tracking Compton Telescope for a Survey of the Deep Universe by MeV gamma-rays
Photon imaging for MeV gammas has serious difficulties due to huge
backgrounds and unclearness in images, which are originated from incompleteness
in determining the physical parameters of Compton scattering in detection,
e.g., lack of the directional information of the recoil electrons. The recent
major mission/instrument in the MeV band, Compton Gamma Ray
Observatory/COMPTEL, which was Compton Camera (CC), detected mere
persistent sources. It is in stark contrast with 2000 sources in the GeV
band. Here we report the performance of an Electron-Tracking Compton Camera
(ETCC), and prove that it has a good potential to break through this stagnation
in MeV gamma-ray astronomy. The ETCC provides all the parameters of
Compton-scattering by measuring 3-D recoil electron tracks; then the Scatter
Plane Deviation (SPD) lost in CCs is recovered. The energy loss rate (dE/dx),
which CCs cannot measure, is also obtained, and is found to be indeed helpful
to reduce the background under conditions similar to space. Accordingly the
significance in gamma detection is improved severalfold. On the other hand, SPD
is essential to determine the point-spread function (PSF) quantitatively. The
SPD resolution is improved close to the theoretical limit for multiple
scattering of recoil electrons. With such a well-determined PSF, we demonstrate
for the first time that it is possible to provide reliable sensitivity in
Compton imaging without utilizing an optimization algorithm. As such, this
study highlights the fundamental weak-points of CCs. In contrast we demonstrate
the possibility of ETCC reaching the sensitivity below erg
cm s at 1 MeV.Comment: 19 pages, 12 figures, Accepted to the Astrophysical Journa
New readout and data-acquisition system in an electron-tracking Compton camera for MeV gamma-ray astronomy (SMILE-II)
For MeV gamma-ray astronomy, we have developed an electron-tracking Compton
camera (ETCC) as a MeV gamma-ray telescope capable of rejecting the radiation
background and attaining the high sensitivity of near 1 mCrab in space. Our
ETCC comprises a gaseous time-projection chamber (TPC) with a micro pattern gas
detector for tracking recoil electrons and a position-sensitive scintillation
camera for detecting scattered gamma rays. After the success of a first balloon
experiment in 2006 with a small ETCC (using a 101015 cm
TPC) for measuring diffuse cosmic and atmospheric sub-MeV gamma rays (Sub-MeV
gamma-ray Imaging Loaded-on-balloon Experiment I; SMILE-I), a (30 cm)
medium-sized ETCC was developed to measure MeV gamma-ray spectra from celestial
sources, such as the Crab Nebula, with single-day balloon flights (SMILE-II).
To achieve this goal, a 100-times-larger detection area compared with that of
SMILE-I is required without changing the weight or power consumption of the
detector system. In addition, the event rate is also expected to dramatically
increase during observation. Here, we describe both the concept and the
performance of the new data-acquisition system with this (30 cm) ETCC to
manage 100 times more data while satisfying the severe restrictions regarding
the weight and power consumption imposed by a balloon-borne observation. In
particular, to improve the detection efficiency of the fine tracks in the TPC
from 10\% to 100\%, we introduce a new data-handling algorithm in
the TPC. Therefore, for efficient management of such large amounts of data, we
developed a data-acquisition system with parallel data flow.Comment: 11 pages, 24 figure
CANGAROO-III Observation of TeV Gamma Rays from the vicinity of PSR B1 706-44
Observation by the CANGAROO-III stereoscopic system of the Imaging Cherenkov
Telescope has detected extended emission of TeV gamma rays in the vicinity of
the pulsar PSR B170644. The strength of the signal observed as
gamma-ray-like events varies when we apply different ways of emulating
background events. The reason for such uncertainties is argued in relevance to
gamma-rays embedded in the "off-source data", that is, unknown sources and
diffuse emission in the Galactic plane, namely, the existence of a complex
structure of TeV gamma-ray emission around PSR B170644.Comment: 10 pages, 13 figures, to be published in Ap
CANGAROO-III observation of TeV gamma rays from the unidentified gamma-ray source HESS J1614-518
We report the detection, with the CANGAROO-III imaging atmospheric Cherenkov
telescope array, of a very high energy gamma-ray signal from the unidentified
gamma-ray source HESS J1614-518, which was discovered in the H.E.S.S. Galactic
plane survey. Diffuse gamma-ray emission was detected above 760 GeV at the 8.9
sigma level during an effective exposure of 54 hr from 2008 May to August. The
spectrum can be represented by a power-law:
8.2+-2.2_{stat}+-2.5_{sys}x10^{-12}x (E/1TeV)^{-Gamma} cm^{-2} s^{-1} TeV^{-1}
with a photon index Gamma of 2.4+-0.3_{stat}+-0.2_{sys}, which is compatible
with that of the H.E.S.S. observations. By combining our result with
multi-wavelength data, we discuss the possible counterparts for HESS J1614-518
and consider radiation mechanisms based on hadronic and leptonic processes for
a supernova remnant, stellar winds from massive stars, and a pulsar wind
nebula. Although a leptonic origin from a pulsar wind nebula driven by an
unknown pulsar remains possible, hadronic-origin emission from an unknown
supernova remnant is preferred.Comment: 9 pages, 7 figures, accepted for publication in Ap
A Bayesian assessment of an approximate model for unconfined water flow in sloping layered porous media
The prediction of water table height in unconfined layered porous media is a difficult modelling problem that typically requires numerical simulation. This paper proposes an analytical model to approximate the exact solution based on a steady-state Dupuit–Forchheimer analysis. The key contribution in relation to a similar model in the literature relies in the ability of the proposed model to consider more than two layers with different thicknesses and slopes, so that the existing model becomes a special case of the proposed model herein. In addition, a model assessment methodology based on the Bayesian inverse problem is proposed to efficiently identify the values of the physical parameters for which the proposed model is accurate when compared against a reference model given by MODFLOW-NWT, the open-source finite-difference code by the U.S. Geological Survey. Based on numerical results for a representative case study, the ratio of vertical recharge rate to hydraulic conductivity emerges as a key parameter in terms of model accuracy so that, when appropriately bounded, both the proposed model and MODFLOW-NWT provide almost identical results
Readout technologies for directional WIMP Dark Matter detection
The measurement of the direction of WIMP-induced nuclear recoils is a compelling but technologically challenging strategy to provide an unambiguous signature of the detection of Galactic dark matter. Most directional detectors aim to reconstruct the dark-matter-induced nuclear recoil tracks, either in gas or solid targets. The main challenge with directional detection is the need for high spatial resolution over large volumes, which puts strong requirements on the readout technologies. In this paper we review the various detector readout technologies used by directional detectors. In particular, we summarize the challenges, advantages and drawbacks of each approach, and discuss future prospects for these technologies
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