44,622 research outputs found
AliEnFS - a Linux File System for the AliEn Grid Services
Among the services offered by the AliEn (ALICE Environment
http://alien.cern.ch) Grid framework there is a virtual file catalogue to allow
transparent access to distributed data-sets using various file transfer
protocols. (AliEn File System) integrates the AliEn file catalogue as
a new file system type into the Linux kernel using LUFS, a hybrid user space
file system framework (Open Source http://lufs.sourceforge.net). LUFS uses a
special kernel interface level called VFS (Virtual File System Switch) to
communicate via a generalised file system interface to the AliEn file system
daemon. The AliEn framework is used for authentication, catalogue browsing,
file registration and read/write transfer operations. A C++ API implements the
generic file system operations. The goal of AliEnFS is to allow users easy
interactive access to a worldwide distributed virtual file system using
familiar shell commands (f.e. cp,ls,rm ...) The paper discusses general aspects
of Grid File Systems, the AliEn implementation and present and future
developments for the AliEn Grid File System.Comment: 9 pages, 12 figure
The AliEn system, status and perspectives
AliEn is a production environment that implements several components of the
Grid paradigm needed to simulate, reconstruct and analyse HEP data in a
distributed way. The system is built around Open Source components, uses the
Web Services model and standard network protocols to implement the computing
platform that is currently being used to produce and analyse Monte Carlo data
at over 30 sites on four continents. The aim of this paper is to present the
current AliEn architecture and outline its future developments in the light of
emerging standards.Comment: Talk from the 2003 Computing in High Energy and Nuclear Physics
(CHEP03), La Jolla, Ca, USA, March 2003, 10 pages, Word, 10 figures. PSN
MOAT00
Atomic hydrogen maser active oscillator cavity and bulb design optimization
The performance characteristics and reliability of the active oscillator atomic hydrogen maser depend upon oscillation parameters which characterize the interaction region of the maser, the resonant cavity and atom storage bulb assembly. With particular attention to use of the cavity frequency switching servo (1) to reduce cavity pulling, it is important to maintain high oscillation level, high atomic beam flux utilization efficiency, small spin exchange parameter and high cavity quality factor. It is also desirable to have a small and rigid cavity and bulb structure and to minimize the cavity temperature sensitivity. Curves for a novel hydrogen maser cavity configuration which is partially loaded with a quartz dielectric cylinder and show the relationships between cavity length, cavity diameter, bulb size, dielectric thickness, cavity quality factor, filling factor and cavity frequency temperature coefficient are presented. The results are discussed in terms of improvement in maser performance resulting from particular design choices
The production of glow precursors by oxidative erosion of spacecraft surfaces
Erosion rates of organic materials are measured during a recent flight of the shuttle (STS-8). Several forms of carbon and a variety of thermosetting and thermoplastic polymers are exposed to the ram beam of atomic oxygen. Arrhenius energies of about 1000 to 2000 cal/mole were measured from the rate dependencies on temperature. If some simple assumptions are made about the chemical nature of the desorbed species, the data can be used to estimate production rates at surfaces in orbit under different conditions of temperature, oxygen atom flux, and material surface conditions
Multi-Task Policy Search for Robotics
© 2014 IEEE.Learning policies that generalize across multiple tasks is an important and challenging research topic in reinforcement learning and robotics. Training individual policies for every single potential task is often impractical, especially for continuous task variations, requiring more principled approaches to share and transfer knowledge among similar tasks. We present a novel approach for learning a nonlinear feedback policy that generalizes across multiple tasks. The key idea is to define a parametrized policy as a function of both the state and the task, which allows learning a single policy that generalizes across multiple known and unknown tasks. Applications of our novel approach to reinforcement and imitation learning in realrobot experiments are shown
Massive Black Holes in Star Clusters. II. Realistic Cluster Models
We have followed the evolution of multi-mass star clusters containing massive
central black holes through collisional N-body simulations done on GRAPE6. Each
cluster is composed of between 16,384 to 131,072 stars together with a black
hole with an initial mass of M_BH=1000 Msun. We follow the evolution of the
clusters under the combined influence of two-body relaxation, stellar mass-loss
and tidal disruption of stars.
The (3D) mass density profile follows a power-law distribution \rho \sim
r^{-\alpha} with slope \alpha=1.55. This leads to a constant density profile of
bright stars in projection, which makes it highly unlikely that core collapse
clusters contain intermediate-mass black holes (IMBHs). Instead globular
clusters containing IMBHs can be fitted with standard King profiles.
The disruption rate of stars is too small to form an IMBH out of a M_BH
\approx 50 Msun progenitor black hole, unless a cluster starts with a central
density significantly higher than what is seen in globular clusters.
Kinematical studies can reveal 1000 Msun IMBHs in the closest clusters. IMBHs
in globular clusters are only weak X-ray sources since the tidal disruption
rate of stars is low and the star closest to the IMBH is normally another black
hole. For globular clusters, dynamical evolution can push compact stars near
the IMBH to distances small enough that they become detectable through
gravitational radiation. If 10% of all globular clusters contain IMBHs,
extragalactic globular clusters could be one of the major sources for {\it
LISA}. (abridged)Comment: 20 pages, 16 figures, ApJ in pres
Building research capacity in a practitioner network: the National Action Research Network on researching and evaluating Personal Development Plannin
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