259 research outputs found
Manoeuvring Experiments Using the MUN Explorer AUV
Autonomous Underwater Vehicles (AUVs) are
self-propelled robotic platforms that can perform a
predetermined mission completely unmanned. A series of
manoeuvring experiments were performed using the MUN
Explorer AUV during the summer of 2006 and a selection of
these experimental results are presented in this paper. The
purpose of these experiments was to collect a set of useful
data for validating a hydrodynamic model of the dynamic
performance of the vehicle. This paper aims at explaining the
methods and measures adopted in accomplishing this task.
Apart from providing a data set for validation of the
hydrodynamic model, the data record also shows the ability
of the AUV to perform extreme manoeuvres and the accuracy
with which it can follow a pre-planned mission
Manoeuvring Trials with the MUN Explorer AUV: Data Analysis and Observations
Manoeuvring trials are usually performed to
determine the manoeuvring characteristics of a marine vehicle. It
is through certain standard manoeuvres we evaluate the
robustness, performance and limitations of the vehicle control
system. A series of open-water manoeuvring trials were
performed using the MUN Explorer AUV in the summer of 2006.
The actual purpose of these experiments was to collect a set of
experimental data in order to validate a hydrodynamic model of
the dynamic performance of the vehicle. This paper presents the
results and observations from the analysis of a set of
manoeuvring trials data: in particular the results from straightline
(acceleration – deceleration) tests and turning circles. It
outlines briefly the method by which these tests were conducted
and discusses the results and observations made. Apart from
providing a data set for validation purposes, the results also
indicate the ability of the vehicle to follow a pre-planned mission
with precision
Pepper-pot emittance measurement of laser-plasma wakefield accelerated electrons
The transverse emittance is an important parameter governing the brightness of an electron beam. Here we present the first pepper-pot measurement of the transverse emittance for a mono-energetic electron beam from a laser-plasma wakefield accelerator, carried out on the Advanced Laser-Plasma High Energy Accelerators towards X-Rays (ALPHA-X) beam line. Mono-energetic electrons are passed through an array of 52 mu m diameter holes in a tungsten mask. The pepper-pot results set an upper limit for the normalised emittance at 5.5 +/- 1 pi mm mrad for an 82 MeV beam
Role of surface roughness in hard x-ray emission from femtosecond laser produced copper plasmas
The hard x-ray emission in the energy range of 30-300 keV from copper plasmas
produced by 100 fs, 806 nm laser pulses at intensities in the range of
10 W cm is investigated. We demonstrate that surface
roughness of the targets overrides the role of polarization state in the
coupling of light to the plasma. We further show that surface roughness has a
significant role in enhancing the x-ray emission in the above mentioned energy
range.Comment: 5 pages, 4 figures, to appear in Phys. Rev.
Exclusion Limits on the WIMP-Nucleon Cross-Section from the First Run of the Cryogenic Dark Matter Search in the Soudan Underground Lab
The Cryogenic Dark Matter Search (CDMS-II) employs low-temperature Ge and Si
detectors to seek Weakly Interacting Massive Particles (WIMPs) via their
elastic scattering interactions with nuclei. Simultaneous measurements of both
ionization and phonon energy provide discrimination against interactions of
background particles. For recoil energies above 10 keV, events due to
background photons are rejected with >99.99% efficiency. Electromagnetic events
very near the detector surface can mimic nuclear recoils because of reduced
charge collection, but these surface events are rejected with >96% efficiency
by using additional information from the phonon pulse shape. Efficient use of
active and passive shielding, combined with the the 2090 m.w.e. overburden at
the experimental site in the Soudan mine, makes the background from neutrons
negligible for this first exposure. All cuts are determined in a blind manner
from in situ calibrations with external radioactive sources without any prior
knowledge of the event distribution in the signal region. Resulting
efficiencies are known to ~10%. A single event with a recoil of 64 keV passes
all of the cuts and is consistent with the expected misidentification rate of
surface-electron recoils. Under the assumptions for a standard dark matter
halo, these data exclude previously unexplored parameter space for both
spin-independent and spin-dependent WIMP-nucleon elastic scattering. The
resulting limit on the spin-independent WIMP-nucleon elastic-scattering
cross-section has a minimum of 4x10^-43 cm^2 at a WIMP mass of 60 GeV/c^2. The
minimum of the limit for the spin-dependent WIMP-neutron elastic-scattering
cross-section is 2x10^-37 cm^2 at a WIMP mass of 50 GeV/c^2.Comment: 37 pages, 42 figure
EMITTANCE AND ENERGY SPREAD MEASUREMENTS OF RELATIVISTICS ELECTRONS FROM LASER-DRIVEN ACCELERATOR
Abstract In this paper, we present a single-shot transverse emittance measurement for 125 ± 3 MeV electron beam using pepper-pot technique. A normalised transverse emittance as low as 1.1 ± 0.1 -mm-mrad was measured using this method. Considering 60 consecutive shots, an average normalised emittance of ε rms,x,y =2.2 ± 0.7, 2.3 ± 0.6 -mm-mrad was obtained, which is comparable to a conventional linear accelerator. We also obtained high energy monoenergetic electron beam with relative energy spread less than 1%. The measured transverse emittance characterises the quality of an electron beam generated from laser-driven accelerator. Brightness, parallelism and focusability are all functions of the emittance. The low emittance and energy spread indicates that this type of accelerator is suitable for compact free electron laser driver
Exercise Capacity and All-Cause Mortality in African American and Caucasian Men With Type 2 Diabetes
OBJECTIVE - The purpose of this study was to assess the association between exercise capacity and mortality in African Americans and Caucasians with type 2 diabetes and to explore racial differences regarding this relationship. RESEARCH DESIGN AND METHODS - African American (n = 1,703; aged 60 ± 10 years) and Caucasian (n = 1,445; aged 62 ± 10 years) men with type 2 diabetes completed a maximal exercise test between 1986 and 2007 at the Veterans Affairs Medical Centers in Washington, DC, and Palo Alto, California. Three fitness categories were established (low-, moderate-, and high-fit) based on peak METs achieved. Subjects were followed for all-cause mortality for 7.3 ± 4.7 years. RESULTS - The adjusted mortality risk was 23% higher in African Americans than in Caucasians (hazard ratio 1.23 [95% CI 1.1-1.4]). A graded reduction in mortality risk was noted with increased exercise capacity for both races. There was a significant interaction between race and METs (P \u3c 0.001) and among race and fitness categories (P \u3c 0.001). The association was stronger for Caucasians. Each 1-MET increase in exercise capacity yielded a 19% lower risk for Caucasians and 14% for African Americans (P \u3c 0.001). Similarly, the risk was 43% lower (0.57 [0.44- 0.73]) for moderate-fit and 67% lower (0.33 [0.22-0.48]) for high-fit Caucasians. The comparable reductions in African Americans were 34% (0.66 [0.55-0.80]) and 46% (0.54 [0.39-0.73]), respectively. CONCLUSIONS - Exercise capacity is a strong predictor of all-cause mortality in African American and Caucasian men with type 2 diabetes. The exercise capacity-related reduction in mortality appears to be stronger and more graded for Caucasians than for African Americans. © 2009 by the American Diabetes Association
Gamma-ray production from resonant betatron oscillations of accelerated electrons in a plasma wake
The laser-plasma wakefield accelerator is a novel ultra-compact particle accelerator. A very intense laser pulse focused onto plasma can excites plasma density waves. Electrons surfing these waves can be accelerated to very high energies with unprecedented accelerating gradients in excess of 1 GV/cm. While accelerating, electrons undergo transverse betatron oscillations and emit synchrotron-like x-ray radiation into a narrow on-axis cone, which is enhanced when electrons interact with the electromagnetic field of the laser. In this case, the laser can resonantly drive the electron motion, lading to direct laser acceleration. This occurs when the betatron frequency matches the Doppler down-shifted frequency of the laser. As a consequence, the number of photons emitted is strongly enhanced and the critical photon energy is increases to 100’s of ke
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