273 research outputs found
Logarithmic oscillators: ideal Hamiltonian thermostats
A logarithmic oscillator (in short, log-oscillator) behaves like an ideal
thermostat because of its infinite heat capacity: when it weakly couples to
another system, time averages of the system observables agree with ensemble
averages from a Gibbs distribution with a temperature T that is given by the
strength of the logarithmic potential. The resulting equations of motion are
Hamiltonian and may be implemented not only in a computer but also with
real-world experiments, e.g., with cold atoms.Comment: 5 pages, 3 figures. v4: version accepted in Phys. Rev. Let
Two-photon excited LIF determination of H-atom concentrations near a heated filament in a low pressure H2 environment
Meier U, Kohse-Höinghaus K, Schäfer L, Klages C-P. Two-photon excited LIF determination of H-atom concentrations near a heated filament in a low pressure H2 environment. Applied Optics. 1990;29(33):4993-4999.With respect to the investigation of low pressure filament-assisted chemical vapor deposition processes for diamond formation, absolute concentrations of atomic hydrogen were determined by two-photon laserinduced fluorescence in the vicinity of a heated filament in an environment containing H2 or mixtures of H2 and CH4. Radial H concentration profiles were obtained for different pressures and filament temperatures, diameters, and materials. The influence of the addition of various amounts of methane on the H atom concentrations was examined
Modeling of Atmospheric-Pressure Dielectric Barrier Discharges in Argon with Small Admixtures of Tetramethylsilane
A time-dependent, spatially one-dimensional fluid-Poisson model is applied to analyze the impact of small amounts of tetramethylsilane (TMS) as precursor on the discharge characteristics of an atmospheric-pressure dielectric barrier discharge (DBD) in argon. Based on an established reaction kinetics for argon, it includes a plasma chemistry for TMS, which is validated by measurements of the ignition voltage at the frequency f=86.2kHz for TMS amounts of up to 200 ppm. Details of both a reduced Ar-TMS reaction kinetics scheme and an extended plasma-chemistry model involving about 60 species and 580 reactions related to TMS are given. It is found that good agreement between measured and calculated data can be obtained, when assuming that 25% of the reactions of TMS with excited argon atoms with a rate coefficient of 3.0×10−16m3/s lead to the production of electrons due to Penning ionization. Modeling results for an applied voltage Ua,0=4kV show that TMS is depleted during the residence time of the plasma in the DBD, where the percentage consumption of TMS decreases with increasing TMS fraction because only a finite number of excited argon species is available to dissociate and/or ionize the precursor via energy transfer. Main species resulting from that TMS depletion are presented and discussed. In particular, the analysis clearly indicates that trimethylsilyl cations can be considered to be mainly responsible for the film formation
A GPU-based Correlator X-engine Implemented on the CHIME Pathfinder
We present the design and implementation of a custom GPU-based compute
cluster that provides the correlation X-engine of the CHIME Pathfinder radio
telescope. It is among the largest such systems in operation, correlating
32,896 baselines (256 inputs) over 400MHz of radio bandwidth. Making heavy use
of consumer-grade parts and a custom software stack, the system was developed
at a small fraction of the cost of comparable installations. Unlike existing
GPU backends, this system is built around OpenCL kernels running on
consumer-level AMD GPUs, taking advantage of low-cost hardware and leveraging
packed integer operations to double algorithmic efficiency. The system achieves
the required 105TOPS in a 10kW power envelope, making it among the most
power-efficient X-engines in use today.Comment: 6 pages, 5 figures. Accepted by IEEE ASAP 201
Calibrating CHIME, A New Radio Interferometer to Probe Dark Energy
The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is a transit
interferometer currently being built at the Dominion Radio Astrophysical
Observatory (DRAO) in Penticton, BC, Canada. We will use CHIME to map neutral
hydrogen in the frequency range 400 -- 800\,MHz over half of the sky, producing
a measurement of baryon acoustic oscillations (BAO) at redshifts between 0.8 --
2.5 to probe dark energy. We have deployed a pathfinder version of CHIME that
will yield constraints on the BAO power spectrum and provide a test-bed for our
calibration scheme. I will discuss the CHIME calibration requirements and
describe instrumentation we are developing to meet these requirements
Canadian Hydrogen Intensity Mapping Experiment (CHIME) Pathfinder
A pathfinder version of CHIME (the Canadian Hydrogen Intensity Mapping
Experiment) is currently being commissioned at the Dominion Radio Astrophysical
Observatory (DRAO) in Penticton, BC. The instrument is a hybrid cylindrical
interferometer designed to measure the large scale neutral hydrogen power
spectrum across the redshift range 0.8 to 2.5. The power spectrum will be used
to measure the baryon acoustic oscillation (BAO) scale across this poorly
probed redshift range where dark energy becomes a significant contributor to
the evolution of the Universe. The instrument revives the cylinder design in
radio astronomy with a wide field survey as a primary goal. Modern low-noise
amplifiers and digital processing remove the necessity for the analog
beamforming that characterized previous designs. The Pathfinder consists of two
cylinders 37\,m long by 20\,m wide oriented north-south for a total collecting
area of 1,500 square meters. The cylinders are stationary with no moving parts,
and form a transit instrument with an instantaneous field of view of
100\,degrees by 1-2\,degrees. Each CHIME Pathfinder cylinder has a
feedline with 64 dual polarization feeds placed every 30\,cm which
Nyquist sample the north-south sky over much of the frequency band. The signals
from each dual-polarization feed are independently amplified, filtered to
400-800\,MHz, and directly sampled at 800\,MSps using 8 bits. The correlator is
an FX design, where the Fourier transform channelization is performed in FPGAs,
which are interfaced to a set of GPUs that compute the correlation matrix. The
CHIME Pathfinder is a 1/10th scale prototype version of CHIME and is designed
to detect the BAO feature and constrain the distance-redshift relation.Comment: 20 pages, 12 figures. submitted to Proc. SPIE, Astronomical
Telescopes + Instrumentation (2014
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