109,708 research outputs found
A Fast Method to Measure the Volume of a Large Cavity
How to quickly and accurately measure the volume of a large cavity is challenging. This paper presents an efficient method to measure the volume of a large conducting cavity. The proposed method is based on statistical wave theory. By measuring the Q factor in the time and frequency domains, the volume of the cavity can be extracted. In the time domain, the Q factor can be extracted directly from the time domain response, while in the frequency domain, the Q factor depends on the volume of the cavity and the transferred power; the transferred power can be measured directly. By correcting the frequency domain Q with the radiation efficiency of antennas, the Q factors obtained from both the time and frequency domains are equal in a well-stirred chamber; this provides an opportunity to measure the volume of the cavity. Measurements are conducted to verify the proposed method. Although the measurement is conducted using electromagnetic waves, acoustic waves can also be used; in this case, the approach can be applied to any cavity, not limited to a conducting cavity. The advantages and the limitations of the proposed method are also discussed
Absolute absorption and fluorescence measurements over a dynamic range of 10 with cavity-enhanced laser-induced fluorescence
We describe a novel experimental setup that combines the advantages of both
laser-induced fluorescence and cavity ring-down techniques. The simultaneous
and correlated measurement of the ring-down and fluorescence signals yields
absolute absorption coefficients for the fluorescence measurement. The combined
measurement is conducted with the same sample in a single, pulsed laser beam.
The fluorescence measurement extends the dynamic range of a stand-alone cavity
ring-down setup from typically three to at least six orders of magnitude. The
presence of the cavity improves the quality of the signal, in particular the
signal-to-noise ratio. The methodology, dubbed cavity-enhanced laser-induced
fluorescence (CELIF), is developed and rigorously tested against the
spectroscopy of 1,4-bis(phenylethynyl)benzene in a molecular beam and density
measurements in a cell. We outline how the method can be utilised to determine
absolute quantities: absorption cross sections, sample densities and
fluorescence quantum yields.Comment: 12 pages, 6 figures, submitted to J. Chem. Phy
Magnetization dynamics in the single-molecule magnet Fe8 under pulsed microwave irradiation
We present measurements on the single molecule magnet Fe8 in the presence of
pulsed microwave radiation at 118 GHz. The spin dynamics is studied via time
resolved magnetization experiments using a Hall probe magnetometer. We
investigate the relaxation behavior of magnetization after the microwave pulse.
The analysis of the experimental data is performed in terms of different
contributions to the magnetization after-pulse relaxation. We find that the
phonon bottleneck with a characteristic relaxation time of 10 to 100 ms
strongly affects the magnetization dynamics. In addition, the spatial effect of
spin diffusion is evidenced by using samples of different sizes and different
ways of the sample's irradiation with microwaves.Comment: 14 pages, 12 figure
Experiment for transient effects of sudden catastrophic loss of vacuum on a scaled superconducting radio frequency cryomodule
Safe operation of superconducting radio frequency (SRF) cavities require
design consideration of a sudden catastrophic loss of vacuum (SCLV) adjacent
with liquid helium (LHe) vessels and subsequent dangers. An experiment is
discussed to test the longitudinal effects of SCLV along the beam line of a
string of scaled SRF cavities. Each scaled cavity includes one segment of beam
tube within a LHe vessel containing 2 K saturated LHe, and a riser pipe
connecting the LHe vessel to a common gas header. At the beam tube inlet is a
fast acting solenoid valve to simulate SCLV and a high/low range orifice plate
flow-meter to measure air influx to the cavity. The gas header exit also has an
orifice plate flow-meter to measure helium venting the system at the relief
pressure of 0.4 MPa. Each cavity is instrumented with Validyne pressure
transducers and Cernox thermometers. The purpose of this experiment is to
quantify the time required to spoil the beam vacuum and the effects of
transient heat and mass transfer on the helium system. Heat transfer data is
expected to reveal a longitudinal effect due to the geometry of the experiment.
Details of the experimental design criteria and objectives are presented.Comment: 8 pp. Cryogenic Engineering Conference and International Cryogenic
Materials Conference CEC-ICMC. 13-17 June 2011. Spokane, Washingto
Layered architecture for quantum computing
We develop a layered quantum computer architecture, which is a systematic
framework for tackling the individual challenges of developing a quantum
computer while constructing a cohesive device design. We discuss many of the
prominent techniques for implementing circuit-model quantum computing and
introduce several new methods, with an emphasis on employing surface code
quantum error correction. In doing so, we propose a new quantum computer
architecture based on optical control of quantum dots. The timescales of
physical hardware operations and logical, error-corrected quantum gates differ
by several orders of magnitude. By dividing functionality into layers, we can
design and analyze subsystems independently, demonstrating the value of our
layered architectural approach. Using this concrete hardware platform, we
provide resource analysis for executing fault-tolerant quantum algorithms for
integer factoring and quantum simulation, finding that the quantum dot
architecture we study could solve such problems on the timescale of days.Comment: 27 pages, 20 figure
Three-Dimensional Simulations of Magnetized Superbubbles: New Insights into the Importance of MHD Effects on Observed Quantities
We present three-dimensional magnetohydrodynamic (MHD) simulations of
superbubbles, to study the importance of MHD effects in the interpretation of
images from recent surveys of the Galactic plane. These simulations focus
mainly on atmospheres defined by an exponential density distribution and the
Dickey & Lockman (1990) density distribution. In each case, the magnetic field
is parallel to the Galactic plane and we investigate cases with either infinite
scale height (constant magnetic field) or a constant ratio of gas pressure to
magnetic pressure. The three-dimensional structure of superbubbles in these
simulations is discussed with emphasis on the axial ratio of the cavity as a
function of magnetic field strength and the age of the bubble. We investigate
systematic errors in the age of the bubble and scale height of the surrounding
medium that may be introduced by modeling the data with purely hydrodynamic
models. Age estimates derived with symmetric hydrodynamic models fitted to an
asymmetric magnetized superbubble can differ by up to a factor of four,
depending on the direction of the line of sight. The scale height of the
surrounding medium based on the Kompaneets model may be up to 50% lower than
the actual scale height. We also present the first ever predictions of Faraday
rotation by a magnetized superbubble based on three-dimensional MHD
simulations. We emphasize the importance of MHD effects in the interpretation
of observations of superbubbles.Comment: 21 journal pages. 17 figures. 5 tables (added extensive discussion on
the effect of cooling on the bubbles); Accepted for publications in the
Astrophysical Journal. Related animations can be accessed via
http://www.capca.ucalgary.c
Shaping the learning landscape in neural networks around wide flat minima
Learning in Deep Neural Networks (DNN) takes place by minimizing a non-convex
high-dimensional loss function, typically by a stochastic gradient descent
(SGD) strategy. The learning process is observed to be able to find good
minimizers without getting stuck in local critical points, and that such
minimizers are often satisfactory at avoiding overfitting. How these two
features can be kept under control in nonlinear devices composed of millions of
tunable connections is a profound and far reaching open question. In this paper
we study basic non-convex one- and two-layer neural network models which learn
random patterns, and derive a number of basic geometrical and algorithmic
features which suggest some answers. We first show that the error loss function
presents few extremely wide flat minima (WFM) which coexist with narrower
minima and critical points. We then show that the minimizers of the
cross-entropy loss function overlap with the WFM of the error loss. We also
show examples of learning devices for which WFM do not exist. From the
algorithmic perspective we derive entropy driven greedy and message passing
algorithms which focus their search on wide flat regions of minimizers. In the
case of SGD and cross-entropy loss, we show that a slow reduction of the norm
of the weights along the learning process also leads to WFM. We corroborate the
results by a numerical study of the correlations between the volumes of the
minimizers, their Hessian and their generalization performance on real data.Comment: 37 pages (16 main text), 10 figures (7 main text
The many sides of RCW 86: a type Ia supernova remnant evolving in its progenitor's wind bubble
We present the results of a detailed investigation of the Galactic supernova
remnant RCW 86 using the XMM-Newton X-ray telescope. RCW 86 is the probable
remnant of SN 185 A.D, a supernova that likely exploded inside a wind-blown
cavity. We use the XMM-Newton Reflection Grating Spectrometer (RGS) to derive
precise temperatures and ionization ages of the plasma, which are an indication
of the interaction history of the remnant with the presumed cavity. We find
that the spectra are well fitted by two non-equilibrium ionization models,
which enables us to constrain the properties of the ejecta and interstellar
matter plasma. Furthermore, we performed a principal component analysis on EPIC
MOS and pn data to find regions with particular spectral properties. We present
evidence that the shocked ejecta, emitting Fe-K and Si line emission, are
confined to a shell of approximately 2 pc width with an oblate spheroidal
morphology. Using detailed hydrodynamical simulations, we show that general
dynamical and emission properties at different portions of the remnant can be
well-reproduced by a type Ia supernova that exploded in a non-spherically
symmetric wind-blown cavity. We also show that this cavity can be created using
general wind properties for a single degenerate system. Our data and
simulations provide further evidence that RCW 86 is indeed the remnant of SN
185, and is the likely result of a type Ia explosion of single degenerate
origin.Comment: Accepted for publication in MNRAS. 16 pages, 13 figure
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