4,637 research outputs found
The effect of toroidal flows on the stability of ITGs in MAST
The free energy in the large temperature and density gradients in tokamaks
can drive microinstabilities, which in turn drive turbulence. This turbulence is
responsible for the transport of energy and particles over and above that predicted
by neoclassical theory. Sheared toroidal rotation can suppress the turbulence and
stabilise the underlying microinstabilities, thereby reducing the transport. This
thesis investigates how variation of the equilibrium temperature and density profiles,
over the same scales associated with the microinstabilities, affects how the
ow shear
stabilises the linear modes and suppresses the turbulence. A global gyrokinetic code
is employed in this investigation, which retains the profile variation and simulates
the full 3D domain of a tokamak plasma.
How much
ow shear is needed to stabilise the linear ion temperature gradient
(ITG) mode is found to be dependent on its poloidal wavenumber, with longer
wavelength modes needing more
ow shear than the fastest growing mode. This
dependence is present whether the
ow shear is constant across the radius or if it
has the variation typical in an experimental rotation profile. There is an asymmetry
with respect to the sign of the
ow shear in the effectiveness of the stabilisation,
with the maximum linear growth rate occurring at finite negative shearing rates for
the plasma studied here. This asymmetry arises from the profile variation, and is
found to be significant in simulations of MAST L-mode plasmas, especially when
the effects of kinetic trapped electrons are included in the simulations.
Flow shear asymmetry is still present in nonlinear simulations, and the suppression
of fully-developed turbulence depends on the sign of the shearing rate.
With the experimental rotation profile, the heat
ux arising from ITG turbulence
is reduced by an amount comparable to the reduction in the linear growth rates.
When the direction of the rotation profile is reversed, such that the sign of the
ow
shear is
ipped while the magnitude remains the same, the turbulence is almost
completely suppressed. A new diagnostic on MAST, beam emission spectroscopy
(BES), is used to make a direct comparison between density
fluctuations from simulation,
and from experiment. Collisionless, electrostatic simulations with rotation
are found to disagree significantly with experiment in the level of ITG turbulence
activity and the correlation times and lengths of the turbulence. The inclusion
of electron-electron and electron-ion collisions into static simulations is enough to
bring the level of turbulent density
uctuations down to within a factor two of the
experimental levels, with the correlation lengths becoming comparable, while the
correlation times remain an order of magnitude too large
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Quantifying the contribution of different cloud types to the radiation budget in southern West Africa
The contribution of cloud to the radiation budget of southern West Africa (SWA) is poorly understood yet is important for understanding regional monsoon evolution and for evaluating and improving climate models, which have large biases in this region. Radiative transfer calculations applied to atmospheric profiles obtained from the CERES-CloudSat-CALIPSO-MODIS (CCCM) dataset are used to investigate the effects of 12 different cloud types (defined by their vertical structure) on the regional energy budget of SWA (5â10 °N, 8 °W-8 °E) during June-September. We show that the large regional mean cloud radiative effect in SWA is due to non-negligible contributions from many different cloud types; 8 cloud types have a cloud fraction larger than 5 % and contribute at least 5 % of the regional mean shortwave cloud radiative effect at the top of atmosphere. Low-clouds, which are poorly observed by passive satellite measurements, were found to cause net radiative cooling of the atmosphere, which reduces the heating from other cloud types by approximately 10 %. The sensitivity of the radiation budget to underestimating low-cloud cover is also investigated. The radiative effect of missing low-cloud is found to be up to approximately â25 W m-2 for upwelling shortwave irradiance at the top of atmosphere and 35 W m-2 for downwelling shortwave irradiance at the surface
Dynamical Stability of Witten Rings
The dynamical stability of cosmic rings, or vortons, is investigated for the
particular equation of state given by the Witten bosonic model. It is found
that there exists a finite range of the state parameter for which the vorton
states are actually stable against dynamical perturbations. Inclusion of the
electromagnetic self action into the equation of state slightly shrinks the
stability region but otherwise yields no qualitative difference. If the Witten
bosonic model represents a good approximation for more realistic string models,
then the cosmological vorton excess problem can only be solved by assuming
either that strings are formed at low energy scales or that some quantum
instability may develop at a sufficient rate.Comment: 11 pages, LaTeX-ReVTeX (v.3), 2 figures available upon request, DAMTP
R-94/1
TCT-294: Long-Term (Three Years) Follow-Up of the Patients with Multiple Sirolimus Eluting Stent Implantation (Full-metal Jacket Patients)
1. The cost, usability and power efficiency of available wildlife monitoring equipment currently inhibits full ground-level coverage of many natural systems. Developments over the last decade in technology, open science, and the sharing economy promise to bring global access to more versatile and more affordable monitoring tools, to improve coverage for conservation researchers and managers.
2. Here we describe the development and proof-of-concept of a low-cost, small-sized and low-energy acoustic detector: 'AudioMoth'. The device is open-source and programmable, with diverse applications for recording animal calls or human activity at sample rates of up to 384kHz. We briefly outline two ongoing real-world case studies of large-scale, long-term monitoring for biodiversity and exploitation of natural resources. These studies demonstrate the potential for AudioMoth to enable a substantial shift away from passive continuous recording by individual devices, towards smart detection by networks of devices flooding large and inaccessible ecosystems.
3. The case studies demonstrate one of the smart capabilities of AudioMoth, to trigger event logging on the basis of classification algorithms that identify specific acoustic events. An algorithm to trigger recordings of the New Forest cicada (Cicadetta montana) demonstrates the potential for AudioMoth to vastly improve the spatial and temporal coverage of surveys for the presence of cryptic animals. An algorithm for logging gunshot events has potential to identify a shotgun blast in tropical rainforest at distances of up to 500 m, extending to 1km with continuous recording.
4. AudioMoth is more energy efficient than currently available passive acoustic monitoring (PAM) devices, giving it considerably greater portability and longevity in the field with smaller batteries. At a build cost of ~US$43 per unit, AudioMoth has potential for varied applications in large-scale, long-term acoustic surveys. With continuing developments in smart, energy-efficient algorithms and diminishing component costs, we are approaching the milestone of local communities being able to afford to remotely monitor their own natural resources
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A multi-satellite climatology of clouds, radiation and precipitation in southern West Africa and comparison to climate models
Southern West Africa (SWA) has a large population that relies on highly variable monsoon rainfall, yet climate models show little consensus over projected precipitation in this region. Understanding of the current and future climate of SWA is further complicated by rapidly increasing anthropogenic emissions and a lack of surface observations. Using multiple satellite observations, the ERA-Interim reanalysis, and four climate models, we document the climatology of cloud, precipitation and radiation over SWA in June-July, highlight discrepancies among satellite products, and identify shortcomings in climate models and ERA-Interim. Large differences exist between monthly mean cloud cover estimates from satellites, which range from 68 to 94 %. In contrast, differences among satellite observations in top of atmosphere outgoing radiation and surface precipitation are smaller, with monthly means of about 230âWâmâ2 of longwave radiation, 145âWâmâ2 of shortwave radiation and 5.87âmmâdayâ1 of precipitation. Both ERA-Interim and the climate models show less total cloud cover than observations, mainly due to underestimating low cloud cover. Errors in cloud cover, along with uncertainty in surface albedo, lead to a large spread of outgoing shortwave radiation. Both ERA-Interim and the climate models also show signs of convection developing too early in the diurnal cycle, with associated errors in the diurnal cycles of precipitation and outgoing longwave radiation. Clouds, radiation and precipitation are linked in an analysis of the regional energy budget, which shows that inter-annual variability of precipitation and dry static energy divergence are strongly linked
Fermionic massive modes along cosmic strings
The influence on cosmic string dynamics of fermionic massive bound states
propagating in the vortex, and getting their mass only from coupling to the
string forming Higgs field, is studied. Such massive fermionic currents are
numerically found to exist for a wide range of model parameters and seen to
modify drastically the usual string dynamics coming from the zero mode currents
alone. In particular, by means of a quantization procedure, a new equation of
state describing cosmic strings with any kind of fermionic current, massive or
massless, is derived and found to involve, at least, one state parameter per
trapped fermion species. This equation of state exhibits transitions from
subsonic to supersonic regimes while the massive modes are filled.Comment: 27 pages, 15 figures, uses ReVTeX. Shortened version, accepted for
publication in Phys. Rev.
FUSE Measurements of Far Ultraviolet Extinction. I. Galactic Sight Lines
We present extinction curves that include data down to far ultraviolet
wavelengths (FUV; 1050 - 1200 A) for nine Galactic sight lines. The FUV
extinction was measured using data from the Far Ultraviolet Spectroscopic
Explorer. The sight lines were chosen for their unusual extinction properties
in the infrared through the ultraviolet; that they probe a wide range of dust
environments is evidenced by the large spread in their measured ratios of
total-to-selective extinction, R_V = 2.43 - 3.81. We find that extrapolation of
the Fitzpatrick & Massa relationship from the ultraviolet appears to be a good
predictor of the FUV extinction behavior. We find that predictions of the FUV
extinction based upon the Cardelli, Clayton & Mathis (CCM) dependence on R_V
give mixed results. For the seven extinction curves well represented by CCM in
the infrared through ultraviolet, the FUV extinction is well predicted in three
sight lines, over-predicted in two sight lines, and under-predicted in 2 sight
lines. A Maximum Entropy Method analysis using a simple three component grain
model shows that seven of the nine sight lines in the study require a larger
fraction of grain materials to be in dust when FUV extinction is included in
the models. Most of the added grain material is in the form of small (radii <
200 A) grains.Comment: Accepted for publication in the Astrophysical Journal. 31 pages with
7 figure
The Star Formation History of the Large Magellanic Cloud
Using WFPC2 aboard the Hubble Space Telescope, we have created deep
color-magnitude diagrams in the V and I passbands for approximately 100,000
stars in a field at the center of the LMC bar and another in the disk. The
main--sequence luminosity functions (LFs) from 19 mag < V < 23.5 mag, the red
clump and horizontal branch morphologies, and the differential Hess diagram of
the two fields all strongly imply that the disk and bar have significantly
different star-formation histories (SFHs). The disk's SFH has been relatively
smooth and continuous over the last 15 Gyr while the bar's SFH was dominated by
star formation episodes at intermediate ages. Comparison of the LF against
predictions based on Padova theoretical stellar evolution models and an assumed
age-metallicity relationship allows us to identify the dominant stellar
populations in the bar with episodes of star formation that occurred from 4 to
6 and 1 to 2 Gyr ago. These events accounted, respectively, for approximately
25% and 15% of its stellar mass. The disk field may share a mild enhancement in
SF for the younger episode, and thus we identify the 4 to 6 Gyr episode with
the formation of the LMC bar.Comment: 14 pages, 5 figures, Latex, also available at
http://www.ps.uci.edu/physics/smeckerhane.html. Accepted for publication in
Ap
Equation of state of cosmic strings with fermionic current-carriers
The relevant characteristic features, including energy per unit length and
tension, of a cosmic string carrying massless fermionic currents in the
framework of the Witten model in the neutral limit are derived through
quantization of the spinor fields along the string. The construction of a Fock
space is performed by means of a separation between longitudinal modes and the
so-called transverse zero energy solutions of the Dirac equation in the vortex.
As a result, quantization leads to a set of naturally defined state parameters
which are the number densities of particles and anti-particles trapped in the
cosmic string. It is seen that the usual one-parameter formalism for describing
the macroscopic dynamics of current-carrying vortices is not sufficient in the
case of fermionic carriers.Comment: 30 pages, 15 figures, uses ReVTeX, equation of state corrected,
comments and references added. Accepted for publication in Phys. Rev.
Deep three-dimensional solid-state qubit arrays with long-lived spin coherence
Nitrogen-vacancy centers (NVCs) in diamond show promise for quantum computing, communication, and sensing. However, the best current method for entangling two NVCs requires that each one is in a separate cryostat, which is not scalable. We show that single NVCs can be laser written 6â15-”m deep inside of a diamond with spin coherence times that are an order of magnitude longer than previous laser-written NVCs and at least as long as naturally occurring NVCs. This depth is suitable for integration with solid immersion lenses or optical cavities and we present depth-dependent T2 measurements. 200â000 of these NVCs would fit into one diamond
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