4,151 research outputs found
Solid flow drives surface nanopatterning by ion-beam irradiation
Ion Beam Sputtering (IBS) is known to produce surface nanopatterns over
macroscopic areas on a wide range of materials. However, in spite of the
technological potential of this route to nanostructuring, the physical process
by which these surfaces self-organize remains poorly under- stood. We have
performed detailed experiments of IBS on Si substrates that validate dynamical
and morphological predictions from a hydrodynamic description of the
phenomenon. Our results elucidate flow of a nanoscopically thin and highly
viscous surface layer, driven by the stress created by the ion-beam, as a
description of the system. This type of slow relaxation is akin to flow of
macroscopic solids like glaciers or lead pipes, that is driven by defect
dynamics.Comment: 12 pages, 4 figure
Coupling of morphology to surface transport in ion-beam irradiated surfaces. I. Oblique incidence
We propose and study a continuum model for the dynamics of amorphizable
surfaces undergoing ion-beam sputtering (IBS) at intermediate energies and
oblique incidence. After considering the current limitations of more standard
descriptions in which a single evolution equation is posed for the surface
height, we overcome (some of) them by explicitly formulating the dynamics of
the species that transport along the surface, and by coupling it to that of the
surface height proper. In this we follow recent proposals inspired by
``hydrodynamic'' descriptions of pattern formation in aeolian sand dunes and
ion-sputtered systems. From this enlarged model, and by exploiting the
time-scale separation among various dynamical processes in the system, we
derive a single height equation in which coefficients can be related to
experimental parameters. This equation generalizes those obtained by previous
continuum models and is able to account for many experimental features of
pattern formation by IBS at oblique incidence, such as the evolution of the
irradiation-induced amorphous layer, transverse ripple motion with non-uniform
velocity, ripple coarsening, onset of kinetic roughening and other.
Additionally, the dynamics of the full two-field model is compared with that of
the effective interface equation.Comment: 23 pages, 14 figures. Movies of figures 6, 7, and 8 available at
http://gisc.uc3m.es/~javier/Movies
The impact of the Kasatochi eruption on the Moon's illumination during the August 2008 lunar eclipse
The Moon's changeable aspect during a lunar eclipse is largely attributable
to variations in the refracted unscattered sunlight absorbed by the terrestrial
atmosphere that occur as the satellite crosses the Earth's shadow. The
contribution to the Moon's aspect from sunlight scattered at the Earth's
terminator is generally deemed minor. However, our analysis of a published
spectrum of the 16 August 2008 lunar eclipse shows that diffuse sunlight is a
major component of the measured spectrum at wavelengths shorter than 600 nm.
The conclusion is supported by two distinct features, namely the spectrum's
tail at short wavelengths and the unequal absorption by an oxygen collisional
complex at two nearby bands. Our findings are consistent with the presence of
the volcanic cloud reported at high northern latitudes following the 7-8 August
2008 eruption in Alaska of the Kasatochi volcano. The cloud both attenuates the
unscattered sunlight and enhances moderately the scattered component, thus
modifying the contrast between the two contributions.Comment: Accepted for publication in Geophysical Research Letter
Recent ASDEX Upgrade research in support of ITER and DEMO
Recent experiments on the ASDEX Upgrade tokamak aim at improving the physics base for ITER and DEMO to aid the machine
design and prepare efficient operation. Type I edge localized mode (ELM) mitigation using resonant magnetic perturbations
(RMPs) has been shown at low pedestal collisionality (
Μ
â
ped
<
0
.
4
)
. In contrast to the previous high
Μ
â
regime, suppression only
occurs in a narrow RMP spectral window, indicating a resonant process, and a concomitant confinement drop is observed due
to a reduction of pedestal top density and electron temperature. Strong evidence is found for the ion heat flux to be the decisive
element for the LâH power threshold. A physics based scaling of the density at which the minimum
P
LH
occurs indicates that
ITER could take advantage of it to initiate H-mode at lower density than that of the final
Q
=
10 operational point. Core density
fluctuation measurements resolved in radius and wave number show that an increase of
R/L
T
e
introduced by off-axis electron
cyclotron resonance heating (ECRH) mainly increases the large scale fluctuations. The radial variation of the fluctuation level
is in agreement with simulations using the GENE code. Fast particles are shown to undergo classical slowing down in the
absence of large scale magnetohydrodynamic (MHD) events and for low heating power, but show signs of anomalous radial
redistribution at large heating power, consistent with a broadened off-axis neutral beam current drive current profile under these
conditions. Neoclassical tearing mode (NTM) suppression experiments using electron cyclotron current drive (ECCD) with
feedback controlled deposition have allowed to test several control strategies for ITER, including automated control of (3,2) and
(2,1) NTMs during a single discharge. Disruption mitigation studies using massive gas injection (MGI) can show an increased
fuelling efficiency with high field side injection, but a saturation of the fuelling efficiency is observed at high injected mass as
needed for runaway electron suppression. Large locked modes can significantly decrease the fuelling efficiency and increase
the asymmetry of radiated power during MGI mitigation. Concerning power exhaust, the partially detached ITER divertor
scenario has been demonstrated at
P
sep
/R
=
10 MW m
â
1
in ASDEX Upgrade, with a peak time averaged target load around
5MWm
â
2
, well consistent with the component limits for ITER. Developing this towards DEMO, full detachment was achieved
at
P
sep
/R
=
7MWm
â
1
and stationary discharges with core radiation fraction of the order of DEMO requirements (70% instead
of the 30% needed for ITER) were demonstrated. Finally, it remains difficult to establish the standard ITER
Q
=
10 scenario at
low
q
95
=
3 in the all-tungsten (all-W) ASDEX Upgrade due to the observed poor confinement at low
ÎČ
N
. This is mainly due to
a degraded pedestal performance and hence investigations at shifting the operational point to higher
ÎČ
N
by lowering the current
have been started. At higher
q
95
, pedestal performance can be recovered by seeding N
2
as well as CD
4
, which is interpreted as
improved pedestal stability due to the decrease of bootstrap current with increasing
Z
eff
. Concerning advanced scenarios, the
upgrade of ECRH power has allowed experiments with central ctr-ECCD to modify the
q
-profile in improved H-mode scenarios,
showing an increase in confinement at still good MHD stability with flat elevated
q
-profiles at values between 1.5 and 2.European Commission (EUROfusion 633053
Evaluation of susceptibility to powdery mildew (Erysiphe necator) in Vitis vinifera varieties
Susceptibility to grape powdery mildew (Erysiphe necator Schwein.) was studied in 159 Vitis vinifera foreign and native grape varieties grown in Spain. The relationship between morphological features of vines and their susceptibility to the disease was also studied. The infection was evaluated under natural conditions on leaves and bunches. A total of 35 cultivars were very susceptible to the disease (very low to low resistance on bunches), while another 83 showed low susceptibility (high to very high resistance on bunches). Results provide useful information for grape growers and breeders for the selection of varieties less susceptible to powdery mildew
Towards a component-based framework for developing Semantic Web applications
For those outside the research community, to develop Semantic Web applications entails real difficulty. This difficulty is due in part to the lack of usable approaches for planning Semantic Web solutions, even though Semantic Web tools have already reached industrial maturity. We propose here the Semantic Web Framework, a component-based
framework for analysing rapidly the required components, the dependencies between them, and selecting existing solutions. This approach has been tested with a number of industrial partners, which justifies the effort made in this direction
Overview of ASDEX Upgrade results
The ASDEX Upgrade (AUG) programme is directed towards physics input to critical
elements of the ITER design and the preparation of ITER operation, as well as addressing
physics issues for a future DEMO design. Since 2015, AUG is equipped with a new pair of
3-strap ICRF antennas, which were designed for a reduction of tungsten release during ICRF
operation. As predicted, a factor two reduction on the ICRF-induced W plasma content could
be achieved by the reduction of the sheath voltage at the antenna limiters via the compensation
of the image currents of the central and side straps in the antenna frame. There are two main
operational scenario lines in AUG. Experiments with low collisionality, which comprise
current drive, ELM mitigation/suppression and fast ion physics, are mainly done with freshly
boronized walls to reduce the tungsten influx at these high edge temperature conditions. Full
ELM suppression and non-inductive operation up to a plasma current of Ip = 0.8 MA could
be obtained at low plasma density. Plasma exhaust is studied under conditions of high neutral
divertor pressure and separatrix electron density, where a fresh boronization is not required.
Substantial progress could be achieved for the understanding of the confinement degradation
by strong D puffing and the improvement with nitrogen or carbon seeding. Inward/outward
shifts of the electron density profile relative to the temperature profile effect the edge stability
via the pressure profile changes and lead to improved/decreased pedestal performance.
Seeding and D gas puffing are found to effect the core fueling via changes in a region of high
density on the high field side (HFSHD).
The integration of all above mentioned operational scenarios will be feasible and
naturally obtained in a large device where the edge is more opaque for neutrals and higher
plasma temperatures provide a lower collisionality. The combination of exhaust control
with pellet fueling has been successfully demonstrated. High divertor enrichment values of
nitrogen EN 10 have been obtained during pellet injection, which is a prerequisite for the
simultaneous achievement of good core plasma purity and high divertor radiation levels.
Impurity accumulation observed in the all-metal AUG device caused by the strong neoclassical
inward transport of tungsten in the pedestal is expected to be relieved by the higher
neoclassical temperature screening in larger devices.European Commission (EUROfusion 633053
The Simple Non-degenerate Relativistic Gas: Statistical Properties and Brownian Motion
This paper shows a novel calculation of the mean square displacement of a
classical Brownian particle in a relativistic thermal bath. The result is
compared with the expressions obtained by other authors. Also, the
thermodynamic properties of a non-degenerate simple relativistic gas are
reviewed in terms of a treatment performed in velocity space.Comment: 6 pages, 2 figure
Mathematical modelling of water absorption and evaporation in a pharmaceutical tablet during film coating
It is well understood that during the pharmaceutical aqueous film coating process the amount of liquid water that interacts with the porous tablet core can affect the quality of the final product. Therefore, understanding and simulating the mechanisms of water droplet spreading, absorption and evaporation is crucial for controlling the process and optimising the shelf-life of the tablets. The purpose of the work presented in this paper is to define and describe the spreading, absorption and evaporation phenomena after droplet impingement on a tablet. We divided the droplet behaviour into three phases of different dynamics and duration: the kinematic, capillary and evaporation phases. To model the kinematic phase, we combined and modified 1-D spreading models from the literature which solve the kinetic energy balance equation for the first milliseconds of spreading. For the capillary phase, we simplified and solved the continuity and Navier-Stokes equations using the lubrication approximation theory. Finally, for the evaporation phase, we adopted a modelling approach for the second drying stage of slurry droplets inside a spray dryer. During this stage, one can no longer describe the droplet as a liquid system containing solids, having to regard it as a wet particle with a dry crust and a wet core. In our work, we represented in a novel way the crust as the dry surface of the tablet and the wet core as the wetted area inside the porous matrix. We implemented the mathematical model presented in this work in gPROMS, employing the Modelbuilder platform. Our numerical results (droplet height and spreading, wetting, evaporation front profiles) are in good agreement with recent experimental data that we found in the literature
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