12,426 research outputs found
Plasma Wakefield Acceleration with a Modulated Proton Bunch
The plasma wakefield amplitudes which could be achieved via the modulation of
a long proton bunch are investigated. We find that in the limit of long bunches
compared to the plasma wavelength, the strength of the accelerating fields is
directly proportional to the number of particles in the drive bunch and
inversely proportional to the square of the transverse bunch size. The scaling
laws were tested and verified in detailed simulations using parameters of
existing proton accelerators, and large electric fields were achieved, reaching
1 GV/m for LHC bunches. Energy gains for test electrons beyond 6 TeV were found
in this case.Comment: 9 pages, 7 figure
From microscopic to macroscopic descriptions of cell\ud migration on growing domains
Cell migration and growth are essential components of the development of multicellular organisms. The role of various cues in directing cell migration is widespread, in particular, the role of signals in the environment in the control of cell motility and directional guidance. In many cases, especially in developmental biology, growth of the domain also plays a large role in the distribution of cells and, in some cases, cell or signal distribution may actually drive domain growth. There is a ubiquitous use of partial differential equations (PDEs) for modelling the time evolution of cellular density and environmental cues. In the last twenty years, a lot of attention has been devoted to connecting macroscopic PDEs with more detailed microscopic models of cellular motility, including models of directional sensing and signal transduction pathways. However, domain growth is largely omitted in the literature. In this paper, individual-based models describing cell movement and domain growth are studied, and correspondence with a macroscopic-level PDE describing the evolution of cell density is demonstrated. The individual-based models are formulated in terms of random walkers on a lattice. Domain growth provides an extra mathematical challenge by making the lattice size variable over time. A reaction-diffusion master equation formalism is generalised to the case of growing lattices and used in the derivation of the macroscopic PDEs
Morphology of Hydrodynamic Winds: A Study of Planetary Winds in Stellar Environments
Bathed in intense ionizing radiation, close-in gaseous planets undergo
hydrodynamic atmospheric escape, which ejects the upper extent of their
atmospheres into the interplanetary medium. Ultraviolet detections of escaping
gas around transiting planets corroborate such a framework. Exposed to the
stellar environment, the outflow is shaped by its interaction with the stellar
wind and by the planet's orbit. We model these effects using Athena to perform
3-D radiative-hydrodynamic simulations of tidally-locked hydrogen atmospheres
receiving large amounts of ionizing extreme-ultraviolet flux in various stellar
environments for the low-magnetic-field case. Through a step-by-step
exploration of orbital and stellar wind effects on the planetary outflow, we
find three structurally distinct stellar wind regimes: weak, intermediate, and
strong. We perform synthetic Lyman- observations and find unique
observational signatures for each regime. A weak stellar
windwhich cannot confine the planetary outflow, leading to a
torus of material around the starhas a pre-transit, red-shifted
dayside arm and a slightly redward-skewed spectrum during transit. The
intermediate regime truncates the dayside outflow at large distances from the
planet and causes periodic disruptions of the outflow, producing observational
signatures that mimic a double transit. The first of these dips is blue-shifted
and precedes the optical transit. Finally, strong stellar winds completely
confine the outflow into a cometary tail and accelerate the outflow outwards,
producing large blue-shifted signals post-transit. Across all three regimes,
large signals occur far outside of transit, offering motivation to continue
ultraviolet observations outside of direct transit.Comment: 33 pages, 21 figures (7 of which have embedded movies viewable with
Adobe Acrobat Pro), Submitted to Ap
Nonthermal Radio Emission from Planetary Nebulae
In a recent analysis of the radio emission from the planetary nebula A30,
Dgani, Evans & White (1998) claim that the emission, located in the inner
region, is probably dominated by nonthermal emission.
We propose a model to explain this.
We assume that the fast wind, blown by the central star of A30 carries a very
weak magnetic field. The interaction of this wind with a cluster of dense
condensations traps the magnetic field lines for a long time and stretches
them, leading to a strong magnetic field.
If relativistic particles are formed as the fast wind is shocked, then the
enhanced magnetic field will result in nonthermal radio emission.
The typical nonthermal radio flux at 1 GHz can be up to several milli-Jansky.
In order to detect the nonthermal emission, the emitting region should be
spatially resolved from the main optical nebula.
We list other planetary nebulae which may possess nonthermal radio emission.Comment: 11 page
A change in temperature modulates defence to yellow (stripe) rust in wheat line UC1041 independently of resistance gene Yr36
Background Rust diseases are of major importance in wheat production worldwide. With the constant evolution of new rust strains and their adaptation to higher temperatures, consistent and durable disease resistance is a key challenge. Environmental conditions affect resistance gene performance, but the basis for this is poorly understood. Results Here we show that a change in day temperature affects wheat resistance to Puccinia striiformis f. sp tritici (Pst), the causal agent of yellow (or stripe) rust. Using adult plants of near-isogenic lines UC1041 +/- Yr36, there was no significant difference between Pst percentage uredia coverage in plants grown at day temperatures of 18°C or 25°C in adult UC1041 + Yr36 plants. However, when plants were transferred to the lower day temperature at the time of Pst inoculation, infection increased up to two fold. Interestingly, this response was independent of Yr36, which has previously been reported as a temperature-responsive resistance gene as Pst development in adult UC1041 -Yr36 plants was similarly affected by the plants experiencing a temperature reduction. In addition, UC1041 -Yr36 plants grown at the lower temperature then transferred to the higher temperature were effectively resistant and a temperature change in either direction was shown to affect Pst development up to 8 days prior to inoculation. Results for seedlings were similar, but more variable compared to adult plants. Enhanced resistance to Pst was observed in seedlings of UC1041 and the cultivar Shamrock when transferred to the higher temperature. Resistance was not affected in seedlings of cultivar Solstice by a temperature change in either direction. Conclusions Yr36 is effective at 18°C, refining the lower range of temperature at which resistance against Pst is conferred compared to previous studies. Results reveal previously uncharacterised defence temperature sensitivity in the UC1041 background which is caused by a change in temperature and independently of Yr36. This novel phenotype is present in some cultivars but absent in others, suggesting that Pst defence may be more stable in some cultivars than others when plants are exposed to varying temperatures
Effects of deleting cannabinoid receptor-2 on mechanical and material properties of cortical and trabecular bone
Acknowledgements We thank Dr J.S. Gregory for assistance with Image J and Mr K. Mackenzie for assistance with Micro-CT analysis. Funding ABK was funded by a University of Aberdeen, Institute of Medical Sciences studentship and the Overseas Research Students Awards Scheme.Peer reviewedPublisher PD
Mechanical and material properties of cortical and trabecular bone from cannabinoid receptor-1-null (Cnr1-/-) mice
Funding ABK was funded by a studentship from the University of Aberdeen, Institute of Medical Sciences, and the Overseas Research Students Awards Scheme Acknowledgments We are grateful to Dr J.S. Gregory for assistance with Image J and Mr K. Mackenzie for assistance with Micro-CT analysis.Peer reviewedPostprin
Reconciling transport models across scales: the role of volume exclusion
Diffusive transport is a universal phenomenon, throughout both biological and
physical sciences, and models of diffusion are routinely used to interrogate
diffusion-driven processes. However, most models neglect to take into account
the role of volume exclusion, which can significantly alter diffusive
transport, particularly within biological systems where the diffusing particles
might occupy a significant fraction of the available space. In this work we use
a random walk approach to provide a means to reconcile models that incorporate
crowding effects on different spatial scales. Our work demonstrates that
coarse-grained models incorporating simplified descriptions of excluded volume
can be used in many circumstances, but that care must be taken in pushing the
coarse-graining process too far
Do the cosmological observational data prefer phantom dark energy?
The dynamics of expansion and large scale structure formation of the Universe
are analyzed for models with dark energy in the form of a phantom scalar field
which initially mimics a -term and evolves slowly to the Big Rip
singularity. The discussed model of dark energy has three parameters -- the
density and the equation of state parameter at the current epoch,
and , and the asymptotic value of the equation of state parameter at
, . Their best-fit values are determined jointly
with all other cosmological parameters by the MCMC method using observational
data on CMB anisotropies and polarization, SNe Ia luminosity distances, BAO
measurements and more. Similar computations are carried out for CDM
and a quintessence scalar field model of dark energy. It is shown that the
current data slightly prefer the phantom model, but the differences in the
maximum likelihoods are not statistically significant. It is also shown that
the phantom dark energy with monotonically increasing density in future will
cause the decay of large scale linear matter density perturbations due to the
gravitational domination of dark energy perturbations long before the Big Rip
singularity.Comment: 13 pages, 8 figures, 5 tables; comments and references added; version
accepted for publication in Phys.Rev.
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