Hybrid inflation along waterfall trajectories

We identify a new inflationary regime for which more than 60 e-folds are generated classically during the waterfall phase occuring after the usual hybrid inflation. By performing a bayesian Monte-Carlo-Markov-Chain analysis, this scenario is shown to take place in a large part of the parameter space of the model. When this occurs, the observable perturbation modes leave the Hubble radius during waterfall inflation. The power spectrum of adiabatic perturbations is red, possibly in agreement with CMB constraints. A particular attention has been given to study only the regions for which quantum backreactions do not affect the classical dynamics. Implications concerning the preheating and the absence of topological defects in our universe are discussed.Comment: 10 pages, 5 figures, section III-A on quantum backreactions more detailed, comments on transverse field gradient contribution added, version accepted for publication in Phys.Rev.

The ion motion in self-modulated plasma wakefield accelerators

The effects of plasma ion motion in self-modulated plasma based accelerators is examined. An analytical model describing ion motion in the narrow beam limit is developed, and confirmed through multi-dimensional particle-in-cell simulations. It is shown that the ion motion can lead to the early saturation of the self-modulation instability, and to the suppression of the accelerating gradients. This can reduce the total energy that can be transformed into kinetic energy of accelerated particles. For the parameters of future proton-driven plasma accelerator experiments, the ion dynamics can have a strong impact. Possible methods to mitigate the effects of the ion motion in future experiments are demonstrated.Comment: 11 pages, 3 figures, accepted for publication in Phys. Rev. Let

A micro finite-element model for soil behaviour

This paper describes a numerical model that virtualises the fabric of a natural sand obtained from micro computed tomography (μCT) to simulate the mechanical response of the material, termed here a micro finite-element (μFE) model. The grain-to-grain interactions under loading are modelled in a framework of combined discrete–finite-element method. The basis of this approach is that using a true representation of soil fabric and deformable grains will enable a more realistic representation of the physics of granular behaviour. Each individual grain is represented in a numerical mesh and modelled as a continuum body allowed to deform according to a prescribed constitutive model with appropriate friction contact conditions. An important feature of this model is the ability to compute the map of stress distribution inside the grains. A case study of an intact sand subjected to oedometer compression is presented to demonstrate the insights that can be gained into the stress transmission mechanisms and yield initiation within the grains. The displacement field, inertia tensor and active contact number are used to quantify grain kinematics as the virtual fabric deforms. By coupling contact dynamics with contact topology, this approach provides a robust numerical tool to infer important grain scale parameters that link the micro phenomena to the macro response of soil

Dynamical star-disk interaction in the young stellar system V354 Mon

The main goal of this work is to characterize the mass accretion and ejection processes of the classical T Tauri star V354 Mon, a member of the young stellar cluster NGC 2264. In March 2008, photometric and spectroscopic observations of V354 Mon were obtained simultaneously with the CoRoT satellite, the 60 cm telescope at the Observat\'orio Pico dos Dias (LNA - Brazil) equipped with a CCD camera and Johnson/Cousins BVRI filters, and the SOPHIE \'echelle spectrograph at the Observatoire de Haute-Provence (CNRS - France). The light curve of V354 Mon shows periodical minima (P = 5.26 +/- 0.50 days) that vary in depth and width at each rotational cycle. From the analysis of the photometric and spectroscopic data, it is possible to identify correlations between the emission line variability and the light-curve modulation of the young system, such as the occurrence of pronounced redshifted absorption in the H_alpha line at the epoch of minimum flux. This is evidence that during photometric minima we see the accretion funnel projected onto the stellar photosphere in our line of sight, implying that the hot spot coincides with the light-curve minima. We applied models of cold and hot spots and a model of occultation by circumstellar material to investigate the source of the observed photometric variations. We conclude that nonuniformly distributed material in the inner part of the circumstellar disk is the main cause of the photometric modulation, which does not exclude the presence of hot and cold spots at the stellar surface. It is believed that the distortion in the inner part of the disk is created by the dynamical interaction between the stellar magnetosphere, inclined with respect to the rotation axis, and the circumstellar disk, as also observed in the classical T Tauri star AA Tau and predicted by magnetohydrodynamical numerical simulations.Comment: Accepted by Astronomy and Astrophysic

A micro finite element model for soil behaviour: Numerical validation

A micro finite-element (μFE) model capable of handling arbitrary shapes and deformable grains has been developed by the authors. The basis of this μFE model is to use a virtualised soil fabric obtained from micro computed tomography (μCT) of real sand to simulate grain-to-grain interaction in a framework of combined discrete–finite-element method. By incorporating grain deformation into the model, the contact response emerges from the interaction of contacting bodies and each irregular contact area will produce a unique response. A detailed numerical description of grain morphology and contact topology of a natural sand and the subsequent simulation are presented in the original paper. The present study focuses on the numerical validation of the constitutive contact behaviour against existent theories, for a single sphere and an assembly of spheres. The ability of the model to simulate elastic–plastic behaviour making use of the deformability of the grains is demonstrated. The unloading–reloading behaviour associated with the geometrical arrangement of the grains for a granular assembly under triaxial compression is examined in terms of energy dissipation quantities

A laboratory-based technique for grain size and shape characterisation

The significance of grain shape dependent behaviour is widely reported in the literature. Quantification of grain shape is, however, not part of routine laboratory characterisation and this can be in part attributed to the lack to accessible equipment. In this paper, we discuss the potential of a novel imaging technique to capture the three-dimensional outline of grains. This technique enables the volumetric description of the grain to be obtained by reconstructing the planar projections of the grain acquired at different angles of rotation using a standard camera. The imaging setup is very simple and can be easily implemented in any laboratory. It includes a camera, a lens (optional) and a stepper motor to rotate the object in controllable and precise increments. The greater the number of acquired projections, the better the detail of the reconstructed volume will be. Results from single grain compression tests on Leighton Buzzard sand and a shelly carbonate sand from the Persian Gulf are presented to demonstrate the dependency of the tensile strength on the grain shape. The simplicity and easy access to this laboratory-based technique have the potential to enhance laboratory and physical experiments of geomaterials

Cavity cooling a single charged nanoparticle

The development of laser cooling coupled with the ability to trap atoms and ions in electromagnetic fields, has revolutionised atomic and optical physics, leading to the development of atomic clocks, high-resolution spectroscopy and applications in quantum simulation and processing. However, complex systems, such as large molecules and nanoparticles, lack the simple internal resonances required for laser cooling. Here we report on a hybrid scheme that uses the external resonance of an optical cavity, combined with radio frequency (RF) fields, to trap and cool a single charged nanoparticle. An RF Paul trap allows confinement in vacuum, avoiding instabilities that arise from optical fields alone, and crucially actively participates in the cooling process. This system offers great promise for cooling and trapping a wide range of complex charged particles with applications in precision force sensing, mass spectrometry, exploration of quantum mechanics at large mass scales and the possibility of creating large quantum superpositions.Comment: 8 pages, 5 figures Updated version includes additional references, new title, and supplementary information include