Reduced thermodynamic description of phase separation in a quasi-one-dimensional granular gas

We describe simulations of a quasi-one-dimensional, vibrated granular gas which exhibits an apparent phase separation into a liquidlike phase and a gaslike phase. In thermal equilibrium, such a phase separation in one dimension is prohibited by entropic considerations. We propose that the granular gas minimizes a function of the conserved mechanical variables alone: the particle number and volume. Simulations in small cells can be used to extract the equation of state and predict the coexisting pressure and densities, as confirmation of the minimization principle. Fluctuations in the system manifest themselves as persistent density waves but they do not destroy the phase-separated state

A competitive scheme for storing sparse representation of X-Ray medical images

A competitive scheme for economic storage of the informational content of an X-Ray image, as it can be used for further processing, is presented. It is demonstrated that sparse representation of that type of data can be encapsulated in a small file without affecting the quality of the recovered image. The proposed representation, which is inscribed within the context of data reduction, provides a format for saving the image information in a way that could assist methodologies for analysis and classification. The competitiveness of the resulting file is compared against the compression standards JPEG and JPEG200

A history of high-power laser research and development in the United Kingdom

The first demonstration of laser action in ruby was made in 1960 by T. H. Maiman of Hughes Research Laboratories, USA. Many laboratories worldwide began the search for lasers using different materials, operating at different wavelengths. In the UK, academia, industry and the central laboratories took up the challenge from the earliest days to develop these systems for a broad range of applications. This historical review looks at the contribution the UK has made to the advancement of the technology, the development of systems and components and their exploitation over the last 60 years

The Response to an External AC Drive of the Interface Between Solid and Liquid Helium Near the Roughening Transition

The theory of the roughening transition developed by Nozières uses the notion of a stopped renormalisation scheme when dealing with the effect of fluctuations of the crystal-liquid interface. For a faceting direction, the renormalisation is stopped at the correlation length in the absence of any external driving force on the interface. However the renormalisation process needs to be stopped earlier if there is a shorter length scale in the problem. Nozières considered such finite size effects arising from either vicinal surfaces or from the effect of an external DC driving force which causes the interface to grow (or melt) at a finite velocity. Here we consider the effect of an applied AC driving force which introduces a new length scale; this length is related to the distance a disturbance in the interface diffuses in a time $\omega^{-1}$, where $\omega$ is the angular frequency of the applied drive. We have extended the recursion relations to deal with the AC case. Provided the correlation length is larger than the other length scales we can solve the recursion relations and obtain the response of the interface, an approach which is exact to second order in the pinning potential. When the correlation length is smaller than one of the other lengths, we solve the recursion relations for length scales shorter than correlation length; for longer length scales we solve the equation of motion renormalised the correlation length by ignoring the random noise as being small. This crude approximation allows us to describe semi-quantitatively the crossover from the mobile to the pinned behavior of the interface.La théorie de la transition rugueuse développée par Nozières utilise un programme de renormalisation tronquée pour traiter de l'effet des fluctuations de l'interface liquide-cristal. Pour une direction facettée, la renormalisation s'arrête à la longueur de corrélation en l'absence d'une force extérieure appliquée à l'interface. Cependant, le processus de renormalisation doit être arrêté plus tôt s'il y a une longueur plus courte dans le problème. Nozières a examiné de tels effets de taille finie soit pour des surfaces vicinales, lorsqu'une force extérieure continue est appliquée pour provoquer une croissance (ou une fusion) de l'interface à vitesse finie. Ici on considère l'effet d'une force extérieure alternative qui introduit une nouvelle échelle de longueur ; c'est la distance de diffusion d'une perturbation pendant le temps $\omega^{-1}$, où $\omega$ est la pulsation de la force appliquée. Nous avons étendu les relations de récurrence pour traiter le cas de courant alternatif. Pourvu que la longueur de correlation soit plus grande que les autres échelles de longueur, nous pouvons résoudre les relations de récurrence et obtenir la réponse de l'interface, une approche qui est exacte au deuxième ordre en potentiel d'ancrage. Quand la longueur de corrélation est plus petite que les autres longueurs, nous résolvons les relations de récurrence pour des échelles de longueur plus courtes que la longueur de corrélation ; pour des échelles de longueur plus grandes nous résolvons l'equation de mouvement renormalisée sans tenir compte du bruit aléatoire. Cette approximation grossière nous permet de décrire semi-quantitativement le passage du comportement mobile au comportement ancré de l'interface

Theory of the kinetic coefficients of the atomically rough surface of 4He crystals

The growth coefficient K (the velocity of growth per unit chemical potential difference) and the Onsager cross-coefficients b1 and b2, coupling growth and heat flow, are calculated for atomically rough surfaces of hcp 4He crystals. The calculation is based on the premise, suggested by Andreev and Parshin, that growth is limited by the collision of phonons and rotons with the interface. The calculated K is compared with that obtained by Keshiskev et al. from the damping of melting-freezing waves. The theory assumes that the excitations are in the ballistic regime where their mean free path is large compared to the wavelength of the melting-freezing waves. In the experiment only the phonons satisfy this condition, yet the theory agrees with the data even when roton scattering is important Irreversible thermodynamics requires that the cross coefficients b1 and b 2 be equal. This is shown by direct calculation. The value of b1 and b2 depends on the ratio of two integrals over the phonon transmission coefficient and it is evaluated for two models of the transmission. The theory agrees fairly well with a recent measurement of b1. A calculation of the dissipation in the hydrodynamic regime, where the free path is short, shows that the damping of melting-freezing waves should have a different dependence on frequency compared to the ballistic regime.Le coefficient de croissance K (vitesse de croissance divisée par la différence de potentiel chimique) et les coefficients croisés de Onsager b1 et b2, couplant croissance et transfert de chaleur, sont calculés pour les surfaces rugueuses des cristaux 4He hexagonaux. Le calcul est fondé sur l'hypothèse proposée par Andreev et Parshin, que la croissance est contrôlée par la diffusion des phonons et rotons à l'interface. Le K théorique est comparé à l'expérience de Keshishev et al. sur l'amortissement des ondes de cristallisation. La théorie est valable quand les excitations sont dans le régime balistique où leur libre parcours moyen est plus important que la longueur des ondes de cristallisation. Dans l'expérience seuls les phonons sont dans ce régime, néanmoins la théorie et l'expérience sont en accord même quand, la diffusion de rotons est la plus importante. La thermodynamique des processus irréversibles indique que les coefficients b1 et b2 sont égaux. Ce résultat est obtenu directement dans le calcul. La vapeur de b1 et b2 dépend du rapport de deux intégrales, de la probabilité de transmission des phonons, et elle est calculée pour deux modèles de transmission. Il y a un accord satisfaisant avec une mesure récente de b1. Un calcul dans le régime hydrodynamique, où le libre parcours moyen est court, montre que l'amortissement des ondes de cristallisation a une dépendance en fréquence différente de celle du régime balistique

Quantum turbulence in 4He, oscillating grids, and where do we go next?

Experimental approaches to the study of quantum turbulence (QT) in superfluid 4He in the low temperature limit, where the normal fluid density is effectively zero, are considered. A succinct general introduction covers liquid 4He, superfluidity, critical velocities for the onset of dissipation, quantized vortex lines and QT. The QT can be created mechanically by the oscillation of wires or grids above characteristic critical velocities. The interesting dynamics of the oscillating grid are discussed. It exhibits an enhanced effective mass due to backflow, as expected from classical hydrodynamics. It is found that the critical velocity attributable to the onset of QT production rises with increasing temperature. Oscillating objects like grids or wires create QT that is not well-characterized in terms of length scale, and the QT is not spatially homogeneous. The QT can be detected by the trapping of negative ions on vortex cores. Although the corresponding capture cross-section has not yet been measured, it is evidently very small, so that the technique cannot be expected to be a very sensitive one. In the future it is hoped to create well-characterized, homogeneous QT by means of a drawn grid. Improved sensitivity in the detection of QT is being sought through calorimetric techniques that monitor the temperature rise of the liquid caused by the decay of the vortex lines

ILL experimental reports and theory college activities 1988

CNRS RP 350 (48) / INIST-CNRS - Institut de l'Information Scientifique et TechniqueSIGLEFRFranc