Lawson criterion for ignition exceeded in an inertial fusion experiment

For more than half a century, researchers around the world have been engaged in attempts to achieve fusion ignition as a proof of principle of various fusion concepts. Following the Lawson criterion, an ignited plasma is one where the fusion heating power is high enough to overcome all the physical processes that cool the fusion plasma, creating a positive thermodynamic feedback loop with rapidly increasing temperature. In inertially confined fusion, ignition is a state where the fusion plasma can begin "burn propagation" into surrounding cold fuel, enabling the possibility of high energy gain. While "scientific breakeven" (i.e., unity target gain) has not yet been achieved (here target gain is 0.72, 1.37 MJ of fusion for 1.92 MJ of laser energy), this Letter reports the first controlled fusion experiment, using laser indirect drive, on the National Ignition Facility to produce capsule gain (here 5.8) and reach ignition by nine different formulations of the Lawson criterion

Pêches profondes aux casiers sur les pentes océaniques des îles de Polynésie française (N/O Marara - 1986/1989)

De 1986 à 1989, environ 3000 casiers ont été posés sur les flancs des îles polynésiennes, à des profondeurs comprises entre 100 et 1120 m. La faune carcinologique s'avère extrêmement originale avec la capture de plusieurs espèces nouvelles pour la science. Une liste provisoire des espèces concernées est établie. Les crevettes #Pandalidae des genres #Heterocarpus et #Plesionika constituent l'essentiel des captures. La CPUE est de 121 g/casier/nuit. Ce résultat est nettement plus faible que tous ceux mentionnés jusqu'à présent dans le Pacifique. Les meilleures zones de pêche en Polynésie sont les archipels des Marquises (266 g/casier/nuit) et de la Société (277 g/casier/nuit). La profondeur de pêche la plus favorable est comprise entre 550 et 649 m. Elle est susceptible de varier en fonction de l'espèce concernée, du lieu de pêche, et dans une moindre mesure de la saison. Les possibilités d'exploitations commerciales de ces stocks apparaissent limitées. Quelques apects de la biologie des huit principales espèces (#H. aff. ensifer, P. fenneri, H. laevigatus, H. sibogae, H. amacula, H. parvispina, P. edwardsii et #P. ensis$) sont précisées. (Résumé d'auteur

Experimental generation of spherical converging shock waves

International audienceSpherical converging shock waves with incoming Mach number 1.18 are generated in a conical test chamber fitted to a conventional square-section shock tube. A wave cutter is employed to ensure a proper shock transition from the square to a cylindrical straight pipe installed upstream of the convergent section. The incident planar wave is transformed into a spherical shock by the use of an ellipsoidal membrane acting as a gas lens separating two gases with different densities. The transmitted shock wave propagates within the conical section, and accelerates towards the apex. The trajectory and the shape of the shock wave are both characterized by planar Mie scattering. The spherical shock follows the similarity solution proposed by Guderley (1942), whose trajectory agrees very well with the numerical simulation of the Chester-Chisnell-Whitham (CCW) theory for the geometry considered. The procedure demonstrated here provides a potential method for future investigations of shock focusing and Richtmyer-Meshkov instability in spherical geometry

An experimental and numerical investigation of the dependency on the initial conditions of the Richtmyer-Meshkov instability

International audienceThe nonlinear evolution of 2D single-mode Richtmyer-Meshkov instabilities is investigated through experiments in shock tube and numerical simulations. In our shock tube, the interface is materialized by a thin membrane attached to a stereo-lithographed grid. The purpose of this study is to compare experimental and numerical results, verify that using a higher Mach number for the incident shock wave (M-isw) than in a previous study [C. Mariani, M. Vandenboomgaerde, G. Jourdan, D. Souffland, and L. Houas, ``Investigation of the Richtmyer-Meshkov instability with stereolithographed interfaces,'' Phys. Rev. Lett. 100, 254503 (2008)] drastically reduces the deleterious effects of the membrane remnants, explore the effect of a high initial amplitude at the interface on the growth of the perturbation, and understand the lack of roll-up structures in the nonlinear phase of the instability. Using grayscale gradient rather than gray level, a new processing of the raw pictures is developed. Numerical simulations run with the TRICLADE code show that adding short-wavelength (swl) perturbations to the single-mode interface is necessary to understand the morphology of the experimental pictures. Through these comparisons, a minimal description of the swl perturbations is proposed. The experimental growth rates are correctly predicted by the TRICLADE code. Positive agreements are also obtained by models only if they take into account the effect of a high initial amplitude. A first result is the agreement that we obtain between experiment, simulations, and models indicates that the membrane remnants no longer influence the global dynamics of the instability. This is due to the increase of M-isw from 1.15 to 1.45. Another result of this investigation is that swl perturbations disrupt the spikes which grow at the interface and prevent the appearance of roll-up structures. Numerical simulations show the importance of a diffuse transition layer at the initial interface as a stabilizing process of the swl perturbations. In our case, only such a layer would allow the roll-up structures to grow. Finally, our analysis confirms that, when they are present at the same time, the swl perturbations distort the long wavelength perturbations, and that the latter decreases the growth of the former. (C) 2014 AIP Publishing LLC