MESURE DES GRADIENTS DE DENSITÉ DANS L'INTERACTION LASER-PLASMA

Nous étudions la densité électronique et son gradient dans l'interaction d'un laser à CO2 avec une cible plane de polyéthylène, pour un flux laser de 1012 W/cm2. L'étude par interférométrie Jamin des profils de densité et la mesure strioscopique de son gradient, couplées à l'étude de l'image en rayons X du plasma conduisent à une valeur de 220 ± 80 µm de la longueur du gradient de densité. Ceci est confirmé par des mesures indirectes obtenues à partir du bilan optique de l'interaction et du seuil de génération de l'harmonique 3/2 ω0 du laser à CO2. Les conséquences de ces résultats sur les conditions de l'interaction sont discutées

Long Range Hydration Effects in Electrolytic Free Suspended Black Films

The force law within free suspended black films made of negatively charged Aerosol-OT (AOT) with added LiCl or CsCl is studied accurately using X-ray reflectivity (ca. 1{\AA}). We find an electrolyte concentration threshold above which a substantial additional repulsion is detected in the LiCl films, up to distances of 100 {\AA}. We interpret this phenomenon as an augmentation of the Debye screening length, due to the local screening of the condensed hydrophilic counterions by the primary hydration shell.Comment: 4 pages, 4 figures, to be published Phys. Rev. Let

Observation of the Smectic C -- Smectic I Critical Point

We report the first observation of the smectic C--smectic I (C--I) critical point by Xray diffraction studies on a binary system. This is in confirmity with the theoretical idea of Nelson and Halperin that coupling to the molecular tilt should induce hexatic order even in the C phase and as such both C and I (a tilted hexatic phase) should have the same symmetry. The results provide evidence in support of the recent theory of Defontaines and Prost proposing a new universality class for critical points in layered systems.Comment: 9 pages Latex and 5 postscript figures available from [email protected] on request, Phys.Rev.Lett. (in press

Parametric timed model checking for guaranteeing timed opacity

Information leakage can have dramatic consequences on systems security. Among harmful information leaks, the timing information leakage is the ability for an attacker to deduce internal information depending on the system execution time. We address the following problem: given a timed system, synthesize the execution times for which one cannot deduce whether the system performed some secret behavior. We solve this problem in the setting of timed automata (TAs). We first provide a general solution, and then extend the problem to parametric TAs, by synthesizing internal timings making the TA secure. We study decidability, devise algorithms, and show that our method can also apply to program analysis.Comment: This is the author (and extended) version of the manuscript of the same name published in the proceedings of ATVA 2019. This work is partially supported by the ANR national research program PACS (ANR-14-CE28-0002), the ANR-NRF research program (ProMiS) and by ERATO HASUO Metamathematics for Systems Design Project (No. JPMJER1603), JS

Population pharmacokinetics of colistin and the relation to survival in critically ill patients infected with colistin susceptible and carbapenem-resistant bacteria

Objectives: The aim was to analyse the population pharmacokinetics of colistin and to explore the relationship between colistin exposure and time to death. Methods: Patients included in the AIDA randomized controlled trial were treated with colistin for severe infections caused by carbapenem-resistant Gram-negative bacteria. All subjects received a 9 million units (MU) loading dose, followed by a 4.5 MU twice daily maintenance dose, with dose reduction if creatinine clearance (CrCL) 2 mg/L in 94% (195/208) and 44% (38/87) of patients with CrCL ≤120 mL/min, and >120 mL/min, respectively. Colistin methanesulfonate sodium (CMS) and colistin clearances were strongly dependent on CrCL. High colistin exposure to MIC ratio was associated with increased hazard of death in the multivariate analysis (adjusted hazard ratio (95% CI): 1.07 (1.03–1.12)). Other significant predictors included SOFA score at baseline (HR 1.24 (1.19–1.30) per score increase), age and Acinetobacter or Pseudomonas as index pathogen. Discussion: The population pharmacokinetic model predicted that >90% of the patients had colistin concentrations

Magnetic reconnection driven by electron dynamics

Magnetic reconnections play essential roles in space, astrophysical, and laboratory plasmas, where the anti-parallel magnetic field components re-connect and the magnetic energy is converted to the plasma energy as Alfvénic out flows. Although the electron dynamics is considered to be essential, it is highly challenging to observe electron scale reconnections. Here we show the experimental results on an electron scale reconnection driven by the electron dynamics in laser-produced plasmas. We apply a weak-external magnetic field in the direction perpendicular to the plasma propagation, where the magnetic field is directly coupled with only the electrons but not for the ions. Since the kinetic pressure of plasma is much larger than the magnetic pressure, the magnetic field is distorted and locally anti-parallel. We observe plasma collimations, cusp and plasmoid like features with optical diagnostics. The plasmoid propagates at the electron Alfvén velocity, indicating a reconnection driven by the electron dynamics

Soft X-ray imaging with multilayer optics in laser fusion experiments

The development of multilayered mirrors for the soft X-ray range allows us to bulld diagnostics capable of imaging dense plasmas at radiation energies less than or equal to 100 eV. Multilayered mirrors have particular applications in laser fusion research. In this area, such instrumentation allow us to study the overcritical zone of laser created plasmas. In this zone where the electromagnetic wave associated with the laser cannot penetrate, the processes of energy transport take place. By the analysis of the radiation emitted from this zone, it is possible to characterize the transport mechanisms. We have used such mirrors to image this zone. The experiments to the described illustrate the use of multilayered mirrors for the study of dense plasmas.Le développement des miroirs multicouches dans le domaine des rayons X mous a permis de mettre au point des diagnostics autres que la spectroscopie pour les plasmas denses et de température inférieure ou égale à 100 eV. En particulier, ces nouvelles surfaces ont été utilisées dans les recherches sur la fusion inertielle par laser. Dans ce domaine, de tels instruments ont particulièrement aidé à l'étude de la zone surcritique des plasmas créés par laser. Dans cette zone où l'onde laser ne peut pénétrer ont lieu les processus de transport de l'énergie. Par l'intermédiaire du rayonnement émis à partir de cette zone, il est possible de caractériser les mécanismes de transport. C'est à l'aide de dispositifs à miroirs faisant l'imagerie de cette zone que nous illustrons l'usage de dispositifs à multicouches pour l'étude des plasmas denses

Optical measurement of the expansion of a laser plasma created on a planar microtarget

Interferometric density measurements in the corona of a laser created plasma require small targets in order to reach the highest density possible. Here we have chosen to create the plasma on the end of microcylinders which diameter is of the size of the focal spot of the laser. With this type of target, the plasma created outside the focal spot of the laser by lateral conduction is located at the periphery of the microcylinder and can be optically probed separately from the main plasma. Using such targets illuminated with a 1-3 J, 50 ps duration neodymium laser pulse, we have evidenced during the laser pulse non linear effects such as density profile modifications and filamentation. After the laser pulse, we have measured the density distribution showing that the density contours are nearly spherical. A few percent of the incident laser energy is found in a lateral plasma whose temperature is a few electron volts and which expands about 100 μm along the microcylinder.Pour mesurer par interférométrie la densité dans la couronne d'un plasma créé par laser, il faut utiliser de petites cibles afin d'atteindre la densité la plus élevée possible. Ici, nous avons choisi de créer un plasma au bout de micro-cylindres de même diamètre que la tache focale du laser. Avec ce type de cible, le plasma créé en dehors de la tache focale par conduction latérale est situé sur la périphérie du micro-cylindre et peut être sondé optiquement indépendamment du plasma principal. En utilisant de telles cibles illuminées par une impulsion laser à néodyme de 1 à 3 J et de durée 50 ps, nous avons mis en évidence pendant l'impulsion laser, des effets non linéaires tels que modification du profil de densité et filamentation. Après l'impulsion laser, nous avons mesuré la distribution de densité, montrant que les isodensités ont une forme sphérique. Quelques pourcents de l'énergie laser incidente sont retrouvés dans l'expansion latérale d'un plasma de quelques électrons volts de température qui s'étend sur environ 100 μm le long du micro-cylindre