Coulomb-Volkov approach of ionization by extreme ultraviolet laser pulses in the subfemtosecond regime

In conditions where the interaction betweeen an atom and a short high-frequency extreme ultraviolet laser pulse is a perturbation, we show that a simple theoretical approach, based on Coulomb-Volkov-type states, can make reliable predictions for ionization. To avoid any additional approximation, we consider here a standard case : the ionization of hydrogen atoms initially in their ground state. For any field parameter, we show that the method provides accurate energy spectra of ejected electrons, including many above threshold ionization peaks, as long as the two following conditions are simultaneously fulfilled : (i) the photon energy is greater than or equal to the ionization potential ; (ii) the ionization process is not saturated. Thus, ionization of atoms or molecules by the high order harmonic laser pulses which are generated at present may be addressed through this Coulomb-Volkov treatment.Comment: 19 pages including 5 figures and figure caption

Modeling laser wakefield accelerators in a Lorentz boosted frame

Modeling of laser-plasma wakefield accelerators in an optimal frame of reference \cite{VayPRL07} is shown to produce orders of magnitude speed-up of calculations from first principles. Obtaining these speedups requires mitigation of a high-frequency instability that otherwise limits effectiveness in addition to solutions for handling data input and output in a relativistically boosted frame of reference. The observed high-frequency instability is mitigated using methods including an electromagnetic solver with tunable coefficients, its extension to accomodate Perfectly Matched Layers and Friedman's damping algorithms, as well as an efficient large bandwidth digital filter. It is shown that choosing the frame of the wake as the frame of reference allows for higher levels of filtering and damping than is possible in other frames for the same accuracy. Detailed testing also revealed serendipitously the existence of a singular time step at which the instability level is minimized, independently of numerical dispersion, thus indicating that the observed instability may not be due primarily to Numerical Cerenkov as has been conjectured. The techniques developed for Cerenkov mitigation prove nonetheless to be very efficient at controlling the instability. Using these techniques, agreement at the percentage level is demonstrated between simulations using different frames of reference, with speedups reaching two orders of magnitude for a 0.1 GeV class stages. The method then allows direct and efficient full-scale modeling of deeply depleted laser-plasma stages of 10 GeV-1 TeV for the first time, verifying the scaling of plasma accelerators to very high energies. Over 4, 5 and 6 orders of magnitude speedup is achieved for the modeling of 10 GeV, 100 GeV and 1 TeV class stages, respectively

Constitutive modelling of the creep behaviour of single crystal superalloys under non-isothermal conditions inducing phase transformations

The prediction of the viscoplastic behaviour of Ni-based single crystal superalloy is still a challenging issue due to the non-isothermal loadings which can be encountered by aeronautic engines components such as high pressure turbine blades and vanes. Under particular in-service conditions, these materials may experience temperature cycles which promote the dissolution of the strengthening g’-phase of the material on (over)heating, and subsequent precipitation on cooling, leading to a transient viscoplastic behaviour. New internal variables representing the microstructural changes under those specific thermal loadings have been introduced in the framework of crystal plasticity using a macroscopic approach (no representation of the g/g’ microstructure of the alloy) to account for the transient creep behaviour induced by microstructure changes. This modelling approach captures first order effects on the creep behaviour due to (a) g’ precipitates volume fraction evolution of each kind of particles of a bimodal distribution of precipitates (which evolves according to thermal history), (b) recovery of the dislocation density and, (c) material orientation. In addition, a damage law keeping in memory all the thermal history and recovery processes has been introduced to account for the unconventional post-overheating creep life. This model is calibrated on non-isothermal creep experiments on [001] oriented single crystals made of MC2 alloy. It is able to predict creep strain (primary, secondary, tertiary), whatever the temperature history of the material

Speeding up simulations of relativistic systems using an optimal boosted frame

It can be computationally advantageous to perform computer simulations in a Lorentz boosted frame for a certain class of systems. However, even if the computer model relies on a covariant set of equations, it has been pointed out that algorithmic difficulties related to discretization errors may have to be overcome in order to take full advantage of the potential speedup. We summarize the findings, the difficulties and their solutions, and show that the technique enables simulations important to several areas of accelerator physics that are otherwise problematic, including self-consistent modeling in three-dimensions of laser wakefield accelerator stages at energies of 10 GeV and above.Comment: To be published in the proceedings of DPF-2009, Detroit, MI, July 2009, eConf C09072

Effects of Hyperbolic Rotation in Minkowski Space on the Modeling of Plasma Accelerators in a Lorentz Boosted Frame

Laser driven plasma accelerators promise much shorter particle accelerators but their development requires detailed simulations that challenge or exceed current capabilities. We report the first direct simulations of stages up to 1 TeV from simulations using a Lorentz boosted calculation frame resulting in a million times speedup, thanks to a frame boost as high as gamma=1300. Effects of the hyperbolic rotation in Minkowski space resulting from the frame boost on the laser propagation in the plasma is shown to be key in the mitigation of a numerical instability that was limiting previous attempts

Geological setting and petrochemistry of early Middle Devonian volcanic and gabbroic rocks in the Guysborough area, Nova Scotia

Mapping, petrological studies, and U-Pb dating of volcanic and plutonic units have provided a new understanding of the stratigraphy and paleotectonic setting of rocks in the Guysborough area. From base to top, the stratigraphic sequence is interpreted to consist of: (1) volcanic flows and pyroclastic rocks, with minor interlayered sedimentary rocks (Sunnyville Formation), (2) varied conglomerate (Glenkeen Formation), (3) quartz wacke with minor interlayered quartz arenite and conglomerate, and (4) massive to shaly or laminated siltstone. These volcanic and sedimentary units arc intruded by small gabbroic plutons, sills, and dykes. All the units are early Middle Devonian, based on a U-Pb (zircon) age of 389 ± 2 Ma for rhyolitic tuff from the Sunnyville Formation, and a preliminary U-Pb (baddelcyite) age of ca. 38S Ma for one of the gabbroic plutons in the siltstone unit. Based on whole-rock chemistry, the mafic volcanic and gabbroic rocks are similar, and formed in a continental within-plate setting. However, the rocks in the eastern part of the map area are more alkalic than those in the west, which are dominantly tholeiitic. The mafic magmas are interpreted to have formed by partial melting of the subcontinental upper mantle and to have evolved by crystal fractionation processes; the more alkalic magmas in the east may represent lesser amounts of partial melting in that area. This interpretation is consistent with the presence of felsic volcanic rocks only in the western part of the area. They may represent crustal melts formed as a result of mafic magma underplating of the crust. The regional tectonic significance of these igneous rocks is not yet resolved, but they are clearly older than Late Devonian to Carboniferous igneous units elsewhere in northern mainland Nova Scotia and in Cape Breton Island with which they were previously assumed to be correlative. RÉSUMÉ Des travaux de cartographic, des études pétrologiques et la datation au U-Pb d'unités volcaniques et plutoniques ont permis une meilleure compréhension de la stratigraphic et du cadre paléotcctonique des roches du secteur de Guysborough. On interprèts la composition de la séquence stratigraphique comme suit, de sa base à son sommet : 1) des écoulements volcaniques et des roches pyroclastiques, avec une proportion mineure de roches sédimentaires intercalécs (Formation Sunnyville); 2) un conglomerat diversify (Formation Glenkeen); 3) de la wacke qurtzeuse avec une présence mineure de conglomerat et de quartzite sédimentaire interstratifies; et 4) des siltstones massifs à schistcux ou feuilletés. Ces unités volcaniques et sédimentaires sont pénétries par de petits dykes, filons-couches et plutons gabbrolques. Toutes les unités remontent au début du Dévonien moyen d'apres une datation au U-Pb (zircon) situant a 389+2 Ma le tuf rhyolitique de la Formation Sunnyville ainsi qu'une datation au U-Pb (baddeleyite) situant provisoirement l'un des plutons gabbrolques de l'unité de siltstones a environ 38S Ma. D'après la chimie des roches, les roches volcanomaflques et gabbrolques sont semblabes et elles se sont formées dans un cadre intra-plaque continental. Les roches de la partie orientate du secteur cartographique sont cependant plus alcalines que celles de l'ouest, en predominance tholéeiitiques. Selon l'interprétation avancée, les magmas mafiques se seraient formes par suite d'une fonte partielle du manteau sous-continental supérieur et ils auraient évolué au moyen de processus de différenciation magmatique; les magmas plus alcalins de l'est pourraient représentor une fonte partielle moins importante dans ce secteur. Cette interprétation est compatible avec la présence de roches volcano-felsiques limitée a la partie ouest du secteur. Elles pourraient représentor des éléments crustaux fondus formés par suite d'une remontée de magma mafique sous les plaques de la croûte. On n'a pas encore déterminé ('importance tectonique régionale dc ces roches ignées, mais elles sont clairement plus anciennes que les unités ignées du Dévonien supérieur au Carbonifère des autres régions de L'intérieur de la Nouvelle-Écosse et de l'ile du Cap-Breton avec lesquelles on les supposait auparavant corrélatives. [Traduit par la rédaction

Développement d’un test d’analyse auditive en français : normes et validation de construit

Deux études ont permis de développer, en français, un instrument qui sert à mesurer les habiletés de segmentation des mots prononcés en leurs éléments sonores, chez des enfants de la maternelle à la sixième année, habiletés jugées cruciales pour l'apprentissage de la lecture. Une première étude qui portait sur un large échantillon d'items (n = 70) a servi à la création de deux listes de 42 items chacune, formant deux versions d'un même test, pour une deuxième étude. Les résultats des deux études montrent que les caractéristiques psychométriques - fidélité et validité - de l'instrument développé sont très élevées. De plus, dans la deuxième étude, la performance des sujets ne différait pas selon la version du test. Ces résultats appuient l'utilisation de ce test dans les milieux scolaires pour l'identification de troubles d'analyse auditive chez les enfants qui ont des problèmes de lecture.This article reports on two studies used to develop a French language test to measure oral word-segmentation ability in children from Kindergarten to Grade 6; this ability being considered important in learning to read. The first study of a large sample of items (n = 70) was used to create two lists of 42 items each, to form equivalent versions of the same test, and then used in a second study. The results of the two studies show that the psychometric characteristics of the instrument, that of reliability and validity, are very high. As well, it was found that performance of subjects did not differ with the test version used. These results support use of this test in school settings to identify auditive analysis difficulties in children who have reading difficulties.Dos estudios, han permitido crear un instrumento en francès, para medir la habilidad de los nifios para separar las palabras en fonemas. Esta habilidades considerada importante en el proceso del aprendizaje de la lectura. El instrumento puede aplicarse a nifios disde la escuela de parvulos hasta el 6° ano de primari. El primer estudio comprendié un gran numéro de cuestiones (n = 70) y sirvio para la confection de dos listas de 42 preguntas cada una; se obtuveron asi dos versiones de un mismo test que constituyeron la base del segundo estudio. Los resultados de los dos estudios muestran que las caractensticas psocométricas - fidelidad y validez - del instrumento son bastante elevadas. Ademâs, en el segundo estudio se destaca que el resultado obtenido por no deflene mucho entre las dos versiones. Creemos que esos resultados avalan la utilizaciôn de este test en el medio escolar para identificar problemas de analisis auditivo en nifios con dificultades en el aprendizaje de la lectura.Zwei Studien haben erlaubt, fur die franzôsische Sprache ein Instrument zu entwickeln zur Messung der Fâhigkeit, gesprochene Worter in ihre Lautelemente zu zerlegen, und zwar bei Kindern von der Vorschule vis zur 6.Volksschulklasse. Dièse Fâhigkeit gilt als àuÊerst wichtig fur das Lesenlernen. Eine erste Untersuchung iiber eine grofie Anzahl von Einheiten (n = 70) fiihrte zur Erstellung zweier Listen mit je 42 Einheiten, welche zwei Versionen des gleichen Tests darstellten fiir eine zweite Untersuchung. Die Ergebnisse der beiden Studien zeigen, dass die psychometrischen Qualitâten, nâmlich Genauigkeit und Giiltigkeit, des entwickelten MeCinstrumentes sehr hoch sind. Aufierdem war die Leistung der Versuchspersonen in den beiden Tests nicht verschieden. Dièse Ergebnisse sprechen fur die Anwendung dieses Tests im Schulmilieu, um Schwierigkeiten in der Lautanalyse bei Kindern mit Leseschwierigkeiten zu identifizieren

On the gravitational production of superheavy dark matter

The dark matter in the universe can be in the form of a superheavy matter species (WIMPZILLA). Several mechanisms have been proposed for the production of WIMPZILLA particles during or immediately following the inflationary epoch. Perhaps the most attractive mechanism is through gravitational particle production, where particles are produced simply as a result of the expansion of the universe. In this paper we present a detailed numerical calculation of WIMPZILLA gravitational production in hybrid-inflation models and natural-inflation models. Generalizing these findings, we also explore the dependence of the gravitational production mechanism on various models of inflation. We show that superheavy dark matter production seems to be robust, with Omega_X h^2 ~ (M_X / (10^11 GeV))^2 (T_RH / (10^9 GeV)), so long as M_X < H_I, where M_X is the WIMPZILLA mass, T_RH is the reheat temperature, and H_I is the expansion rate of the universe during inflation.Comment: 26 pages, 7 figures; LaTeX; submitted to Physical Review D; minor typographical error correcte