Interference effects in two-photon ATI by multiple orders high harmonics with random or locked phases

We numerically study 2-photon processes using a set of harmonics from a Ti:Sapphire laser and in particular interference effects in the Above Threshold Ionization spectra. We compare the situation where the harmonic phases are assumed locked to the case where they have a random distribution. Suggestions for possible experiments, using realistic parameters are discussed.Comment: 11 pages, 13 figures, LaTe

Effects of soil insecticide treatments on northern corn rootworm, Diabrotica barberi [Coleoptera : Chrysomelidae] populations and on corn yield

Durant 3 ans, des essais au champ ont permis d'evaluer les effets de trois insecticides appliqués au sol lors des semis sur les populations de la chrysomèle des racines du maïs (Diabrotica barberi). Dans chacune des parcelles, une cage d'émergence a été placée au-dessus d'un plant de maïs (Zea mays) et les populations d'adultes ont été suivies hebdomadairement. Les dommages faits par les larves aux racines de maïs ont été évalués et le rendement en grains a été déterminé à la fin de la saison de croissance des plantes. Les traitements insecticides ont réduit significativement le nombre d'adultes émergeant durant la première année seulement. La force d'arrachage était reliée négativement au nombre d'adultes émergeant par plant. Chaque année le rendement en grains des parcelles traitées n'était pas significativement différent de celui des parcelles non traitées. Les traitements insecticides peuvent donc réduire le nombre d'adultes émergeant sans toutefois augmenter les rendements.A 3-yr field study was conducted to determine the effects of three soil applied insecticides on northern corn rootworm (Diabrotica barberi) populations. In each plot, an emergence cage was placed over a corn (Zea mays) plant, and adult populations were monitored weekly. Larval damage to corn roots was evaluated and corn yield was recorded at the end of the growing season. Insecticide treatments significantly reduced the number of emerging adults in the first yr, but no difference was observed in the following yr. Root strength was negatively correlated to the number of emerging adults per plant. Each yr, corn yields in treated plots did not differ significantly from yield in untreated plots, which suggests that insecticide treatments can reduce the number of emerging adults without increasing yields

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

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

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

Améliorer les apprentissages en sciences en milieu francophone minoritaire : résultats de l’expérimentation d’un modèle pédagogique

Selon les postulats théoriques, les faibles résultats scolaires chez les populations linguistiques minoritaires s’expliqueraient par des facteurs socioculturels et économiques, on recommande donc une pédagogie qui composerait avec ces éléments. Dans le cadre de cette recherche, les auteurs ont créé et expérimenté un modèle pédagogique pour enseigner les sciences en milieu minoritaire tenant compte de ces facteurs. Lors d’une intervention de deux mois, des élèves de cinquième année ont étudié les marais salés à partir de ce modèle pédagogique. L’expression écrite et orale de leurs observations demeurait une priorité. Les résultats ont révélé une attitude positive envers l’approche, un apprentissage scientifique important et une progression lexicale aux niveaux oral et écrit.Socio-cultural and economic factors may in part explain poor academic results for linguistic minorities. In such a case, pedagogical considerations in light of such factors should be put in place. This research created and experimented a pedagogical model for teaching science to minority language students, while considering socio-cultural factors. During a two-month intervention, grade five students studied salt-water marshes in a unit conceived within this model. Oral and written expression of their observations constituted an important part of the experiment. Results indicate a favourable attitude towards the pedagogical approach and progression in science and lexical learning in both written and oral language.Los postulados teóricos indican que los bajos resultados escolares en las poblaciones de lenguas minoritarias se justifican por factores socioculturales y económicos. Por lo tanto, se requiere de una pedagogía que tome en cuenta estos elementos. En el marco de esta investigación, los autores han creado y experimentado un modelo pedagógico para enseñar las ciencias en un ámbito minoritario tomando en cuenta estos factores. En una intervención de una duración de dos meses, alumnos de quinto grado estudiaron las marismas saladas a partir de un modelo pedagógico. Quedó prioritaria la expresión escrita y oral de sus observaciones. Los resultados revelaron una actitud positiva hacia el enfoque, un aprendizaje científico importante así como una progresión lexical a nivel oral y escrito

Structure and dynamics of Oxide Melts and Glasses : a view from multinuclear and high temperature NMR

Solid State Nuclear Magnetic Resonance (NMR) experiments allow characterizing the local structure and dynamics of oxide glasses and melts. Thanks to the development of new experiments, it now becomes possible to evidence not only the details of the coordination state of the network formers of glasses but also to characterize the nature of polyatomic molecular motifs extending over several chemical bonds. We present results involving 31P homonuclear experiments that allow description of groups of up to three phosphate units and 27Al/17O heteronuclear that allows evidencing μ3 oxygen bridges in aluminate glasses and rediscussion of the structure of high temperature melts.Comment: Journal of Non-Crystalline Solids (2007) in press; Also available online at: http://crmht.cnrs-orleans.fr/Intranet/Publications/?id=207

Time-dependent calculation of ionization in Potassium at mid-infrared wavelengths

We study the dynamics of the Potassium atom in the mid-infrared, high intensity, short laser pulse regime. We ascertain numerical convergence by comparing the results obtained by the direct expansion of the time-dependent Schroedinger equation onto B-Splines, to those obtained by the eigenbasis expansion method. We present ionization curves in the 12-, 13-, and 14-photon ionization range for Potassium. The ionization curve of a scaled system, namely Hydrogen starting from the 2s, is compared to the 12-photon results. In the 13-photon regime, a dynamic resonance is found and analyzed in some detail. The results for all wavelengths and intensities, including Hydrogen, display a clear plateau in the peak-heights of the low energy part of the Above Threshold Ionization (ATI) spectrum, which scales with the ponderomotive energy Up, and extends to 2.8 +- 0.5 Up.Comment: 15 two-column pages with 15 figures, 3 tables. Accepted for publication in Phys. Rev A. Improved figures, language and punctuation, and made minor corrections. We also added a comparison to the ADK theor