Étude spectroscopique des phénomènes de résonance dans les plasmas micro-ondes

Dans les plasmas produits par des champs électromagnétiques de haute fréquence, le chauffage des électrons peut provenir du chauffage ohmique (collisionnel) ou encore de processus non collisionnels. Dans ce régime, la production d'électrons suprathermiques peut résulter de la conversion de l'onde électromagnétique en ondes électrostatiques au point de résonance (lorsque la permittivité du plasma εp→0) et leur amortissement par effet Landau. L'objectif de ce mémoire de maîtrise est de caractériser expérimentalement l'effet de processus non collisionnels associés à la présence d'une résonance sur la fonction de distribution en énergie des électrons (FDEE). L'étude a été effectuée par spectroscopie optique d'émission de gaz rares dans divers plasmas susceptibles de présenter des phénomènes de résonance. Pour des plasmas tubulaires à basse pression (<50 mTorr) produits par des ondes électromagnétiques de surface à 600MHz, la FDEE est caractérisée par deux températures : TeAll décrit l'énergie moyenne et TeTail représente les électrons de la queue de la distribution. Une étude paramétrique de ces températures en fonction de la position axiale le long de la colonne de plasma et de la pression du gaz a permis de corréler l'augmentation du rapport TeTail / TeAll avec la longueur caractéristique du gradient de densité électronique dans le plasma L. Des mesures et analyses similaires ont aussi été réalisées dans une source à plasma micro-onde magnétisée entretenue à la résonance cyclotronique électronique. Dans ces conditions, la production d'électrons suprathermiques provient non seulement des phénomènes de résonance et des interactions ondes-particules évoqués plus haut mais aussi de la résonance cyclotronique électronique.In plasmas produced by high-frequency electromagnetic fields, electron heating can result from ohmic heating (collisional) or from collisionless processes. In this regime, the production of suprathermal electrons can result from the conversion at the resonant point (when the plasma permittivity εp→0) of the propagating electromagnetic wave into electrostatic waves and their absorption by Landau damping. The objective of this Master's thesis is to experimentally characterize the effect of collisionless processes associated with the presence of resonance phenomena on the electron energy distribution function (EEDF). The study was carried out by trace-rare-gases optical emission spectroscopy (TRG-OES) in various plasmas capable of exhibiting resonance phenomena. For low-pressure tubular plasmas (< 50 mTorr) produced by an electromagnetic surface wave at 600 MHz, EEDF is characterized by two temperatures: TeAldescribes the mean energy and TeTail represents the high-energy tail of the distribution. A parametrical study of these temperatures as a function of the axial distance and the gas pressure revealed that the ratio TeTail / TeAlincreases with the characteristic length of the electron density gradient L. Similar measurements and analyses were also performed in a magnetized microwave plasma source sustained at the electron cyclotron resonance. In such conditions, the production of suprathermal electrons results not only from the resonance phenomena and wave-particle interactions mentioned above but also from the electron cyclotron resonanc

Réaction fonctionnelle des arbres et des peuplements à l'éclaircie commerciale en forêt résineuse

Globalement, on connait bien les objectifs et les effets théoriques de l'éclaircie commerciale. Cependant, peu de résultats expérimentaux sont disponibles au Québec pour confirmer que l'application d'un tel traitement permet d'atteindre les objectifs et de bien répondre aux attentes. Pour pallier à ce manque, nous avons étudié des peuplements traités il y a 7 à 10 ans et localisés dans des forêts résineuses du sud du Québec. Nous avons évalué la réaction des arbres et des peuplements par le biais de variables agissant sur leurs processus de croissance. Pour ce faire, nous sommes retournés dans des placettes échantillons permanentes établies dans les années 1980 et 2000 pour y caractériser la taille des cimes et l'environnement compétitif d'arbres éclaircis et non éclaircis en plus des mesures dendrométriques usuelles. Nous avons aussi prélevé des carottes dendrométriques qui ont permis de reconstituer la surface foliaire des arbres à partir de leur surface d'aubier au moment de la coupe. L'utilisation de modèles linéaires mixtes a permis de démontrer que les accroissements en surface terrière et en surface foliaire des arbres sont fortement liés à leur localisation par rapport au sentier de débardage le plus près. La compétition, quantifiée par un indice indépendant de la distance, a expliqué une proportion importante de la réaction des arbres. Par rapport aux peuplements témoins, l'éclaircie a augmenté la quantité de bois produite par unité de surface foliaire sans toutefois mener à un accroissement supérieur en volume marchand à l'hectare étant donné le plus faible nombre d'arbres. Ces résultats viennent donc éclairer notre compréhension des processus agissant sur la croissance des arbres après un traitement d'éclaircie commerciale. Ils pourront aider à déterminer l'aptitude de peuplements à l'éclaircie et servir à l'élaboration d'outils d'aide à la décision lors du choix des arbres à récolter

Natural Health Products, Modulation of Immune Function and Prevention of Chronic Diseases

The immune system is increasingly found to be involved in the development of several chronic illnesses, for which allopathic medicine has provided limited tools for treatment and especially prevention. In that context, it appears worthwhile to target the immune system in order to modulate the risk of certain chronic illnesses. Meanwhile, natural health products (NHPs) are generating renewed interest, particularly in the prevention and treatment of several chronic diseases. Over 20 scientists from fields related to immune function and NHPs were thus convened to establish the state of knowledge on these subjects and to explore future research directions. This review summarizes the result of discussions held during the symposium. It thus seeks to be thought provoking rather than to comprehensively cover such broad areas of research. Notably, a brief overview of the immune system is presented, including potentially useful targets and strategies to keep it in an equilibrated state, in order to prevent certain disorders. The pertinence and limitations of targeting the immune system to prevent chronic diseases is also discussed. The paper then discusses the usefulness and limitations of current experimental tools available to study the immune modulating effects of NHPs. Finally, a concise review of some of the most studied NHPs showing promising immunomodulatory activity is given, and avenues for future research are described

Adjustment trajectories during the college transition : types, personal and family antecedents, and academic outcomes

This longitudinal study was aimed to describe and understand student adjustment trajectories during the college transition. Participants came from a large random sample of Quebec high school students. They completed a multidimensional measure of adjustment at two times before entering college and at two other times after college admission. Group-based trajectory analysis showed decreased adjustment for 6% (social adjustment) to 66.1% (academic adjustment) of students over this period, versus improved adjustment for 4.5% (social adjustment) to 11.6% (emotional and academic adjustment). All changes were linear, suggesting progressive changes from Secondary 4 to the second year of college. Multivariate and contingency analyses showed that personal anxiety, academic success, and attention problems in high school were significant determinants for adjustment trajectories, and that these trajectories were subsequently related to perseverance and college graduation

Forecasting in dynamic factor models using Bayesian model averaging

This paper considers the problem of forecasting in dynamic factor models using Bayesian model averaging. Theoretical justifications for averaging across models, as opposed to selecting a single model, are given. Practical methods for implementing Bayesian model averaging with factor models are described. These methods involve algorithms which simulate from the space defined by all possible models. We discuss how these simulation algorithms can also be used to select the model with the highest marginal likelihood (or highest value of an information criterion) in an efficient manner. We apply these methods to the problem of forecasting GDP and inflation using quarterly U.S. data on 162 time series. For both GDP and inflation, we find that the models which contain factors do out-forecast an AR(p), but only by a relatively small amount and only at short horizons. We attribute these findings to the presence of structural instability and the fact that lags of dependent variable seem to contain most of the information relevant for forecasting. Relative to the small forecasting gains provided by including factors, the gains provided by using Bayesian model averaging over forecasting methods based on a single model are appreciable

A flexible approach to parametric inference in nonlinear and time varying time series models

Many structural break and regime-switching models have been used with macroeconomic and …nancial data. In this paper, we develop an extremely flexible parametric model which can accommodate virtually any of these speci…cations and does so in a simple way which allows for straightforward Bayesian inference. The basic idea underlying our model is that it adds two simple concepts to a standard state space framework. These ideas are ordering and distance. By ordering the data in various ways, we can accommodate a wide variety of nonlinear time series models, including those with regime-switching and structural breaks. By allowing the state equation variances to depend on the distance between observations, the parameters can evolve in a wide variety of ways, allowing for everything from models exhibiting abrupt change (e.g. threshold autoregressive models or standard structural break models) to those which allow for a gradual evolution of parameters (e.g. smooth transition autoregressive models or time varying parameter models). We show how our model will (approximately) nest virtually every popular model in the regime-switching and structural break literatures. Bayesian econometric methods for inference in this model are developed. Because we stay within a state space framework, these methods are relatively straightforward, drawing on the existing literature. We use arti…cial data to show the advantages of our approach, before providing two empirical illustrations involving the modeling of real GDP growth

Cognitive mediators of the effect of peer victimization on loneliness

The impact of stress on psychological adjustment may be mediated by cognitive interpretations (i.e., appraisals) of events for individuals. Defining characteristics of loneliness suggest that appraisals of blame, threat, and perceived control may be particularly important in this domain. AIMS: To evaluate the extent to which cognitive appraisals (perceived control, threat, and blame) can mediate the effect of peer victimization on loneliness. SAMPLE: One hundred and ten children (54 boys, 56 girls) aged 8-12 years attending mainstream schools in Scotland. METHOD: Self-report measures of peer victimization, appraisal, and loneliness. RESULTS: Perceived control partially mediated the effects of peer victimization on loneliness, but neither blame nor threat were mediators. All three measures of control were significantly associated with loneliness at the bivariate level, but only perceived control was significant when the appraisals were entered as predictors in a hierarchical multiple linear regression. CONCLUSIONS: The results highlight the importance of research designs assessing multiple categories of appraisal. Furthermore, they suggest that intervention efforts aiming to combat feelings of loneliness within a peer victimization context should address children's appraisals of perceived control

E119D Neuraminidase Mutation Conferring Pan-Resistance to Neuraminidase Inhibitors in an A(H1N1)pdm09 Isolate From a Stem-Cell Transplant Recipient

Background. An influenza A(H1N1)pdm09 infection was diagnosed in a hematopoietic stem cell transplant recipient during conditioning regimen. He was treated with oral oseltamivir, later combined with intravenous zanamivir. The H275Y neuraminidase (NA) mutation was first detected, and an E119D NA mutation was identified during zanamivir therapy. Methods. Recombinant wild-type (WT) E119D and E119D/H275Y A(H1N1)pdm09 NA variants were generated by reverse genetics. Susceptibility to NA inhibitors (NAIs) was evaluated with a fluorometric assay using the 2′-(4-methylumbelliferyl)-α-d-N-acetylneuraminic acid (MUNANA) substrate. Susceptibility to favipiravir (T-705) was assessed using plaque reduction assays. The NA affinity and velocity values were determined with NA enzymatic studies. Results. We identified an influenza A(H1N1)pdm09 E119D mutant that exhibited a marked increase in the 50% inhibitory concentrations against all tested NAIs (827-, 25-, 286-, and 702-fold for zanamivir, oseltamivir, peramivir, and laninamivir, respectively). The double E119D/H275Y mutation further increased oseltamivir and peramivir 50% inhibitory concentrations by 790- and >5000-fold, respectively, compared with the WT. The mutant viruses remained susceptible to favipiravir. The NA affinity and velocity values of the E119D variant decreased by 8.1-fold and 4.5-fold, respectively, compared with the WT. Conclusions. The actual emergence of a single NA mutation conferring pan-NAI resistance in the clinical setting reinforces the pressing need to develop new anti-influenza strategie

Preservation of plant-wax biomarkers in deserts:implications for Quaternary environment and human evolutionary studies

Analysis of plant‐wax biomarkers from sedimentary sequences can enable past environmental and hydrological reconstruction and provide insights into past hominin adaptations. However, biomarker preservation in desert contexts has been considered unlikely given the sparse nature of the vegetation within the landscape. Here we evaluate the preservation of n‐alkanes and fatty acids collected from four depositional sequences associated with archaeological contexts in the Nefud Desert, Saudi Arabia, and the Thar Desert, India. Pleistocene and Holocene samples were selected to understand the effects of age on preservation. The results of molecular distribution patterns and indices, particularly the high carbon preference index and average chain length, show the preservation of plantwax biomarkers in both the Holocene and Pleistocene desert sequences, while δ13C values and organic content provide insights into the vegetation contributing to the plant‐wax organic pool. This study provides a baseline for understanding human–environment interactions and for reconstructing changes in arid land habitats of relevance to hominins during the Quaternary.peer-reviewe