11,035 research outputs found
Palaeogeography and palaeoecology of early Floian (Early Ordovician) cephalopods from the Upper Fezouata Formation, Anti-Atlas, Morocco
High-quality ion beams by irradiating a nano-structured target with a petawatt laser pulse
We present a novel laser based ion acceleration scheme, where a petawatt
circularly polarized laser pulse is shot on an ultra-thin (nano-scale)
double-layer target. Our scheme allows the production of high-quality light ion
beams with both energy and angular dispersion controllable by the target
properties. We show that extraction of all electrons from the target by
radiation pressure can lead to a very effective two step acceleration process
for light ions if the target is designed correctly. Relativistic protons should
be obtainable with pulse powers of a few petawatt. Careful analytical modeling
yields estimates for characteristic beam parameters and requirements on the
laser pulse quality, in excellent agreement with one and two-dimensional
Particle-in Cell simulations.Comment: 18 pages, 7 figures, accepted in New. J. Phy
An improved reconstruction procedure for the correction of local magnification effects in three-dimensional atom-probe
A new 3DAP reconstruction procedure is proposed that accounts for the
evaporation field of a secondary phase. It applies the existing cluster
selection software to identify the atoms of the second phase and, subsequently,
an iterative algorithm to homogenise the volume laterally. This Procedure,
easily implementable on existing reconstruction software, has been applied
successfully on simulated and real 3DAP analyses
Influence de la préoxydation sur la coagulation par le chlorure ferrique de la matiÚre organique
La coagulation par le Fe(III) de substances humiques aquatiques est optimale Ă pH = 5,5 pour un rapport massique Fe/COT de l'ordre de 2, quant Ă l'Ă©limination du COT. Le but de cet article est d'Ă©tudier l'influence d'une prĂ©oxydation sur l'Ă©limination de la matiĂšre organique par coagulation dans ces conditions.La prĂ©oxydation (ozone, bioxyde de chlore ou chlore) Ă faibles doses induit une lĂ©gĂšre dĂ©gradation des rendements de coagulation des acides fulviques. Un taux de prĂ©oxydant fort (â 0,5 mg oxydant par mg de carbone organique initial), pratiquĂ© avant la coagulation, implique de mettre en oeuvre une dose de coagulant plus Ă©levĂ©e [â 3 mg Fe(III) par mg de COT]. Cette augmentation est moins marquĂ©e dans le cas d'une prĂ©chloration.Les substances humiques ne reprĂ©sentant qu'une partie de la matiĂšre organique des eaux naturelles, des manipulations sur des eaux brutes ont donc Ă©tĂ© entreprises. L'effet de faibles doses d'ozone conduit Ă une lĂ©gĂšre amĂ©lioration de la coagulation. Cependant de fortes doses d'oxydant (0,5 mg d'ozone ou de bioxyde de chlore par mg COT) nĂ©cessitent l'emploi de plus fortes doses de coagulant. De plus, le rendement d'Ă©limination est aussi affectĂ©.We present here some works which take place in the particular frame of the study of the drinking water treatment of reservoir waters containing organics at high concentration, mainly humic substances. Previous studies (LEFEBVRE and LEGUBE, 1990) on coagulation of fulvic acid solutions have proven that the optimal removal of organic matter was reached at pH = 5.5 with 2 mg of ferric iron per mg of organic carbon. The main question in this study is to know the impact of preoxidation (ozone, chlorine dioxide, chlorine) on the removal of organics by iron(III) coagulation.The reconstituted waters obtained by dissolving fulvic acids (table 1) in a solution of salts in high-purity water (table 2), are defined by both a low inorganic content (as calcium, sulfate and bicarbonate ions) and a high organics concentration, main characteristics of real impounded waters (table 3). The experimental procedures of coagulation-flocculation-clarification followed the treatment lines shown in figure 1. Ozonation was carried out in a semi-continuous system (batch solution), by bubbling ozone (fig. 2). Chlorination and preoxidation with chlorine dioxide were performed in a batch reactor. The preoxidation steps were carried out at pH of reconstituted waters or of raw waters. A Dohrmann DC 80 was used for the determination of TOC. The accuracy of TOC measurement was found to be ± 0.05 mg/l C (for the range of studied concentration). Residual iron was analysed by atomic absorption on a Perkin Elmer 2380 apparatus (oxy-acetylene flame).As shown in table 4, preozonation induced a slight decrease of Cebron fulvic acid removal by iron(III) coagulation at high ozone dose applied (= 0.5 mg O3 per mg TOCi). In order to try to understand why ozone partially inhibits the efficiency of iron coagulation to remove fulvic acid at acidic pH, we determined the optimum dosage of coagulant required to obtain the best percent of removal on preozonated fulvic acid solutions. Figure 4 shows that preozonation at about 0.5 mg O3 per mg TOCi appears to have shifted the region of the optimal TOC removal towards the higher Fe/TOCi mass ratios (â 3 mg Fe(III) per mg TOCi, for two fulvic acids).Prechlorination (> 1 mg Cl2/mg TOCi) induced a decrease of Cebron fulvic acid removal by iron(III) coagulation (table 5). Hence, we examined the optimum dosage of coagulant required to obtain the best percent of removal on prechlorinated fulvic acid solutions (fig. 6). The region of the optimal TOC removal was still obtained for a Fe/TOCi mass ratio of 2. However, the difference between the percent TOC remaining for inital fulvic acid of 10 and 15 mg/l was not significant (respectively 61.4 % and 61.9 %). A greater prechlorinatlon dosage (> 0.5 mg Cl2/mg TOCi) would probably give a shift of the optimal mass ratio.Preoxidation with chlorine dioxide (even without ClO2 residual) before a coagulation induced a decrease at TOC (table 4). So, we studied the shift of optimum dosage of coagulant required to remove the organic matter. Figure 5 shows that preoxidation (â 0.5 mg ClO2/mg TOCi) shifted the region of optimal TOC removal towards higher Fe/TOCi mass ratio (â 3 mg per mg for Cebron fulvic acid, in figure 5).The fulvic acids represent only an amount of TOC of a raw waters. So we carried out experiments on real impounded waters. For the study of the influence of the preozonation on the coagulation efficiency, we chose to work at the optimal coagulant dose determined from previous experiments. Several runs were conducted with different dosages of ozone between 0 and 2 mg/l O3. Results reported in tables 6 and 7 indicate that for these two raw waters (Moulin Papon and Cebron), the preozonation slightly improved the efficiency of iron coagulation at acidic pH. These results can only be compared with Cebron fulvic acid when applied ozone dose was 0.2 mg O3/mg TOCi.Consequently, other experiments were carried out at a high ozone dose (- 0.5 mg O3/mg TOCi). They showed a shift in the region of the optimal TOC removal towards the higher coagulant dose (fig. 7), as already observed with the fulvic acid solutions.Only one experiment was made with chlorine dioxide in the case of Moulin Papou raw water. This preoxidation (â 0.5 mg ClO2/mg TOCi) induced a shift to higher coagulant dosage but also a decrease of the efficiency of iron coagulation whatever the applied coagulant dosage (fig. 8).As compared to humic chlorination literature, little information exists concerning ozonation of humics. Furthermore, in spite of small amounts of identified by-products in the literature, ozonation was usually found to increase smaller size materials in humic substances (GLOOR et al., 1981; VEENSTRA et al., 1983; FLOGSTAD and ODEGAARD, 1985; ANDERSON et al., 1986; AMY et al., 1987; LEGUBE et al., 1989). Moreover, analyses of carboxyl-group on the Cebron fuivic acid allowed us to show that the carboxyl content increased as ozone dosage increased (fig. 9).Some works (VAN BREEMEN et al., 1979) have proven the rate of carboxyl groups in the stoichiometry of coagulation reaction of humics with iron(III) at slightly acidic pH. Consequently, it is not surprising that preozonation of fulvic acids appears to have shifted the region of TOC removal up into the higher iron dose range, according to the enhancement of the carboxyl content in the fulvic acids by ozone. This effect of preozonation was already reported by others (JEKEL, 1983; RECKHOW and SINGER, 1983).Chlorination (GLAZE and PEYTON, 1978) or oxidation with chlorine dioxide (NORWOOD et al., 1983; COLCLOUGH Of al., 1983) led also to a decrease of the molecular weight of aquatic organic matter. The oxidative action of chlorine and chlorine dioxide gave many products like aromatic and aliphatic acids (chlorinated or not chlorinated) and probably increased the carboxyl content of humic substances. Hence, those oxidants inhibit the coagulation-flocculation of organic matter
Spin-Exchange Interaction in ZnO-based Quantum Wells
Wurtzitic ZnO/(Zn,Mg)O quantum wells grown along the (0001) direction permit
unprecedented tunability of the short-range spin exchange interaction. In the
context of large exciton binding energies and electron-hole exchange
interaction in ZnO, this tunability results from the competition between
quantum confinement and giant quantum confined Stark effect. By using
time-resolved photoluminescence we identify, for well widths under 3 nm, the
redistribution of oscillator strengths between the A and B excitonic
transitions, due to the enhancement of the exchange interaction. Conversely,
for wider wells, the redistribution is cancelled by the dominant effect of
internal electric fields, which dramatically reduce the exchange energy.Comment: 14 pages, 3 figure
Early out-of-equilibrium beam-plasma evolution
We solve analytically the out-of-equilibrium initial stage that follows the
injection of a radially finite electron beam into a plasma at rest and test it
against particle-in-cell simulations. For initial large beam edge gradients and
not too large beam radius, compared to the electron skin depth, the electron
beam is shown to evolve into a ring structure. For low enough transverse
temperatures, the filamentation instability eventually proceeds and saturates
when transverse isotropy is reached. The analysis accounts for the variety of
very recent experimental beam transverse observations.Comment: to appear in Phys. Rev. Letter
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