Triggering and guiding of an upward positive leader from a ground rod with an ultrashort laser pulse. I. Experimental results

Using a plasma channel produced by an ultrashort laser pulse, we have studied the laser triggering and guiding of a positive leader from the tip of a 2-m vertical rod standing on the bottom plane of a 7-m plane-plane gap. The purpose of this setup was to reproduce in the laboratory the electric field conditions leading to the onset of a positive upward leader from a ground rod as a downward negative leader is approaching during a thunderstorm, in order to demonstrate the working principle of a possible future laser lightning rod. The leader triggering properties of the laser-created plasma channel have been studied as a function of the synchronization of the laser pulse with the voltage impulse applied to the gap. We show that the laser pulse reduces the inception voltage of the leader compared to its normal value and that the laser plasma channel guides the propagation of the upward leader at a velocity ten times higher than that of an ordinary leader, with a significantly lower charge per unit length. We show that laser guiding of the leader significantly reduces the breakdown voltage of the gap and that the effect of the laser channel at the end of a lightning rod can be compared quite favorably with the effect of an additional metal rod of the same length.     

Approche synthétique efficace pour l’assemblage d’α-méthylène-γ-butyrolactones diversement fonctionnalisées

A new synthetic approach for the elaboration of diversely functionalized α-methylene-γ-butyrolactones has been developed. The first step was the preliminary construction of the lactone ring system equipped with easily manageable functionalities. As the key step, the ultimate deprotonation/methoxymethylation/β-elimination sequence allowed installation of the exocyclic olefinic moiety and then permitted chemical transformation of the above-mentioned functionalities to complete the synthesis of the title compounds.     

Relative permeability relations: A key factor for a drying model

In the modelling of heat, mass and momentum transfer phenomena which occur in a capillary porous medium during drying, the liquid and gas flows are usually described by the generalised Darcy laws. Nevertheless, the question of how to determine experimentally the relative permeability relations remains unanswered for most materials that consist of water and humid air, and as a result, arbitrary functions are used in the drying codes. In this paper, the emphasis is on deducing from both numerical and experimental studies a method for estimating pertinent relations for these key parameters. In the first part, the sensitivity of liquid velocity and, consequently, of drying kinetics in the variation of the relative permeabilities is investigated numerically by testing various forms. It is concluded that in order to predict a realistic liquid velocity behaviour, relative permeabilities can be linked to a measurable quantity: the capillary pressure. An estimation technique, based on simulations coupled with experimental measurements of capillary pressure, together with moisture content kinetics obtained for low or middle temperature convective drying, is deduced. In the second part, the proposed methodology is applied to pine wood. It is shown that the obtained relations provide closer representation of physical reality than those commonly used.     

Spanners of additively weighted point sets

We study the problem of computing geometric spanners for (additively) weighted point sets. A weighted point set is a set of pairs (p,r) where p is a point in the plane and r is a real number. The distance between two points (pi,ri) and (pj,rj) is defined as |pipj|−ri−rj. We show that in the case where all ri are positive numbers and |pipj|?ri+rj for all i, j (in which case the points can be seen as non-intersecting disks in the plane), a variant of the Yao graph is a (1+?)-spanner that has a linear number of edges. We also show that the Additively Weighted Delaunay graph (the face-dual of the Additively Weighted Voronoi diagram) has a spanning ratio bounded by a constant. The straight-line embedding of the Additively Weighted Delaunay graph may not be a plane graph. Given the Additively Weighted Delaunay graph, we show how to compute a plane straight-line embedding that also has a spanning ratio bounded by a constant in time.     

Nouvelle méthode de synthèse de thiazolopyridines

It has been established that the direct condensation between aromatic and aliphatic thioesters and the variously generated anion of amino-chloropyridines represents the best method for the synthesis of thiazolo-pyridines. The transient thioamides, sometimes isolated, can be easily converted chemically or photochemically into the desired fused heterocycles.     

Heavy fermion contributions to the anomalous magnetic dipole and quadrupole moments of theW-boson

The contribution of a massivet-quark to the anomalous magnetic moment (Δκ) and the quadrupole moment (ΔQ) of theW-boson is calculated. An upper bound of Δκ=1.5×10^?2 and ΔQ=2.5×10^?3 for the standard model is obtained. Additional contributions from extraE ₆ fermions is also discussed.     

Orbits and Lattices for Linear Random Number Generators with Composite Moduli

In order to analyze certain types of combinations of multiple recursive linear congruential generators (MRGs), we introduce a generalized spectral test. We show how to apply the test in large dimensions by a recursive procedure based on the fact that such combinations are subgenerators of other MRGs with composite moduli. We illustrate this with the well-known RANMAR generator. We also design an algorithm generalizing the procedure to arbitrary random number generators.

     

Parametric analysis of the energy levels of Nd3+ in frozen dilute aqueous solutions of neodymium chloride

A parametric analysis of the Stark-split spectra of Nd³⁺ in frozen dilute aqueous solutions of neodymium chloride has been carried out.The similarity of the spectra of Nd³⁺ in frozen dilute solutions and in crystals, where the ion is nine-fold coordinated and of approximate D_3h symmetry (Nd³⁺: LaCl₃, neodymium ethylsulfate and bromate), was used to obtain initial parameter values. Simultaneous diagonalization of the free ion and crystal field hamiltonians resulted in a r.m.s. deviation σ of 11 cm^?1 for 50 levels. The resulting crystal field parameters are B²₀ = 278 ± 19, B⁴₀ = ?201 ± 39, B^b₀ = ?946 ± 55 and B⁶₆ = 663 ± 43 cm^?1. Selection rules for D_3h are approximately obeyed. The small value of σ, the number of levels accounted for and the fact that the final parameters are not very different from those in the crystals indicate a nine-fold coordination of approximate D_3h symmetry for Nd³⁺ in frozen aqueous solutions. The spectra of liquid solutions show by their similarity to the spectra of frozen solutions that the same surroundings exist in the liquid.