Fundamental processes in long air gap discharges

The development of atmospheric lightning is initiated and sustained by the formation in virgin air of ‘streamer corona’ and ‘leader’ discharges, very similar to those observed in laboratory long sparks. Therefore, the experimental and theoretical investigations of these laboratory discharges have become of large interest to improve the physical knowledge of the lightning process and to develop self-consistent models that could be applied to new protection concepts.In the present paper the fundamental processes of the subsequent phases of long air gap discharges are analyzed, from the first corona inception and development to the leader channel formation and propagation. For all these processes simulations models are discussed that have been essentially derived and simplified by the authors, in order to develop sequential time-dependent simulation of the laboratory breakdown, with both positive and negative voltages. The possibility of extending these models to the case of natural lightning is discussed in the companion paper, presented in this same volume. To cite this article: I. Gallimberti et al., C. R. Physique 3 (2002) 1335–1359.     

基于分形理论的飞机雷击初始附着点的数值模拟

飞机雷击附着点的确定能够为飞机防雷设计提供依据,是飞机雷击区域划分和飞机各部件进行雷电试验鉴定的先决条件.本文提出了一种基于分形理论的飞机雷击初始附着点数值模拟的新方法.该方法首先依据标准SAE-ARP5416中有关飞机雷击附着点试验的规定,确定放电间隙、雷电起始坐标、飞机姿态和放电次数等参数,然后根据分形理论,使用电介质击穿模型模拟符合自然界雷电物理机理和几何特征的雷电先导分形发展过程,同时考虑飞机自身触发双向先导的情况,最终得到飞机的雷击附着点分布.通过本文方法仿真模拟得到飞机F-4雷击附着点的分布概率,并分别与该飞机飞行实验和实验室高压放电实验测试得到的真实雷击附着点的概率分布情况比较,结果基本吻合,验证了该方法的有效性.研究结果为飞机雷击附着点仿真模拟提供了一个有潜力的方法,可作为飞机防雷设计和今后开展相关研究工作的基础.     

Physical processes during development of lightning flashes

The objective of this paper is to review our present understanding of the physical processes in lightning flashes during their development within or outside a cloud, following lightning initiation. This represents the ‘big picture’ of lightning development, in the scale of the cloud dimensions themselves. Since the acceptance of the bi-directional, zero-net-charge leader concept, significant changes have occurred in our understanding of the key physical processes of which a lightning flash is comprised, and in the analytical relationship between the electrical structure of a cloud and lightning parameters. These changes are discussed with an emphasis on the unifying nature of the bi-directional leader concept. To cite this article: V. Mazur, C. R. Physique 3 (2002) 1393–1409.     

Observations and modeling of lightning leaders

The development of atmospheric lightning is initiated by a ‘leader’ phase during which ionized channels appear in virgin air. The use of rapid cameras, the measure of fields and currents associated with the discharge allow one to compare the propagation of laboratory leaders with those of natural or artificially triggered lightning. The corresponding physical processes can be analyzed with the help of models developed for laboratory leaders provided that the non linear effects due to the intense current circulation leading to lightning leader thermalization are taken into account. A self-coherent simulation of triggered lightning leaders for both polarities is presented is this paper. Furthermore, these models make it possible to define the ‘stabilization field’ concept, equal to the minimum ambient field allowing the stable progress of a leader from a ground structure, expressed as a height and curvature function of this structure. This concept can be validated through triggered lightning tests. Finally, the stabilization field analysis is completed by a simplified analytical model based upon an electrostatic approach of propagation equilibrium. To cite this article: P. Lalande et al., C. R. Physique 3 (2002) 1375–1392.     

Some microphysical and electrical aspects of a Cloud Resolving Model: description and thunderstorm case study

An electrification scheme, consistent with the mixed-phase microphysical parameterization, has been developed for the French cloud resolving model MésoNH. There are four successive steps: (i) charge separation is assumed to result only from non-inductive processes; (ii) electrical charges carried by the different hydrometeor species are transported along the air flow and redistributed according to the microphysical processes; (iii) the electric field is deduced from the integration of a modified Poisson equation; (iv) a lightning parameterization simulates triggering, propagation and pseudo-fractal branching of the flashes and associated charge neutralization. Two numerical experiments are conducted firstly to evaluate the performances of the lightning scheme, secondly to test the simulated evolution of the electrical characteristics of a idealized supercellular storm. To cite this article: G. Molinié et al., C. R. Physique 3 (2002) 1305–1324.     

The physical origin of the land–ocean contrast in lightning activity

New tests and older ideas are explored to understand the origin of the pronounced contrast in lightning between land and sea. The behavior of islands as miniature continents with variable area supports the traditional thermal hypothesis over the aerosol hypothesis for lightning control. The substantial land–ocean contrast in updraft strength is supported globally by TRMM (Tropical Rainfall Measuring Mission) radar comparisons of mixed phase radar reflectivity. The land–ocean updraft contrast is grossly inconsistent with the land–ocean contrast in CAPE (Convective Available Potential Energy), from the standpoint of parcel theory. This inconsistency is resolved by the scaling of buoyant parcel size with cloud base height, as suggested by earlier investigators. Strongly electrified continental convection is then favored by a larger surface Bowen ratio, and by larger, more strongly buoyant boundary layer parcels which more efficiently transform CAPE to kinetic energy of the updraft in the moist stage of conditional instability. To cite this article: E. Williams, S. Stanfill, C. R. Physique 3 (2002) 1277–1292.     

A lightning initiation mechanism: application to a thunderstorm electrification model

The use of numerical models has greatly increased our understanding of the electrical and microphysical process within electrified clouds. We use the University of Washington, 1.5-dimensional thunderstorm model to examine the effects of including a runaway electron based lightning initiation mechanism. We find that this mechanism can significantly alter the electrification history of modeled storms and produce vertical electric field profiles that are very similar to those of observed storms. To cite this article: R. Solomon et al., C. R. Physique 3 (2002) 1325–1333.     

The control of lightning using lasers: properties of streamers and leaders in the presence of laser-produced ionization

This paper summarizes the research this team has performed over the past few years investigating laboratory electrical breakdown discharges in the presence of a plasma cylinder created by a single ultrashort laser pulse. This work is part of a feasibility study about the control of lightning using laser systems. Our experimental investigations have included discharges (i) in modest (30 cm) air gaps mediated by streamers, and (ii) in large (several meters) ambient air gaps for which the discharge took place through the formation of a leader, the mechanism relevant to large scale natural discharges such as lightning. In order to understand the observations, various physical models have been used, the main results of which are discussed in this paper. To cite this article: F. Vidal et al., C. R. Physique 3 (2002) 1361–1374.     

Transparent Boundary Conditions for Split-Step Padé Approximations of the One-Way Helmholtz Equation

In this paper, we generalize the nonlocal discrete transparent boundary condition introduced by F. Schmidt and P. Deuflhard (1995, Comput. Math. Appl.29, 53–76) and by F. Schmidt and D. Yevick (1997, J. Comput. Phys.134, 96–107) to propagation methods based on arbitrary Padé approximations of the two-dimensional one-way Helmholtz equation. Our approach leads to a recursive formula for the coefficients appearing in the nonlocal condition, which then yields an unconditionally stable propagation method.     

X-rays and matter – the basic interactions

In this introductory article we attempt to provide the theoretical basis for developing the interaction between X-rays and matter, so that one can unravel properties of matter by interpretation of X-ray experiments on samples. We emphasize that we are dealing with the basics, which means that we shall limit ourselves to a discussion of the interaction of an X-ray photon with an isolated atom, or rather with a single electron in a Hartree–Fock atom. Subsequent articles in this issue deal with more complicated – and interesting – forms of matter encompassing many atoms or molecules. To cite this article: J. Als-Nielsen, C. R. Physique 9 (2008).     

Nucleation problems in metallurgy of the solid state: recent developments and open questions

Nucleation processes play a key role in the microstructure evolution of metallic alloys during thermomechanical treatments. These processes can involve phase transformations (such as precipitation) and structural instabilities (such as recrystallisation). Although the word ‘nucleation’ is used in both cases, the situation is profoundly different for precipitation and for recrystallisation on which this article is focussed. In the case of precipitation, species are conserved and the underlying physics is stochastic fluctuations, allowing the apparition of critical germs of the new phase. In the case of recrystallisation, the underlying physical phenomenon is the progressive growth of subgrain structures leading to an unstable configuration, allowing a dislocation free grain to grow at the expense of a dislocated one. The two cases require different types of modelling which are presented in the article. To cite this article: Y. Bréchet, G. Martin, C. R. Physique 7 (2006).     

HgCdTe technology in France

SOFRADIR is one of the leading companies worldwide for the production of second generation InfraRed (IR) detectors. This success is due to the top level quality of the unique and production oriented French HgCdTe technology for manufacturing IR focal plane arrays based on an HgCdTe array and a CMOS readout and multiplexed silicon array. This technology and main products are presented in this paper. Finally, in order to prepare for future military and industrial needs, SOFRADIR has been working in close relationship with CEA-LETI/LIR on third generation developments based on HgCdTe material using Molecular Beam Epitaxy (MBE) growth. To cite this article: P. Tribolet, C. R. Physique 4 (2003).     

De la microstructure du polymère à ses propriétés rhéologiquesFrom the microstructure of polymers towards their rheological properties

We extend the previous work by Benallal et al. on the relationship between structure and rheological properties of linear polymer melts. The aim of this paper is to quantify the effect of the chemical structure on the viscoelastic properties. We show that these properties are governed by the monomer dimensions and the interaction energy. To cite this article: A. Allal et al., C. R. Physique 3 (2002) 1451–1458.     

Caractérisation et modélisation de composants matriciels infrarougeCharacterization and modelling of IR sensors

The objective of this paper is to describe the first behavioural models of cooled (based on HgCdTe photodetectors) infrared sensors which were designed at CEA-LETI/SLIR. In this way, the interest of such an approach in the evaluation and improvement of optronic systems will be shown. The paper first presents the modelling approach (architecture of the models, choice of parameters, tools for modelling and calibration, …). Then models are compared to measurements on real components in order to verify the efficiency of the modelling approach. To cite this article: P. Castelein, C. R. Physique 4 (2003).     

D-branes from matrix factorizations

B-type D-branes can be obtained from matrix factorizations of the Landau–Ginzburg superpotential. We here review this promising approach to learning about the spacetime superpotential of Calabi–Yau compactifications. We discuss the grading of the D-branes, and present applications in two examples: the two-dimensional torus, and the quintic. To cite this article: K. Hori, J. Walcher, C. R. Physique 5 (2004).

Résumé

Les D-branes de type B peuvent être décrites à partir de factorisations matricielles du super-potentiel de Landau–Ginzburg. On revoit ici cette approche prometteuse pour étudier le super-potentiel en espace-temps de compactifications de Calabi–Yau. On discute la graduation des D-branes, et présente deux exemples : le tore en deux dimensions, ainsi que la quintique. Pour citer cet article : K. Hori, J. Walcher, C. R. Physique 5 (2004).     

Prospective sur les besoins « défense » en détecteurs infrarougeMid-term defence needs for infrared detectors

In this paper, we propose to present the prospects for mid-term needs for infrared detectors. These needs are derived from expected evolutions in imaging techniques as well as from operational requirements. The main trends that shall allow the direct development in infrared detection are as much the pursuit of greater range, a better discrimination of targets, as the efforts to minimize cost, volume, weigh and consumption. These trends will lead to an examination of the specific needs for some kind of ‘smart’ infrared detector. Among these applications, we will investigate more deeply the technological requirements for flash and 3D imaging, hyperspectral and uncooled imaging. To cite this article: J.-C. Peyrard, C. R. Physique 4 (2003).     

Introduction to the physics of nucleation

In this introductory article, I review the theory of nucleation by thermal activation and by quantum tunneling. The effect of heterogeneous nucleation at surfaces is discussed and a brief survey of experimental techniques is given. To cite this article: H.J. Maris, C. R. Physique 7 (2006).     

The Three Dimensional Non-conforming Finite Element Solution of the Chapman–Ferraro Problem

We demonstrate the feasibility of using a non-conforming, piecewise harmonic finite element method on an unstructured grid in solving a magnetospheric physics problem. We use this approach to construct a global discrete model of the magnetic field of the magnetosphere that includes the effects of shielding currents at the outer boundary (the magnetopause). As in the approach of F. R. Toffolettoet al.(1994,Geophys. Res. Lett.21, 7) the internal magnetospheric field model is that of R. V. Hilmer and G.-H. Voigt (1995,J. Geophys. Res.) while the magnetopause shape is based on an empirically determined approximation (1997, J. Shueet al.,J. Geophys. Res.102, 9497). The results is a magnetic field model whose field lines are completely confined within the magnetosphere. The presented numerical results indicate that the discrete non-conforming finite element model is well-suited for magnetospheric field modeling.     

The Fe–Cr system: atomistic modelling of thermodynamics and kinetics of phase transformations

To understand the behaviour of irradiated defects and kinetic pathways of micro-structural evolution in Fe–Cr alloys, we use a combination of density functional theory with statistical approaches involving cluster expansions and Monte Carlo simulations. A lowest negative mixing enthalpy is found at 6.25% Cr that is consistent with our DFT prediction of an ordered Fe₁₅Cr structure. At 50% Cr, it is found that the predicted enthalpy of formation is 4 times smaller than that calculated by the CPA approach. Thermodynamic and precipitation properties are then discussed in term of segregation between the Fe₁₅Cr and α^′-Cr phases and of vacancy-mediated kMC simulation. To cite this article: D. Nguyen-Manh et al., C. R. Physique 9 (2008).