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.     

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.     

Atmospheric ultraviolet and red-infrared flashes from Universitetsky-Tatiana-2 satellite data

Millisecond ultraviolet (240–400 nm) and red-infrared (610–800 nm) flashes were detected in the nighttime atmosphere with the scientific payload installed onboard the Universitetsky-Tatiana-2 micro-satellite. Flashes with various numbers of photons, from 10²⁰ to 10²⁶, were detected within the atmospheric area 300 km in diameter observed by the detector. The flashes differ in duration and temporal profile: from single short flashes ～1 ms in duration to flashes with a complex profile more than 100 ms in duration. Different global geographic distributions are observed for flashes with different numbers of photons. Flashes with fewer than 10²² photons are distributed uniformly over the Earth’s map. Flashes with more than 10²² photons are concentrated near the equator and above the continents. Series of flashes were observed in one turn of the satellite when flying not only over thunderstorm regions but also over cloudless ones. The flash number distribution has been derived from the ratio of the numbers of red-infrared and ultraviolet photons. As applied to discharges in the upper atmosphere, whose glow is dominated by the emission in the first and second positive systems of molecular nitrogen bands (1PN₂ and 2PN₂), this distribution is equivalent to the flash altitude distribution in the atmosphere. The observed ratio of the numbers of photons in red-infrared and ultraviolet flashes agrees with the calculated one for electric discharges at altitudes higher than 50 km. In-orbit measurements of the charged particle flux (with a threshold energy for electrons of 1 MeV) provide no evidence for a synchronous occurrence of an ultraviolet flash and a burst in the particle flux in the orbit.     

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.     

Spatial characteristics of radiation from lightning discharges

The results of measurements of levels of electric and magnetic radiation fields from lightning discharges are presented. Lightning discharges occurred in the region of the terrestrial surface bounded by a sector of ∼90° with a radius of ∼30–300 km. A power spatial dependence of the electromagnetic radiation field components of lightning discharges is established. The results of measurements are interpreted using fractal geometry methods.     

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.     

Estimation of charge neutralized by negative cloud-to-ground flashes in Catalonia thunderstorms

Charges neutralized by lightning flashes have been usually located and inferred from the quasi-static changes in the vertical component of electric field (ΔE) obtained from multiple synchronized measurement stations. In this paper, the charges neutralized by negative cloud-to-ground (?CG) lightning flashes are located and inferred using single station electric field measurements in combination with total lightning (cloud-to-ground and intra-cloud) data from lightning detection networks in the north-eastern region of Spain. The altitude of the negative charge region in 8 thunderstorms examined here was in the range of 5.8–7.2 km as inferred from temperature soundings. Charge locations are assumed to be in the region where the VHF (Very High Frequency) sources associated with the discharge are located. The quantity of charge neutralized is calculated using a point charge model and the measured vertical component of electric field (ΔE) associated with each flash. The results are compared with those previously obtained from measurements carried out in the NASA Kennedy Space Center (KSC) in Florida. The analysis of 260 ?CG flashes from eight thunderstorms reveals a median charge value of ?9.7 C with 95% of the values ranging between ?4.5 C and ?45 C.     

Stretched exponential relaxation and dispersive conductivity behavior in lithium bismuth silicate glasses

Glasses having compositions xLi₂O∙(85 − x)Bi₂O₃∙15SiO₂ (x = 35, 40, and 45 mol%) were prepared by normal melt quenching technique. Electrical relaxation and conductivity in these glasses were studied using impedance spectroscopy in the frequency range from 20 Hz to 1 MHz and in the temperature range from 453 to 603 K. The ac and dc conductivities, activation energy of the dc conductivity and relaxation frequency were extracted from the impedance spectra. The dc conductivity increases with increase in Li₂O content providing modified glass structure and large number of mobile lithium ions. Similar values of activation energy for dc conduction and for conductivity relaxation time indicate that the ions overcome the same energy barrier while conducting and relaxing. The non-exponential character of relaxation processes increases with decrease in stretched exponential parameter ‘β’ as the composition parameter ‘x’ increases. The observed conductivity spectra follow a power law with exponent ‘s’ which increases regularly with frequency and approaches unity at higher frequencies. Nearly constant losses (NCL) characterize this linearly dependent region of the conductivity spectra. A deviation from the ‘master curve’ for various isotherms of conductivity spectra was also observed in the high-frequency region and at low temperatures, which supports the existence of different dynamic processes like NCL in addition to the ion hopping processes in the investigated glass system.     

QWIP detectors and thermal imagers

Standard GaAs/AlGaAs QWIPs (Quantum Well Infrared Photodetector) are now well established for long wave infrared (LWIR) detection. The main advantage of this technology is the duality with the technology of commercial GaAs devices. The realization of large FPAs (up to 640×480) drawing on the standard III–V technological process has already been demonstrated. The second advantage widely claimed for QWIPs is the so-called band-gap engineering, allowing the custom design of the quantum structure to fulfill the requirements of specific applications such as multispectral detection. QWIP technology has been growing up over the last ten years and now reaches an undeniable level of maturity. As with all quantum detectors, the thermal current, particularly in the LWIR range, limits the operating temperature of QWIPs. It is very crucial to achieve an operating temperature as high as possible and at least above 77 K in order to reduce volume and power consumption and to improve the reliability of the detection module. This thermal current offset has three detrimental effects: noise increase, storage capacitor saturation and high sensitivity of FPAs to fluctuations in operating temperature. For LWIR FPAs, large cryocoolers are required, which means volume and power consumption unsuitable for handheld systems. The understanding of detection mechanisms has led us to design and realize high performance ‘standard’ QWIPs working near 77 K. Furthermore, a new in situ skimmed architecture accommodating this offset has already been demonstrated. In this paper we summarize the contribution of THALES Research & Technology to this progress. We present the current status of QWIPs in France, including the latest performances achieved with both standard and skimmed architectures. We illustrate the potential of our QWIPs through features of Thales Optronique's products for third thermal imager generation. To cite this article: E. Costard et al., C. R. Physique 4 (2003).     

The interaction between a lightning flash and an aircraft in flight

Roughly speaking, every commercial airliner is struck by lightning once per year. Thus, the lightning strike to aircraft is not uncommon and it poses an appreciable threat to flight safety. The understanding of the lightning strike to aircraft has been greatly enhanced during the last years thanks to a comprehensive analysis of data collected from instrumented aircraft that have been flown into thunderstorm regions. In this article, we will start with the phenomenology of the lightning strike to aircraft and continue with going deeper into the underlying physics of selected processes during the strike. To cite this article: A. Larsson, C. R. Physique 3 (2002) 1423–1444.     

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.     

Electron band structure detection by high-order optical harmonic generation in solids

As the threshold field strength for the breakdown in air significantly exceeds the maximum measured thundercloud strength 3 kV/cm/atm, the problem of lightning initiation remains unclear. According to the popular idea, lightning can be initiated from streamer discharges developed in the enhanced electric field in a vicinity of hydrometeors. To test the idea, we carry out numerical simulations of positive streamer development around charged water drops at air pressure typical at thundercloud altitudes and at different background fields, drop sizes and charges. With real drop sizes and charges, the electric field required for the streamer formation is stronger than the measured fields; therefore, second mechanism is required to amplify the local field.     

Gamma-ray emission from thunderstorm discharges

Fine features of gamma-ray radiation registered during a thunderstorm at Tien-Shan Mountain Cosmic Ray Station are presented. Long duration (100-600 ms) gamma-ray bursts are found. They are for the first time identified with atmospheric discharges (lighting). Gamma-ray emission lasts all the time of the discharge and is extremely non-uniform consisting of numerous flashes. Its peak intensity in the flashes exceeds the gamma-ray background up to two orders of magnitude. Exclusively strong altitude dependence of gamma radiation is found. The observation of gamma radiation at the height 4-8 km could serve as a new important method of atmospheric discharge processes investigation.     

Diamond particles synthesized with graphite spark method in two seconds

A wide bandgap semiconductor, diamond, has recently emerged as important and promising materials for a wide field of optoelectronic and electronic applications. With graphite spark method in hydrogenic atmosphere, we successfully synthesized diamond particles in 3–5 μm diameter in only ten seconds and in 1–2 μm diameter in two seconds. The resultant particles were observed with SEM (Scanning Electron Microscope) images, and confirmed as diamond by sharp peak on 1331 cm⁻¹ with Raman spectrometer.With this study, we searched for precursors with various experimental conditions, such as hydrogen pressures and/or graphite temperatures. For gaseous species identification, OES (Optical Emission Spectroscopy) results will be reported on this presentation, and the preliminary synthesis mechanism for ‘spark method’ will be presented.     

Oxide ion conductivity and relaxation in CaREZrNbO7 (RE = La, Nd, Sm, Gd, and Y) system

CaREZrNbO₇ (RE = La, Nd, Sm, Gd and Y) system changed from fluorite (F)-type to pyrochlore (P)-type structure when the ionic radius ratios, r(Ca²⁺–RE³⁺)_av/r(Zr⁴⁺–Nb⁵⁺)_av were larger than 1.34. Thus, the La, Nd, and Sm compounds have a cubic P-type structure and the Gd and Y ones have a defect F-type structure. The electrical conductivity was measured using complex-plane impedance analysis over a wide temperature (300–750 °C) and frequency (1 Hz–1 MHz) ranges. The conductivity relaxation phenomenon was observed in these compounds and the relaxation frequencies were found to show Arrhenius-type behavior and activation energies were in good agreement with those obtained from high temperature conductivity plots. These results support the idea that the relaxation process and the conductivity have the same origin. The ionic conductivity of CaREZrNbO₇ (RE = La, Nd, Sm, Gd and Y) system showed the maximum at the phase boundary between the F-type and P-type phases. On the other hand, the activation energy for the conduction decreased in the F-type phase and increased in the P-type phase with increasing ionic radius ratio. Among the prepared compounds, CaGdZrNbO₇ showed the highest ionic conductivity of 9.47 × 10^− 3 S/cm at 750 °C which was about twice as high as that observed in Gd₂Zr₂O₇ (4.2 × 10^− 3 S/cm at 800 °C). The grain morphology observation by scanning electron microscope (SEM) showed well-sintered grains. AC impedance measurements in various atmospheres further indicated that they are predominantly oxide ion conductors at elevated temperatures (> 700 °C).     

Lightning hazard due to overvoltages resulting from flashes differing in polarity and strokes

The paper deals with a calculation of risk, which may appear within a structure due to lightning discharges. An attempt to involve into the calculations not only lightning flashes but also their strokes has been undertaken. A discrepancy between the results achieved for flashes and for strokes has been assessed and discussed. On the basis of such considerations practical conclusions have been formulated.     

Radiation effects in concentrated alloys and compounds: equilibrium and kinetics of driven systems

What organization of condensed matter does resist irradiation, as a function of irradiation conditions? How to characterize the latter? We survey the advances in the field during the past three decades, when irradiation effects reduce to nuclear collisions. While in simple cases (structure defined by a scalar order parameter) one may define a stochastic potential, which yields the stationary states of the compounds under irradiation and their respective stability, in more general cases, we are left with brute force atomistic simulations to explore materials' behaviour as a function of irradiation conditions. Special attention is given to the kinetics of concentration fields under irradiation, a question with several practical implications. We conclude that irradiation conditions are best defined by three parameters: the cascade features (number of displacements and replacements, length of replacement sequences, …), the frequency of cascade occurrence, and the cumulated dose. We suggest cascade features be named ‘(elementary) dose’ and the cascade occurrence frequency ‘dose rate’. To cite this article: G. Martin, P. Bellon, C. R. Physique 9 (2008).     

Energy and momentum relaxation of electrons in bulk and 2D GaN

We present our experimental and theoretical studies regarding the energy and momentum relaxation of hot electrons in n-type bulk GaN and AlGaN/GaN HEMT structures. We determine the non-equilibrium temperatures and the energy relaxation rates in the steady state using the mobility mapping technique together with the power balance conditions as described by us elsewhere [N. Balkan, M.C. Arikan, S. Gokden, V. Tilak, B. Schaff, R.J. Shealy, J. Phys.: Condens. Matter 14 (2002) 3457]. We obtain the e–LO phonon scattering time of 8 fs and show that the power loss of electrons due to optical phonon emission agrees with the theoretical prediction. The drift velocity–field curves at high electric fields indicate that the drift velocity saturates at approximately 3×10⁶ cm/s for the two-dimensional structure and 4×10⁶ cm/s for the bulk material at 77 K. These values are much lower than those predicted by the existing theories. A critical analysis of the observations is given with a model taking into account of the non-drifting non-equilibrium phonon production.     

人工引发雷电先导-回击过程的电场变化特征

 2005年夏季在山东滨州进行了以人工引发雷电为主的综合观测实验，在负电场环境下，成功引发雷电5次，其中传统引发方式3次，空中引发方式2次。详细分析了双向先导-小回击过程和箭式先导-回击过程的垂直电场变化特征。结果表明：2次空中引发雷电的双向先导发展过程在地面60 m处产生的电场变化为4.70~14.60 kV/m，呈负向变化；在550 m处为0.11~0.16 kV/m，呈正向变化。利用60 m处的先导电场变化，估算双向先导电荷线密度分布的斜率为(1.2~3.8)×10^-7 C·m^-2。箭式先导-回击电场变化波形呈不对称V形，V形的底部对应先导的结束和回击的开始。60和550 m处的箭式先导电场几何平均值分别为17.80和1.20 kV·m^-1。随水平距离的增加，箭式先导电场以水平距离的-1.18次方衰减。