Self-similar spatial structure of a streamer-free nanosecond discharge

The microstructure of a current channel is experimentally found under the conditions when homogeneous air gaps are subjected to nanosecond voltage pulses in an electric field insufficient for streamer generation. As a possible mechanism of microstructure formation, instability of the ionization process at the avalanche stage leading to the formation of a self-similar spatial structure is considered. The fractal dimension of this structure is determined. In inhomogeneous gaps, the avalanche is shown to be unstable as well. The energy benefit of structuring is considered. It is demonstrated that the microstructure of streamer discharges in homogeneous gaps can also be treated in terms of the model suggested.     

Electric field perturbation caused by an increase in conductivity related to seismicity-induced atmospheric radioactivity growth

The influence of conductivity perturbation in the lower atmosphere on the DC electric field over a seismic region is investigated. This perturbation is related to the emanation of radon and other radioactive elements into the lower atmosphere as the seismic activity increases. An increase in the level of atmospheric radioactivity results in the appearance of additional ionization sources. The altitude dependence of the ion formation rate is calculated. An ionization source changes the atmospheric conductivity because of the appearance of ions with an equilibrium number density. The perturbation of the atmospheric conductivity is calculated as a function of the altitude. Lower atmosphere conductivity changes disturb the electric current that flows in the global atmosphere-ionosphere circuit. This disturbance is caused by an external current over a seismic region. DC electric field perturbations on the Earth’s surface and in the ionosphere are estimated. Published in Russian in Khimicheskaya Fizika, 2007, Vol. 26, No. 4, pp. 39–44. The text was translated by the authors.     

Electric field distribution and simulation of avalanche formation due to the passage of heavy ions in a parallel grid avalanche counter

Electric field distributions and their role in the formation of avalanche due to the passage of heavy ions in parallel grid avalanche type wire chamber detectors are evaluated using a Monte Carlo simulation. The relative merits and demerits of parallel and crossed wire grid configurations are studied. It is found that the crossed grid geometry has marginally higher gain at larger electric fields close to the avalanche region. The spatial uniformity of response in the two wire grid configurations is also compared.      

Evolution of electron-hole avalanches and streamers in indirect gap semiconductors

A numerical simulation of the origination and evolution of streamers in semiconductors has been performed using the diffusion-drift approximation including the impact and tunnel ionization. It is assumed that an external electric field E ₀ is static and uniform, an avalanche and a streamer are axisymmetric, background electrons and holes are absent, and all their kinetic coefficients are identical. The linear evolution of an electron-hole avalanche, an avalanche-to-streamer transition, and two successive stages of the evolution of the streamer—intermediate “diffusion” and main exponentially self-similar—have been examined in detail. It has been shown that a streamer is similar to a dumbbell with conical weights. The bases of these cones, streamer fronts, are thin shells, which contain almost the entire streamer charge and are close in shape to the halves of ellipsoids of revolution. A front propagates so that its shape and the shape of the weight of the dumbbell, the maximum field on the front, and the electron-hole plasma density in weights remain unchanged. The field strength behind the front is much smaller than E ₀, but increases with approaching the bar of the dumbbell whose diameter increases with the time t owing to the transverse diffusion. The electron and hole densities in the bar increase due to the impact ionization in an almost uniform field, which is only slightly lower than E ₀. At the diffusion stage, the length of the streamer and the curvature radius of its front increase with constant rates, which are determined not only by the impact ionization and drift, but also by diffusion. In relatively low fields (E ₀ ≲ 0.4 MV/cm for silicon) this stage ends due to the appearance of the instability of the front. In higher fields, the tunnel ionization is manifested before the appearance of instability and gives rise to the appearance of a new-type streamer. Its main feature is the stable exponential increase in all spatial scales with the same response time t _R , so that the charge-carrier density and field strength at large times t depend only on one vector variable $ \hat R $ \hat R = Rexp(−t/t _R ). This means that the solution of a Cauchy problem describing the evolution of the streamer in the uniform field is asymptotically exponentially self-similar.     

Electroluminescence in lead titanate single crystals

Electroluminescence in PbTiO₃ single crystals is studied with variation in applied electric field, frequency (20 Hz to 5 kHz) and temperature. The EL onset depends on the rate at which the dipole switches. Extremely sharp upward rising nature of the pulses of micro second duration suggest that there is a self maintained discharge in the dielectric due to secondaryγ _p mechanism. Frequency dependence of EL suggests that both the secondary mechanisms, viz. theγ _p andγ _i are active after the application of a high field and the critical field at which this occurs decreases with increase in the frequency of the applied voltage. Similarly the onset voltage decreases with increase in frequency. The temperature dependence of EL at the applied frequency of 50 Hz shows that the onset voltage is intimately connected with the coercive field of the crystal and it is minimum at the Curie point. The study suggests that EL occurs in the bulk and there is a breakdown in the dielectric due to an avalanche formation.     

初始电子密度对光学薄膜激光损伤机制的影响

光学元件的激光损伤问题是激光器件向高功率密度方向发展中必须认识和克服的问题.基于Forkker-Planck方程,研究了激光与材料相互作用时的雪崩电离机制、多光子电离机制以及联合两种机制的情况.雪崩电离的产生需要一定密度的初始自由电子存在,该自由电子可以是材料中原本就存在的,也可能是光电离产生的.着重分析了材料中的初始自由电子对材料电离机制的影响.结果表明,雪崩过程在激光作用一段时间后会达到一个稳定的电离阶段(以自由电子平均能量不随时间变化为特征,且此时雪崩电离为材料电离的主导机制),该时间与光电离速率、材料中初始自由电子密度有关.材料中的初始自由电子可以在一定程度上掩盖光电离的作用效果.     

初始电子密度对光学薄膜激光损伤机制的影响

光学元件的激光损伤问题是激光器件向高功率密度方向发展中必须认识和克服的问题.基于Forkker-Planck方程,研究了激光与材料相互作用时的雪崩电离机制、多光子电离机制以及联合两种机制的情况.雪崩电离的产生需要一定密度的初始自由电子存在,该自由电子可以是材料中原本就存在的,也可能是光电离产生的.着重分析了材料中的初始自由电子对材料电离机制的影响.结果表明,雪崩过程在激光作用一段时间后会达到一个稳定的电离阶段（以自由电子平均能量不随时间变化为特征,且此时雪崩电离为材料电离的主导机制）,该时间与光电离速率、材料中初始自由电子密度有关.材料中的初始自由电子可以在一定程度上掩盖光电离的作用效果.
关键词:初始电子;激光损伤;光电离;雪崩电离     

激光二极管触发光导开关实验研究

介绍了利用大功率半导体激光二极管触发3 mm间隙GaAs光导开关、产生非线性电脉冲输出的实验,激光二极管输出功率为70 W,上升前沿约20 ns,脉冲半高宽（FWHM）约40 ns。随着开关两端偏置场强增加,输出电压也线性增加,当偏置场强超过一定阈值,增至约2.53 kV/mm时,经过一个较小的电压峰值和时间延迟后,输出电压急剧增加,产生雪崩现象。实验结果表明:GaAs开关非线性输出的产生与载流子聚集和碰撞电离有关,偏置电场的提高增加了开关芯片中载流子聚集数量,加剧了碰撞离化程度,从而使开关从线性模式进入雪崩模式。     

Heating of nonequilibrium electrons by laser radiation in solid transparent dielectrics

A computer simulation of the heating of nonequilibrium electrons by an intense high-frequency electromagnetic field leading to the bulk damage of solid transparent dielectrics under single irradiation has been carried out. The dependences of the avalanche ionization rate on threshold field strength have been derived. Using the Fokker-Planck equation with a flux-doubling boundary condition is shown to lead to noticeable errors even at a ratio of the photon energy to the band gap ∼0.1. The series of dependences of the critical fields on pulse duration have been constructed for various initial lattice temperatures and laser wavelengths, which allow the electron avalanche to be identified as a limiting breakdown mechanism. The ratio of the energy stored in the electron subsystem to the excess (with respect to the equilibrium state) energy of the phonon subsystem by the end of laser pulse action has been calculated both with and without allowance for phonon heating. The influence of phonon heating on the impact avalanche ionization rate is analyzed.     

Mechanisms of coupling of aeroelectric and temperature fields in the lower atmosphere

Based on synchronous observations of the aeroelectric field and the air temperature, performed in August–October 2002 and June–September 2003 at the measuring facility of the mid-latitude geophysical observatory “Borok” (Yaroslavsl’ Region, Russia), we conclude that correlative relations exist between the aeroelectric-field intensity E and the air temperature T in the atmospheric surface layer. We reveal positive correlation between the temperature and electric-field variations under conditions of unstable stratification (late morning-day) and negative correlation under stable stratification (late evening-night). A simultaneous increase in the amplitude of pulsations of the electric field and the temperature occurs most frequently near the local midday. Intense variations in the electric field correspond to variations in the temperature field with a close time period. A positive temporal shift of variations in E with respect to variations in T, which reaches 40 min, is observed during enhanced temperature activity. The structure-temporal analysis of experimental data from remote sensing of the temperature and electric-field variations at five points is performed. We find simultaneous aeroelectric and temperature coherent structures accompanied by short-period pulsations of the field intensity and the air temperature in the atmospheric surface layer. We discuss possible mechanisms of coupling of the aeroelectric-field intensity and the atmospheric air temperature to describe their positive correlation under conditions of unstable stratification (late morning-day) and negative correlation in the late evening and at night. Enhancement of electric activity, which is stipulated by the intensification of turbulent convection and the formation of aeroelectric structures, seems the most significant process. With allowance for nonlocality of the electric field, this process can in particular explain a substantial lead of the aeroelectric-field perturbations compared with the correlated positive temperature perturbations. In this case, the formation of warm “quasi-front,” which is accompanied by the lifting of the near-surface air abundant in moisture and charged particles to the higher atmospheric layers, can play an important role. Other mechanisms of the field-temperature coupling take into account a decrease in the conductivity of near-surface air due to the condensation of water vapor during its cooling, as well as an increase in the mobility of light ions with increasing air temperature. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 1, pp. 35–52, January 2006.     

Generalized equations for the dynamics of the resistive firehose instability of an REB with time-dependent radius and current

Generalized equations are derived that describe the linear stage of the resistive firehose instability of a relativistic electron beam whose radius and current change along the pulse. Such factors as reverse current, the perturbations of the plasma channel, and the evolution of the plasma conductivity due to impact ionization, avalanche ionization, and recombination are taken into account.     

A Basic Predator‐Prey Type Model for Low Frequency Discharge Oscilations in Hall Thrusters

The mechanism of low frequency oscillations in Hall thrusters is usually explained using the predator‐prey type model, but the reasonable boundary conditions for the model have not been given. Analyses on thrusters' model equations show that besides the processes of neutral replenishment and ionization avalanche, the effects of dynamic electric field are also necessary for low frequency oscillations. The dynamic electric field reflects the interaction of ionization zone with acceleration zone, and is embodied in boundary conditions of the predator‐prey type model. Furthermore, a basic predator‐prey type model with reasonable boundary conditions and complete physical mechanism is proposed. And the effects of electric field on low frequency oscillations are verified by experiment (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ionization process in the humid air of the lower troposphere

This paper analyzes the local processes of ionization and electron loss in the early stages of an electric discharge in humid air at altitudes of 0–12 km. The processes of ionization in an external electric field, background ionization, attachment and detachment of electrons from atomic and molecular oxygen, charge transfer, and the conversion of negative ions are considered. The dependences of the rate constants on the pressure are taken into account. It is shown that the electric field strength at which the effective ionization of air begins decreases with increasing altitude.     

Breakdown characteristics of air in the lower atmosphere

This study is devoted to an analysis of the ionization and loss of electrons at early stages of discharge development in air at an altitude of 0–12 km. Ionization in an external electric field and background ionization, electron attachment and detachment from atomic and molecular oxygen, charge exchange, and negative ion conversion are considered. The dependence of the process rate constants on pressure is taken into account. It is demonstrated that the electric field at which effective air ionization begins decreases with increasing altitude.     

High temperature and wavelength dependence of avalanche gain of AlAsSb avalanche photodiodes

The evolution of the dark currents and breakdown at elevated temperatures of up to 450 K are studied using thin AlAsSb avalanche regions. While the dark currents increase rapidly as the temperature is increased, the avalanche gain is shown to only have a weak temperature dependence. Temperature coefficients of breakdown voltage of 0.93 and 1.93 mV/K were obtained from the diodes of 80 and 230 nm avalanche regions (i-regions), respectively. These values are significantly lower than for other available avalanche materials at these temperatures. The wavelength dependence of multiplication characteristics of AlAsSb p-i-n diodes has also been investigated, and it was found that the ionization coefficients for electrons and holes are comparable within the electric field and wavelength ranges measured.     

铯47D精细能级超冷里德堡原子自由演化的动力学研究

用连续窄线宽激光器将超冷铯里德堡原子分别激发到47D_3/2, 47D_5/2精细态, 观察了处于里德堡精细态的铯原子向超冷铯等离子体自由演化的过程, 详细对比了不同精细态的铯里德堡原子预电离时间、电离速率以及等离子体的转化效率. 将里德堡原子快速转化为等离子体的过程解释为局域势阱内由预电离产生的电子与里德堡原子的快速碰撞导致的雪崩电离.     

Monte Carlo simulation of electron avalanches and avalanche size distributions in methane

The simulation of electron avalanches and avalanche size distributions in methane is presented in this paper. A model for electron transport under the influence of a constant electric field based on the Monte Carlo method is described in detail. The model is verified and then used to simulate the avalanche development, to calculate the number of electrons in the avalanche (avalanche size), and to determine the avalanche size distribution. The simulated avalanche size distributions in methane are compared with the experimental results, and a good agreement is observed. The influence of inter‐electrode distance, pressure, and reduced electric field on the shape of the avalanche size distribution is discussed. The assumption from the literature that for a constant reduced electric field the shape of the reduced avalanche size distribution is independent of the mean size of the avalanche is confirmed for a wide range of experimental conditions. The simulations have shown that avalanche size distributions depend only on the reduced electric field, confirming the similarity principle.     

Determination of electric transport properties in the pre- and post-breakdown regime ofp-germanium

Conductivity and Hall-effect measurements were performed on single-crystallinep-doped germanium, electrically driven into low-temperature avalanche breakdown via impurity impact ionization. The electric transport properties were determined as a function of the applied electric field in the pre- and post-breakdown regime. The characteristic field dependence of the carrier density, mobility, and drift velocity was found to be reflected in smooth variations of the integral current flow. The breakdown mechanism was demonstrated to involve a mobility which sensitively depends upon the density of the mobile charge carriers. Our experimental findings are qualitatively explained by simple model approaches developed from established breakdown theories.     

Electric field induced avalanche breakdown and non-volatile resistive switching in the Mott Insulators AM4Q8

The Mott insulator compounds AM₄Q₈ exhibit a new type of volatile and non volatile resistive switchings that are of interest for RRAM application. We found that above a threshold electric field E _TH of the order of a few kV/cm these compounds undergo a volatile resistive switching based on an avalanche process. For electric field much higher than the threshold avalanche breakdown field, the resistive switching turns non volatile. Our EDXS and STEM analyses show that the non volatile resistive switching originating from the avalanche breakdown can neither be ascribed to local chemical modifications nor to a local phase change with symmetry breaking at a resolution better than a few nanometer. This is in strong contrast with non volatile resistive switching reported so far that are all based on chemical or structural changes. Conversely, our results suggest that the avalanche breakdown induce the collapse of the Mott insulating state at the local scale and the formation of a granular conductive filament formed by compressed metallic domains and expanded “superinsulating” domains.