Development Of Avalanche Research Articles

Influence of non-uniform electric field distribution on the atmospheric pressure air dielectric barrier discharge

The dielectric barrier discharge (DBD) in air at atmospheric pressure is not suitable for industrial applications due to its randomly distributed discharge filaments. In this paper, the influence of the electric field distribution on the uniformity of DBD is theoretically analyzed and experimentally verified. It is found that a certain degree of uneven electric field distributions can control the development of electron avalanches and regulate their transition to streamers in the gap. The discharge phenomena and electrical characteristics prove that an enhanced Townsend discharge can be formed in atmospheric-pressure air with a curved-plate electrode. The spectral analysis further confirms that the gas temperature of the plasma produced by the curved-plate electrode is close to room temperature, which is beneficial for industrial applications. This paper presents the relationship between the electron avalanche transition and the formation of a uniform DBD, which can provide some references for the development and applications of the DBD in the future.

Characteristic Features of the Clouds Producing Thunderstorm Ground Enhancements

AbstractThunderstorm ground enhancements (TGEs) are fluxes of energetic particles multiplied in thunderclouds, which may be registered by ground‐based detectors. Using the Weather Research and Forecasting (WRF) Model, the state of the atmosphere with a 1‐km horizontal resolution is modeled for convective events accompanied by TGEs observed at the high altitude Aragats Research Station in Armenia (40.47 N, 44.18 E, 3,200 m above sea level). By comparing the data obtained in observations with the results of simulations using a microphysical parameterization for WRF, a technique is developed for estimating charge distribution parameters. A typical charge distribution in a TGE‐producing cloud is found to be well approximated by a two‐layered charge structure with a lower positive charge region formed by graupel particles and an upper negative charge region formed by snow particles. The characteristic charge density is 0.01 nC/m3 for the graupel cluster and −0.02 nC/m3 for the snow cluster. A vertical distance of about 1–2 km between the lower positive and upper negative layers is sufficient for the development of an energetic particle avalanche.

Clusters and avalanches of fibre breaks in a model of an impregnated unidirectional fibre bundle under tension

Development of clusters and avalanches of damage events is a key characteristic in statistical physics of structures susceptible to a catastrophic failure. This paper applies the concepts of such descriptions to the process of fibre breakage under tension in an impregnated fibre bundle model (IFBM). The following parameters are analysed: (1) susceptibility (damage caused by change in loading); (2) spatial clustering of breaks and its interdependency with avalanches; (3) the size distribution of fibre break clusters and avalanches; (4) fractal dimensions; and (5) correlation lengths of breaks. A systematic workflow and algorithms for this analysis are presented and applied for two realisations of a random fibre placement of carbon fibre/epoxy bundles with fibre volume fractions of 50% and 60%. For these two realisations: (1) the susceptibility divergence near the failure point is affected by the finite-size effect, being nearly constant for the last ~1/20th of strain life; this behaviour can be considered as a failure predictor; (2) the morphology of the break system is a spatially distributed multi-defect occurrence with the dimensionality of 3 for all breaks together and weak planarity demonstrated only by the largest clusters near failure; (3) the exponents of the cluster-size and avalanche-size power law distributions reached 2.5–3 for both statistics near the bundle failure, which may constitute an imminent failure indicator. Damage evolution of the carbon/epoxy IFBM up to the final failure is found to be short-correlated (with correlation length of the order of one fibre break stress redistribution zone) in the absence of collective long-range defect interactions. During the catastrophic avalanche, the spatially distributed morphology of breaks is gradually reorganized into a translaminar fracture mode.

Snow depth retrieval in North-Western Himalayan region using pursuit-monostatic TanDEM-X datasets applying polarimetric synthetic aperture radar interferometry based inversion Modelling

ABSTRACT Synthetic Aperture Radar (SAR) remote sensing is a state-of-the-art tool for snow monitoring and snow parameters estimation. SAR remote sensing-based techniques, such as interferometric SAR (InSAR) and Polarimetric SAR (PolSAR) have already proven useful in the estimation of geophysical parameters of snow. InSAR-based techniques utilize interferometric phase information from repeat-pass datasets for snow parameters retrieval. During the monitoring of snow, the large temporal gap between the repeat passes results in the temporal decorrelation in the snowpack, which leads to the loss of interferometric coherence. Hence, there is a need for a technique for snow parameters estimation, which can work with zero temporal baseline datasets. This study works on the development of a Polarimetric SAR Interferometry (PolInSAR) based modelling approach for snow-depth estimation using TerraSAR-X/TanDEM-X datasets acquired in the pursuit monostatic mode (temporal baseline = 10 seconds). The study area of this work is the Manali region of Himachal Pradesh situated in the Beas basin. Multi-temporal analysis of the snow-depth variation is executed utilizing the two pursuit-monostatic TanDEM-X interferometric quad-pol dataset pairs of the dates 21 January 2015 and 22 January 2015. In this study, the PolInSAR-based Coherence Amplitude Inversion modelling approach is used for the snow-depth retrieval. The magnitude of the complex interferometric coherence is used during the modelling implementation. The snow extinction coefficient is estimated and used as an input during PolInSAR modelling. Further, a comparison of the calculated volume coherence magnitude and the observed volume coherence magnitude is done during model implementation for the snow-depth estimation. The snow depth is estimated at a resolution of 15 m × 15 m in range and azimuth directions respectively. The estimated snow depth for both the dates shows a precise correlation with the ground datasets. The rise in model retrieved snow-depth value from 0.84 m to 1.24 m is observed during the period. The retrieved results were validated using the ground data of snow depth from the Automatic weather station (AWS) of Snow and Avalanche Study Establishment (SASE), Defence Research and Development Organization (DRDO), and Indian Institute of Remote Sensing (IIRS) installed in the Dhundi region of the study area for same dates.

Relativistic runaway electron avalanche

Discussed are the genesis of the concept of the relativistic runaway electron avalanche (RREA) and its mechanism as an analog of the Townsend’s avalanche, but capable of developing, unlike the latter, in weak thundercloud electric fields. Thanks to this, it was possible to overcome difficulties while interpreting results of observations of penetrating emission enhancements in thunderstorm atmospheres. The main inelastic interactions of high-energy electrons with atomic particles participating in the avalanche development are described; in terms of the drag forces, the essence of the runaway process is discussed; and methods of RREA numerical simulation are described. In approximate historical sequence, results of calculations of the spatial and temporal scales of the avalanche enhancement are analyzed and contemporary data on avalanche macroscopic characteristics are given, which is required for numerical simulations of the runaway electrons in the fluid approximation. As an extension to the relativistic range of the mechanism of the classical cathode-directed streamer, relativistic positive feedback is discussed, by means of which a generation of the RREA series, as a self-sustained process, is supported. Laboratory experiments on RREA modeling are described, in one of which the initial stage of the avalanche was produced. The historical background of the issue is often more interesting than the results obtained. I P Pavlov (1849–1936), 1904 Nobel Prize winner in Physiology and Medicine

Monte Carlo simulation for development of electron avalanches in nitrogen at moderate and high reduced electric field

This paper addresses the simulation of electron avalanches in nitrogen by means of the Monte Carlo method. The data have been obtained for the conditions of classical measurements of the impact ionization coefficient in a wide range of reduced electric fields E/p. It is shown that at moderate E/p values, the results are in a good agreement with the generally accepted approaches in the description of the avalanches based on the drift motion of the electrons and on the diffusion. The high E/p is achieved in the vicinity of the minimum of Paschen's curve due to decreasing the gas pressure. Then the process of development of the single electron avalanche changes radically. The reason is that the runaway electrons appear, which leads to essential longitudinal expansion of the electron cloud. It is revealed that the average electron energy at the front of the cloud turns out to be larger than that at the backside of the cloud.

Mass-wasting deposits in the Domuyo Volcanic Center, northern Neuquén Andes (Argentina): An analysis of the controlling factors

A high-mobility mass movement deposit was identified along the Turbio river, which drains the north-facing hillslope of the Domuyo Volcano, in the northern Neuquén Andes. Multiple satellite images and aerial photograph images pre- and post-mass movement have been examined to study its general features and to narrow the time window of the event. Results suggest that, in 1997, a volume of approximately 0.8 × 106 m3 of debris slid nearly 10 km descending a vertical range of ~1400 m and covering a minimum area of 1.5 × 105 m2. The 1997 autumn-winter period was characterized by unusually heavy precipitation in the area (the largest value of precipitation registered in the last 30 years), which was related to a major El Niño Southern Oscillation event. We interpret the deposit as a debris flow event that would have emerged from a moraine whose loose sediments would have set into motion in a context of heavy rainfalls. Furthermore, in the vicinities of the Domuyo Volcano, there is a widespread occurrence of rock avalanches, mostly post-glacial in age, recognized and mapped in this work. Their distribution is coincident with recently published InSAR deformation patterns, which, together with seismic and geophysical data, may be indicating that the volcanic edifice has been subject to inflation processes for at least the last four years. Assuming that similar volcanic unrest episodes occurred in the last thousand years, we can speculate that a volcano-tectonic control could have played a role in the development of these rock avalanches. From there, this work shows that, in a deglacial landscape like the one that defines the Domuyo Volcano area, there are different elements that can act on slope instability. This information intends to be used proactively to advance in the hazard evaluation of these catastrophic processes in the northern Neuquén Andes, a zone with increasing agricultural and tourism activities.

Lower size limit of the simulated biological target by TEG counter

The gas gain in the range of 1.0–2 × 105 was measured for propane-based tissue equivalent gas mixtures in the pressure range of 9–500 hPa. The Zastawny and Mitev et al. models of avalanche growth were used to analyse the obtained data. Both models showed that for low pressures of the working mixture, (i.e., below 80 hPa), there were non–equilibrium states of the development of the electron avalanche. The minimum size of the simulated biological target was determined on the basis of the measurements.

Magnetic flux avalanches in Nb/NbN thin films

Technological applications of NbN thin films may be threatened by the development of magnetic flux avalanches of thermomagnetic origin appearing in a large portion of the superconducting phase. In this work, we describe an approach to substantially suppress the magnetic flux avalanche regime, without compromising the upper critical field. This procedure consists of depositing a thin Nb layer before the reactive deposition of NbN, thus forming a bi-layered system. We use ac susceptibility and dc magnetometry to characterize both the single-layer films, Nb and NbN, and the bi-layered specimen, as well as calibrated magneto-optical imaging to map the instability regime of the studied samples. Magnetic flux imaging reveals interesting features of the dendritic flux avalanches in the bi-layer system, including halo-like patterns and crossing avalanches.

Statistical and numerical analysis of secondary electron avalanches with ion-induced electron emission in air

Statistical and numerical analyses of secondary electron avalanches with ion-induced electron emission and multielectron initiation in air is carried out. Statistical analysis is based on the negative binomial distribution (NBD) and its mixtures and the mean number of electrons in avalanches is determined. The development of successive secondary electron avalanches for ion-induced electron emission from the cathode ($$ \gamma_{i} $$ process) in air is simulated by the fluid model. The statistics of secondary avalanches is described by a mixture of NBDs, and it is found that the mixing weights $$ a_{j} $$ are positively correlated with $$ \mu^{j} $$ for the contribution of secondary avalanche of $$ j $$th generation, where $$ \mu = \gamma_{i} [\exp (\alpha d) - 1] $$ is the avalanche regeneration factor. Gaussian continual approximations for the discrete negative binomial distribution are also applied to the experimental data and the mixing weights are determined. Our models can explain a deviation of the electron number distribution from exponential due to influence of ion feedback processes.

AZIMUTHAL STABILITY OF TRICHEL PULSES AND CATHODE DIRECTED STREAMERS

The numerical simulations of negative corona discharge in Trichel pulse mode are carried out with the calculation of evolution of azimuthal perturbations. It is found the azimuthal instability with increment corresponding to avalanche development. This instability is suppressed at the nonlinear stage and does not lead to the process branching. The result is also applicable to the azimuthal instability of the cathode directed streamer, found earlier. The difference in their increments is followed from the difference in courses of the processess, which is discuss.

Streamer propagation in the atmosphere of Titan and other N2:CH4 mixtures compared to N2:O2 mixtures

Streamers, thin, ionized plasma channels, form the early stages of lightning discharges. Here we approach the study of extraterrestrial lightning by studying the formation and propagation of streamer discharges in various nitrogen-methane and nitrogen-oxygen mixtures with levels of nitrogen from 20% to 98.4%. We present the friction force and breakdown fields Ek in various N2:O2 (Earth-like) and N2:CH4 (Titan-like) mixtures. The strength of the friction force is larger in N2:CH4 mixtures whereas the breakdown field in mixtures with methane is half as large as in mixtures with oxygen. We use a 2.5 dimensional Monte Carlo particle-in-cell code with cylindrical symmetry to simulate the development of electron avalanches from an initial electron-ion patch in ambient electric fields between 1.5Ek and 3Ek. We compare the electron density, the electric field, the front velocities as well as the occurrence of avalanche-to-streamer transition between mixtures with methane and with oxygen. Whereas we observe the formation of streamers in oxygen in all considered cases, we observe streamer inceptions in methane for small percentages of nitrogen or for large electric fields only. For large percentages of nitrogen or for small fields, ionization is not efficient enough to form a streamer channel within the length of the simulation domain. In oxygen, positive and negative streamers move faster for small percentages of nitrogen. In mixtures with methane, electron or streamer fronts move 10–100 times slower than in mixtures with oxygen; the higher the percentage of methane, the faster the fronts move. On Titan with methane percentages between 1.4% and 5%, a successful streamer inception would require a large electric field of 4.2 MV m−1 (3Ek). Such large fields might not be present and explain the non-detection of Titan lightning by the Cassini/Huygens mission.

Generation of atmospheric pressure diffuse dielectric barrier discharge based on multiple potentials in air

In order to achieve atmospheric pressure diffuse dielectric barrier discharge (DBD) in air, a helical–helical electrode structure with a floating-voltage electrode is proposed in this paper. Results from an electric field distribution simulation indicate that strong electric fields are formed where the helical-contact electrodes’ insulating layers are in contact with each other, as well as near the floating-voltage electrode, which contributes to the production of a large number of seed electrons. The electric field within the air gap is weak (<3 × 106 V m−1), which inhibits the rapid development of electron avalanches and the formation of filament discharge. The experimental result shows that a 3.0 mm width diffuse DBD is generated in air. Moreover, based on the study of the helical–helical electrode with a floating-voltage electrode, a three-dimensional electrode structure is presented, and a three-dimensional diffuse discharge is generated in air by adopting this electrode structure. The plasma studied is stable and demonstrates good diffusion characteristics, and therefore has potential applications in the field of exhaust gas treatment and air purification.

Рельєф стартових зон виникнення снігових лавин на Чорногірському хребті Українських Карпат

Досліджено проблему такого небезпечного стихійного явища, як лавини. Виконано аналіз основних чинників лавиноутворення, а саме морфометричних, метеорологічних та антропогенних. Запропоновано використання геоінформаційних технологій для моніторингу та дослідження потенційно лавинонебезпечних територій, а також для визначення локалізації стартових зон лавин та прогнозування напряму їх руху. Створено цифрову модель рельєфу частини Чорногірського хребта. Виконано візуалізацію стартових зон виникнення снігових лавин двох типів – лійкоподібних, або лоткових та площинних. Створено також тривимірну модель рельєфу, яка візуалізує рельєф, дає його наочне об'єктивне зображення. На ній добре видно основні форми рельєфу, напрямки хребтів, долин, а також місця зародження лоткових і площинних лавин. Порівняно з картою, інформативність зображення рельєфу значно вища. Використання такої 3D-моделі рельєфу дає змогу ще наочніше побачити місця можливого виникнення лавин і спрогнозувати напрям їх руху. Проведені дослідження дають змогу виконувати візуалізацію, наочне представлення лавинонебезпечних ділянок та місць можливого зародження лавин, створювати моделі розвитку лавинних процесів залежно від метеорологічних, ботанічних і морфометричних факторів. Результати досліджень є перспективними для туристичної та рекреаційної галузей, оскільки можуть бути використані у проектуванні туристичних маршрутів, будівництві об'єктів рекреації та для прогнозування виникнення снігових лавин.

Study on Generation of Glow Discharge Plasma in Air and Surface Modification of Wool Fabric

A lamellar electrode structure is proposed to achieve the generation of a large-area plasma and the effective processing of the wool fabric with the electrode on only one side of the treated material. The lamellar electrode can form the curved electric field lines passing through the discharge air gap and the wool fabric, creating the condition for the action of plasma. Meanwhile, the micron-scale discharge air gap is constructed through the direct contact between the electrode and the treated material. Specifically, it can effectively inhibit the development of the electron avalanche, thereby achieving a through-type glow discharge in air between the electrode and the wool fabric under the strong electric field. Through the scanning electron microscope and X-ray photoelectron spectroscopy tests, it can be observed that the scales on the surface of wool fiber are effectively destroyed and the polar groups are introduced to the surface of wool fiber after the plasma treatment. Obviously, the wettability of wool fiber is greatly improved, and the wetting time is reduced from more than 1800 s to less than 0.5 s.

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.

The critical avalanche of runaway electrons

The x-ray and gamma-ray flashes observed in the Earth atmosphere during a thunderstorm are usually associated with the generation of runaway electrons (RE) in atmospheric electric fields. It is supposed that runaway electron avalanches initiated by cosmic rays play the main role in high-altitude discharges observed in a thunderstorm atmosphere. We have performed three-dimensional numerical calculations to investigate the mechanism of the development of a critical avalanche and to determine its parameters. It has been shown that the number of electrons in a critical avalanche occurring in air under conditions characteristic of thunderstorm discharges can reach a value of the order of 1018.

Dislocation avalanche mechanism in slowly compressed high entropy alloy nanopillars

Crystals deform by the intermittent multiplication and slip avalanches of dislocations. While dislocation multiplication is well-understood, how the avalanches form, however, is not clear, and the lack of insight in general has contributed to “a mass of details and controversy” about crystal plasticity. Here, we follow the development of dislocation avalanches in the compressed nanopillars of a high entropy alloy, Al0.1CoCrFeNi, using direct electron imaging and precise mechanical measurements. Results show that the avalanche starts with dislocation accumulations and the formation of dislocation bands. Dislocation pileups form in front of the dislocation bands, whose giveaway trigs the avalanche, like the opening of a floodgate. The size of dislocation avalanches ranges from few to 102 nm in the nanopillars, with the power-law distribution similar to earthquakes. Thus, our study identifies the dislocation interaction mechanism for large crystal slips, and provides critical insights into the deformation of high entropy alloys.

Effects of surface roughness on surface charge accumulation characteristics and surface flashover performance of alumina-filled epoxy resin spacers

The accumulated charges on the insulator surface become a key factor to incur surface flashover. The charge accumulation process is closely related to the surface condition. This research investigates the effects of surface roughness on surface charge accumulation behavior and surface flashover performance of alumina-filled epoxy resin spacers in SF6/N2 mixtures under DC voltage. The insulator is prepared with only half surface subjected to rough treatment. The accumulated charges are distributed in two regions. Considerable homo-charges are located near the high voltage (HV) electrode and few hetero-charges are near the grounded (GND) electrode (region I). These charges are injected by electrodes. The bipolar charges between HV and GND electrodes (region II) originate from gas ionization. Surface rough treatment can suppress surface charge accumulation and improve surface insulation strength. When increasing surface roughness, surface charge declines first slowly (stage I) and then rapidly (stage II), which coincides with the two stages of surface flashover voltage increment. Surface charge declination at stage I is mainly due to the increase in surface conductivity, while at stage II, the introduced deep traps also play a role. Surface flashover voltage increment at stage I is mainly due to the extended creepage distance, while at stage II, the declination of surface charge also plays a role. Besides, the block of electron avalanche development by the roughed surface is also responsible for the increase in surface flashover voltage at these two stages.

Wavelength dependence of the mechanisms governing the formation of nanosecond laser-induced damage in fused silica.

The influence of the wavelength on the morphology of nanosecond laser-induced damage on the exit surface of fused silica is investigated. A combination between the typical features of damage sites initiated at 1064 nm and 355 nm is observed at 532 nm, including ring patterns sporadically exhibited, in good agreement with calculations of the development of an electron avalanche at this wavelength. The associated ring appearance speed scales as the cube root of the laser intensity, and is ~10.5 km/s while it is ~20 km/s when initiated by infrared pulses. The whole set of results sheds light on the different wavelength-dependent mechanisms governing damage formation.