Development Of Avalanche Research Articles

On the deterministic and stochastic solutions of Space Charge models and their impact on high resolution timing

RPCs offer unique opportunities to investigate basic processes in gaseous electronics. The growth of a single avalanche can be studied in a regime where it reacts to its own field. This induces a saturation in its development, often described in a deterministic scenario by a nonlinear model. Once reinterpreted in a fully stochastic framework, the same feature corresponds to a negative feedback mechanism, which regulates the avalanche development and preserves its timing properties. Fluctuations are hence mostly produced in the initial phase of the growth. A clear evidence of the action of this stabilizing scheme is observed in data collected for single avalanches of fixed length.

Developments of optical imaging capillary plate gas detector

An optical imaging capillary plate (CP) gas detector has been successfully developed for a gas mixture of Ar+CF 4. Gas gains of up to 10 4 can be achieved with a single CP. Scintillation light simultaneously emitted during the development of electron avalanches can be observed using a photomultiplier tube (PMT) and optical imaging system which consists of a CP gas detector and a cooled CCD camera coupled to lens optics. The energy resolutions obtained for the charge signal and the light signal were 22% and 24% for 10 keV X-rays, respectively. Using the optical imaging system, clearer images of X-rays have been obtained with the gas mixture. Successful operation with this gas mixture has allowed us to realise a novel-imaging device with CP for X-ray imaging, cold neutron imaging, medical imaging, and cellular function analysis.

The role played by self-inhibition in combustion processes

The absence of correlations between the heat of combustion of substances and substance combustion characteristics is shown to be caused by the special features of competition between reaction chain branching and termination, including partial participation of fuels in the inhibition of their oxidation. By way of example, differences in the characteristics of combustion of hydrocarbons and hydrogen at atmospheric pressure and the mechanisms of the development of chain avalanches in these processes are considered.

Specific features of electron avalanche in high gas multiplication mode

The results of studying an electron avalanche in narrow-gap wire chambers in the region of avalanche-to-streamer transition are described. The amplitude and temporal characteristics of the chambers in the high gas multiplication mode (≥107) are given. Due to specific features of the electric field distribution in narrow-gap chambers and the application of a CF4-based working gas, previously unknown processes taking place in a shower and more clearly reflecting the dynamics of avalanche development were discovered—electron velocity distribution, electrostatic oscillations of the avalanche as a whole, decrease in the anode signal duration, increase in the delay time of charge induction on the cathode by the avalanche development time, and, finally, varying shapes of the anode and cathode signal amplitude distributions. The observed processes can be explained if a completed avalanche is represented in the form of a double charged layer. The basis for such a representation is the specific features of a high-current avalanche mentioned above. The model under consideration is simple, provides answers to questions related to streamer formation and growth, and reflects the structure of an ionized channel and its stability mechanism.

Large-scale failures on domes and stratocones situated on caldera ring faults: sand-box modeling of natural examples from Kamchatka, Russia

Edifices of stratocones and domes are often situated eccentrically above shallow silicic magma reservoirs. Evacuation of such reservoirs forms collapse calderas commonly surrounded by remnants of one or several volcanic cones that appear variously affected and destabilized. We studied morphologies of six calderas in Kamchatka, Russia, with diameters of 4 to 12 km. Edifices affected by caldera subsidence have residual heights of 250–800 m, and typical amphitheater-like depressions opening toward the calderas. The amphitheaters closely resemble horseshoe-shaped craters formed by large-scale flank failures of volcanoes with development of debris avalanches. Where caldera boundaries intersect such cones, the caldera margins have notable outward embayments. We therefore hypothesize that in the process of caldera formation, these eccentrically situated edifices were partly displaced and destabilized, causing large-scale landslides. The landslide masses are then transformed into debris avalanches and emplaced inside the developing caldera basins. To test this hypothesis, we carried out sand-box analogue experiments, in which caldera formation (modeled by evacuation of a rubber balloon) was simulated. The deformation of volcanic cones was studied by placing sand-cones in the vicinity of the expected “caldera” rim. At the initial stage of the modeled subsidence, the propagating ring fault of the caldera bifurcates within the affected cone into two faults, the outermost of which is notably curved outward off the caldera center. The two faults dissect the cone into three parts: (1) a stable outer part, (2) a highly unstable and subsiding intracaldera part, and (3) a subsiding graben structure between parts (1) and (2). Further progression of the caldera subsidence is likely to cause failure of parts (2) and (3) with failed material sliding into the caldera basin and with formation of an amphitheater-like depression oriented toward the developing caldera. The mass of material which is liable to slide into the caldera basin, and the shape of the resulted amphitheater are a function of the relative position of the caldera ring fault and the base of the cone. A cone situated mostly outside the ring fault is affected to a minor degree by caldera subsidence and collapses with formation of a narrow amphitheater deeply incised into the cone, having a small opening angle. Accordingly, the caldera exhibits a prominent outward embayment. By contrast, collapse of a cone initially situated mostly inside the caldera results in a broad amphitheater with a large opening angle, i.e. the embayment of the caldera rim is negligible. The relationships between the relative position of an edifice above the caldera fault and the opening angle of the formed amphitheater are similar for the modeled and the natural cases of caldera/cone interactions. Thus, our experiments support the hypothesis that volcanic edifices affected by caldera subsidence can experience large-scale failures with formation of indicative amphitheaters oriented toward the caldera basins. More generally, the scalloped appearance of boundaries of calderas in contact with pre-caldera topographic highs can be explained by the gravitational influence of topography on the process of caldera formation.

Effect of Near-Threshold Ionization on Electron Attachment in Gaseous Dielectrics

It has been predicted that near-threshold ionization (NTI) in a gaseous dielectric inhibits the development of electron avalanche when the gaseous dielectric has a sufficient capability for low-energy electron attachment. The NTI leaves little energy for the primary and secondary electrons involved in the ionization; thus, both electrons can be captured by dielectric gas molecules without further ionization. A computational estimation indicates that this process can occur in SF6.

Detailed RPC avalanche simulations

We report on the progress in the simulation of Resistive Plate Chambers (RPCs). We investigate the influence of the space charge effect on the signal rise and the time resolution. A two-dimensional simulation model allows to study in detail the development of single avalanches.

The physics of Resistive Plate Chambers

Over the last 3 years we investigated theoretical aspects of Resistive Plate Chambers (RPC) in order to clarify some of the outstanding questions on space charge effects, high efficiency of small gap RPCs, charge spectra, signal shape and time resolution. In a series of reports we analyzed RPC performance including all detector aspects covering primary ionization, avalanche multiplication, space charge effects, signal induction in presence of resistive materials, crosstalk along detectors with long strips and front-end electronics. Using detector gas parameters entirely based on theoretical predictions and physical models for avalanche development and space charge effects we are able to reproduce measurements for 2 and 0.3 mm RPCs to very high accuracy without any additional assumptions. This fact gives a profound insight into the workings of RPCs and also underlines the striking difference in operation regime when compared to wire chambers. A summary of this work as well as recent results on three-dimensional electric field distributions inside the avalanches are presented.

An Experimental Investigation of an Avalanche of Relativistic Runaway Electrons under Normal Conditions

The development of avalanches of relativistic runaway electrons is investigated in an experiment similar to the classical Townsend experiment, but in the relativistic energy region. A large-scale laboratory setup is developed with an operating voltage of up to 1.2 MV, which is a chamber with flat electrodes separated by a segmented insulator. A sensitive collector method of analyzing the spectra of high-energy electrons is developed, which is used in electron detectors with a large collecting surface. The multiplication of relativistic electrons in the air is investigated at voltages of up to 1.0 MV. The initial stage of a relativistic electron avalanche is realized for the first time. The results of measurements agree with the results of Monte Carlo simulation.

Study on an atmospheric pressure glow discharge

Experiments were performed on an atmospheric pressure glow discharge (APGD) in an air gap between two dielectric barrier electrodes. While it is possible to get an APGD in a 2 mm air gap, it is possible to get only a filament discharge in a 5 mm air gap. The development of an electron avalanche in such a gap was numerically simulated. It was found that the critical applied field for a 5 mm electron avalanche to transit to a streamer is equal to 35.07 kV cm−1. This calculated critical applied field is in good agreement with the experimental one. The experimental and theoretical results confirm that only a filament discharge, rather than a glow discharge, can be produced in an atmospheric pressure air gap that is not less than 5 mm if it is not possible to lower the breakdown field of air. A resistive barrier discharge (RBD) was theoretically analysed and the development of RBD was numerically simulated. If a kilohertz discharge is required, the parameters of the resistive layer should be in the range ρεr = (109–1011) Ω cm. APGD in a helium gap was realized using 50 Hz line power with a suitably fabricated resistive layer.

The GEM scintillation in He–CF4, Ar–CF4, Ar–TEA and Xe–TEA mixtures

Light emitted during the development of electron avalanches in a gas electron multiplier (GEM) can be efficiently used for optical readout of the detector. We report on measurements of total light yields in Ar/CF4, He/CF4, Ar/TEA and Xe/TEA mixtures. Information on the energy resolution obtained with low-energy X-ray photons is also presented. The spectral distribution of the light produced in the GEM is analysed and the mechanisms associated with the process of light production are considered.

Ferroelectric cathodes in transverse magnetic fields

Experimental investigations of a planar ferroelectric cathode in a transverse magnetic field up to 3 kG are presented. It is shown that the transverse magnetic field differently affects the operation of ferroelectric plasma cathodes in “bright” and “dark” emission modes in vacuum. In the bright mode, when the surface plasma is formed, the application of the transverse magnetic field leads to an increase of the surface plasma density. In the dark mode, the magnetic field impedes the electron emission. This result indicates the similarity of the dark emission mode to the surface preflashover, where the transverse magnetic field inhibits the development of secondary electron avalanches along the surface.

Use of harmonics for femtosecond micromachining in pure dielectrics

A controllable reproducible modification of dielectrics within the bulk of material by focused ultrashort laser pulses provides the opportunity for writing three-dimensional (3D) photonic structures including bit-wise 3D information recording. Here a nonlinear response of material is employed, the multi-photon absorption being the first step. The present article concerns the expedience of the use of harmonics, first of all, second harmonic, for 3D micromodification of transparent dielectrics. If modification is provided by multi-photon transition to the first electronic excited level either due to subsequent photochemical reaction or material heating, then the reduction of the order of multi-photon transition provided by the second harmonic appears to be effective. If modification is connected with ionization of material and creation of free electrons, i.e., by breakdown, then an advantage of second harmonic is less obvious because impact ionization is more effectively provided by the fundamental frequency. Here a combined effect of both the second harmonic and fundamental frequency beams can be effective. Impact ionization is described by the Fokker–Planck equation. A relation between exponent of avalanche development and impact ionization delay time is discussed.

Development of grain avalanches.

We present a new theoretical approach aimed to describe the dynamical properties of avalanches. Starting from simple microscopic considerations, we propose an analytical derivation accounting for the avalanche growth as a function of time. The solution resembles very closely recent experimental observations. We show in particular that the two observed regimes of growth proceed from a singularity in the rear front velocity as a function of the slope.

New experimental evidence for the stabilizing effect of a superthermal electron avalanche during electron cyclotron resonant heating

New experimental evidence for the stabilizing effect of the superthermal electrons [A. Lazaros, Phys. Plasmas 6, 148 (1999)], which are produced through the avalanche effect [A. Lazaros, Phys. Plasmas 8, 1263 (2001)] during electron cyclotron resonant heating (ECRH), is provided by the suppression of the “fishbone” mode with ECRH in the “compact helical system” [S. Okamura et al., Nucl. Fusion 39, 1337 (1999)], in an experimental arrangement which favors the development of a superthermal electron avalanche and excludes any other stabilizing effect. The significance of the radial location of the EC resonance and the electron density, for the development of the superthermal electron avalanche, is confirmed by radial scans of the EC resonance and by density scans.

Possibility of atmospheric pressure glow discharge in air

The atmospheric pressure discharge in an air gap between two dielectric barrier electrodes has been studied experimentally and the development of an electron avalanche in such an air gap was simulated numerically. The results show that, while it is possible to obtain glow discharge in an air gap not longer than 2 mm, the streamer discharge could only be obtained in an air gap longer than 5 mm if no methods could be used to lower the electric field when discharging. The "iontrapping mechanism", which was said to be responsible for the atmospheric pressure glow discharge, was not verified in the experiments.

Gas Ionization by Runaway Electrons

A description of the development of a runaway electron avalanche in helium is presented. It is demonstrated that the ionization coefficient α approaches a steady-state value αst even in a very strong field. The dependences of α and αst and also of the transient length of αst on the field strength in the transient region are obtained. The experimentally observed monotonic increase in α with the field strength and the absence of maximum in this dependence followed by the subsequent decay of α are explained. It is demonstrated that the maximum in the dependence of αst on the field strength is inaccessible for measurements in strong electric fields.

Effective ionization, electron and ion transport in SF6-Hemixtures

This paper reports on the measurement of the electron driftvelocity, the density normalized effective ionization coefficients (α-η)/N,and the ratio η/α (η and α are the attachment and ionization coefficients,respectively), over a wide range of the density-reduced field strength, E/N,from 40 to 560 Td (1 Td = 10-17 V cm2). From the measured values of(α-η)/N and η/α, the density normalized ionization and attachmentcoefficients were obtained. Further analysis of avalanche development past thefirst electron transit allowed us to derive the average mobility of bothpositive and negative ions. The mobility of negative ions was found toagree very well with Blanc's law at small fields, thereby indicating that themajority negative ion in the avalanche is SF6-. The critical field strengthwas found to vary almost linearly with the SF6 content in the mixture,comparing well with previous measurements.

Mechanisms controlling the transition from glow silent discharge to streamer discharge in nitrogen

Low-energy dielectric-barrier controlled discharges in nitrogen are studied by undertaking electrical measurements to determine mechanisms controlling the transition from glow to streamer-like discharge. The highest and the lowest values of the frequency and the amplitude of power supply voltage leading to a glow discharge have been found dependent on the gas flow and the nature of the surface in contact with the discharge. These boundary values have been related to the criteria necessary for initiating a Townsend breakdown rather than a streamer breakdown commonly observed under such conditions. This implies: (1) that the seed electron density just before the breakdown is high enough to allow the development of numerous small avalanches under a low field avoiding the formation of only one large avalanche mechanism at the origin of the streamer formation; and (2) to let the time for ions issued from the first avalanches to reach the cathode before the electrical field becomes large enough to induce the formation of large avalanches. Practically, the transition from a Townsend breakdown to a streamer breakdown is analyzed from electrical measurements data coupled to the visual aspect of the discharge. Without any gas flow, the obtaining of an atmospheric pressure glow discharge (APGD) is mainly limited by the species etched from the surface in contact with the gas. Indeed, these species can be quenchers of the nitrogen metastable molecules, which are the species at the origin of the formation of seed electrons via the Penning effect. This limitation can be overcome by the use of a laminar gas flow. However, this type of gas flow through the discharge induces a depletion of N/sub 2/ metastables and, consequently, influences the electron density at the entrance of the discharge, leading to a tendency on this part of the discharge to transit to a streamer-like one.

Development of a superthermal electron avalanche and tearing mode suppression during electron cyclotron resonant heating

The critical velocity, for electron acceleration up to the superthermal region, is expressed in terms of the heating power per particle, instead of the inductively applied electric field. This expression is used to evaluate the superthermal electron production rate in a superthermal electron avalanche during electron cyclotron resonant heating (ECRH). The comparison between the predicted growth time of the superthermal electron avalanche and the experimental decay time of tearing modes provides an additional confirmation of the recent theory for the suppression of magnetohydrodynamic modes by superthermal electrons during ECRH.