Development Of Electron Avalanche Research Articles

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.

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.

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.

Arc Modeling

ABSTRACT: This paper is an account of how arc modeling has increased our predictive capability of properties of arc plasmas. Basic material functions of, for example, thermal and electrical conductivities, can be calculated for any plasma, given the component elements of the plasma. Then solution of conservation equations gives properties of the plasma. Examples are given of successes of arc modeling. (1) A one dimensional calculation for an arc confined in a cylindrical tube gives the principal electrical properties of an arc lamp. (2) Two dimensional calculations including the arc and the electrodes gives an explanation of the transition from globular to spray modes of metal transfer in arc welding. (3) Also predictions of plasma and electrode temperatures in tungsten arc welding are in good agreement with experiment. (4) Time dependent calculations of the development of electron avalanches in lightning, explain the development of the stepped leader.

A technique for accelerating Monte Carlo simulations of electron avalanches with applications to ionization gaseous detectors

Monte Carlo simulations are widely used for computing statistical growth processes such as the development of electron avalanches in gases, but typically require excessively long computer times. In this paper, a method based on intermediate statistics is developed and shown to give results in agreement with a full Monte Carlo treatment with a reduction by a factor of about 100 in computer time. This method could be extended, for example to the computation of the resolution of proportional counters as a function of energy and so help to resolve the discrepancy among existing experimental data. This method should be used for situations in equilibrium (in space and time).

Monte Carlo studies of electron avalanches in microdosimetric proportional counters

We present preliminary results from a study of electron avalanches in a low pressure cylindrical microdosimetric counter. The technique is based on the Monte Carlo simulation of the motion of electrons in electric field and their interactions with the molecules of gas. All the electrons are traced simultaneously in time and their coordinates and velocities are recorded. It lets us study all temporal and space aspects of the electron avalanche development. In this work we have concentrated on a study of the dependence of gain, radial extent and time extent of the avalanche on the gas pressure. We explain our results by the existence of seeding region in the avalanche where a relatively small number of secondary electrons are created. Each of these electrons gives subsequently rise to a partial avalanche in the proper avalanche region.

Simulation of corona discharge. Positive corona in SF/sub 6/

Positive coronas in SF/sub 6/ under three voltage levels, 3.0, 3.5 and 4.0 kV, are studied for a 0.5 cm gap between a hyperboloid needle and a plane electrode at a gas number density N=2.12/spl times/10/sup 18/ cm/sup 3/. The current in the form of a burst of pulses is found to increase with increasing voltage. The initiation and development of successive avalanches are traced as function of time after voltage increases. The development of electron avalanches and streamers are due to ionization and photoionization and quenching of avalanches is due to space charge. The simulation provides a detailed structure of avalanches from which essential differences between positive and negative corona with regard to total field distribution, propagation of successive avalanches and ion distribution can be discerned.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Simulation of corona discharge. Negative corona in SF/sub 6/

Negative coronas in SF/sub 6/ under three voltage levels are investigated by Monte Carlo simulation. For the purpose of simulation, the gap between a hyperboloidal needle and a plane is divided into two regions: region I where the electric field is very steep and most electrons and ions are accumulated, and region II the rest of the gap. In region I electron motion is simulated by dividing the region into a number of cells and a small cell size improves the accuracy. The magnitude of current pulse increases with increasing voltage and there appears to be more than one peak in each pulse as observed by experiment. The development of electron avalanches is due to ionization and photoionization in the high field region, while the quenching of the avalanches is due to the space charge field suppression. Also the accumulation of positive and negative ions are displayed in detail. The space charge field distortion is studied and found to increase with increasing applied voltage.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

VUV emissions in gaseous detectors

The authors describe the time evolution of VUV photons emitted in Ar/CH4 and Ar/CO2 mixtures, during the development of electron avalanches occurring in cylindrical coaxial radiation detectors. Particular attention is paid to emissions considered as relevant to the photon feedback mechanisms, namely the C I atomic lines at 156.1, 165.7 and 193.1 nm. A correlation is obtained between the time spectra and the corresponding mean anode current signals, both for the proportional and the self-quenching streamer modes of operation. A comparison is also made between the data obtained with two detectors having different cathode diameters (16 and 30 mm).

The impact ionization and electrical breakdown strength for atomic and molecular liquids

It is pointed out that ionization electron avalanche development determines the electrical breakdown strength of the high-electron-mobility liquids Ar, Kr, and Xe. The DC and laser-induced breakdown strengths of these liquids are lower than low-density gas extrapolation values. This difference is the consequence of the absence of inelastic electron energy losses in liquids. In low-electron-mobility molecular liquids the electrical breakdown for a long pulse duration occurs via the bubble mechanism, but with decreasing voltage pulse duration and gap length the ionization avalanche mechanism would become more preferable. Electric strength measurements for high-density supercritical vapors are interpreted on this basis.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The Townsend coefficient of ionization in dense gases and fluids

Contains a discussion of effects influencing the development of electron avalanche in a dense medium, such as non-binary particle collisions, interatomic correlation and lowering of the ionization potential. By allowing for those effects, the Townsend coefficient is increased by several orders of magnitude and it becomes possible to describe the recently obtained experimental data in liquid xenon.

Properties of compound poisson processes with applications in statistical physics

The concept of a compound Poisson process, which has on occasion been successfully applied in the analysis of statistical behaviour in social and biological systems, is shown to be a useful one for the treatment of certain types of random process in physics. Some properties of this class of statistical distribution are derived and illustrated by various applications including the theory of electron avalanche development and the non-steady state distribution of radiation in a cavity. If the elements of a given process form a homogeneous ensemble, then the question of whether the process is or is not a compound Poisson process is decided in general by whether the correlations in the number of elements are positive or negative. It is pointed out that an analogous distinction may be drawn between the two forms of quantum statistics.

Die Entwicklung der Elektronenlawine in den Plasmakanal, untersucht mit Bildverst�rker und Wischverschlu�

The application of high gain image intensifier and image converter streak shutter techniques to investigations of faintly luminous early stages of gaseous discharges discloses in some detail the development of electron avalanches into anode- and cathode-directed streamers as soon as the critical amplification (n≈108) is attained in the gap. The velocities of both of the streamer branches were measured quantitatively as to the dependence on pressure (p), field (E/p) and gas (Nitrogen and various mixtures), typical values being 0.5 ... 1 · 108 cm/sec. We find that streamer propagation is suitably described in a coordinate-system drifting with the electrons (driftvelocityv − ≈ 1 · 107 cm/sec) and that the way it depends on the discharge parameters suggests that ionization by electron impact (in the eigen-spacecharge distorted field) and photoionization are the determining effects. — The cathode-directed streamers show a considerable intensification and acceleration as soon as they are encountered by cathodic secondary avalanches, which however are obviously not essential for the initiation of the process in itself. — The streak images give also an impression of persisting excitation and ionization processes in the region of the streamer-paths and of the transition into the beginning of the bright spark, correlated with several ionizing fronts observed moving towards anode or cathode with velocities even >108 cm/sec. (The paper includes also details of the gaseous near UV-radiation which was utilized for registration with image intensifier or converter, especially with respect to the decaytime of the excited states.) The results obtained are essentially a verification and to some extend completion of predictions made on the basis of early cloud chamber investigations.

Die weitere Entwicklung der Elektronenlawine, untersucht mit Bildverst�rker und Wischverschlu�

By simultaneously applying image intensifier and image converter streak shutter techniques it is possible to investigate the local and temporal development of electron avalanches in a homogeneous field into the weakly ionised streamer channels. Velocities of the anode-directed and cathode-directed streamers are measured (order of cm magnitude: several 107 and approximately 108sec/cm respectively). Persisting processes in the plasmachannels are revealed and it is shown that once the electrodes are bridged by the streamer-channels the high intensity spark starts rapidly.

Mittlere Energien und Driftgeschwindigkeiten von Elektronen in Stickstoff, Argon und Xenon, ermittelt aus Bildverst�rkeraufnahmen von Elektronenlawinen

Local and temporal development of electron avalanches in a pulsed discharge gap (d=3,00 cm) are investigated in N2, Ar, Xe and mixtures of N2 and CH4 by simultaneously applying high gain image intensifier- and photomultiplier techniques. Electron drift velocities are obtained from time-of-flight and way-of-flight measurements in these gases. The mean energy of agitation of the electrons is derived both from electron mobility and avalanche image trace profile (diffusion broadening). The results obtained (for 20°C), being in fair agreement with one another, read N2: (4·6...5·0) eV forE/p=50...200 V/cm Torr; Ar: (9·0...9·5) eV forE/p=24... 45 V/cm Torr; Xe: (4·8...5·0) eV forE/p= 40... 90 V/cm Torr; CH4(10% N2): 6·3 eV forE/p= 89 V/cm Torr. The mean energy of agitation does not change very much withE/p in the ranges investigated. Some results concerning the radiation properties of these gases are included such as lifetime of the excited states, quenching pressure etc.