Townsend avalanche

TOWNSEND AVALANCHE. A term used in gas-filled counter technology to describe a process which is essentially a cascade multiplication of ions. In this process an ion produces another ion by collision, and the new and original ions produce still others by further collisions, resulting finally in an avalanche of ions (or electrons). The terms cumulative ionization and cascade are also used to describe this process. It occurs in a nonself-maintained gas discharge, where ions have sufficient energy. 

Figure 18.4 Schematic illustration of the Townsend avalanche in a gas ionization device. The avalanche occurs very close to the wire in reality. (From Knoll, 2000.)...

The proportional counters are an improvement over the Geiger-Miiller counters A carrier gas (90% Ar +10% CH4 quencher gas) is ionized by the high-energy (a or />) rays in the counter tube, but at a lower bias the generated (Ar+ + e ) ion pairs are coaxed by this bias to form a Townsend avalanche of further ion pairs, whose overall intensity is proportional to the energy of the incoming a or /J rays. 

Figure 3-25. Schematic representation of the events leading to a Townsend avalanche. Filled circles are argon molecules.

If ocj/p data from the gas phase are converted into oc /N data and hypothetical oc /Niiq values are inserted in Equation 109 it turns out that extremely small distances d of the order of im are sufficient to fulfill the breakdown condition. From these considerations it is apparent that conditions characteristic of a Townsend avalanche are only met over very short distances. Therefore, measurements of require either, in the case of a parallel plate electrode geometry, a short electrode separation or confinement of the avalanche to the vicinity of a thin wire, a sharp blade, or tip. In the... 

The growth of an avalanche in a sufficiently high uniform electric field is given by Townsend s law ... 

This shows the electric field adjacent to the emitter is extremely high and it is this stress that excites any free electrons in the immediate vicinity. These fast moving electrons acquire sufficient energy from the applied electric field so as to collide with other gas molecules to produce further free electrons and positive ions. Townsend, working in this area, proposed the concept of a chain reaction or electron avalanche, in which each new electron produced generates new electrons by ionization in ever increasing numbers. 

The two electrons produced by the process axe accelerated by the electrical field and may ionise other gas molecules. An avalanche is formed. The frequency of the ionisation process is characterised by the first Townsend coefficient a. This coefficient depends on the reduced electrical field E/p, where p is the pressure of the gas. The increase dn of the n electrons after a distance dx is given by ... 

The avalanche-to-streamer transition in the APG DBD depends on the level of preionization. The Meek criterion (Section 4.1.3) is related to an isolated avalanche, whereas in the case of interrsive pre-ionizatiorr, avalanches are produced close to each other and interact. If two avalanches occm close enough, their transition to streamers can be electrostatically prevented and the discharge remains uniform (A. Fridman et al., 2005). A modified Meek criterion of the avalanche-to-streamer transition can be obtained by considering two simrrltaneously starting avalanches with maximum radius R, separated by the distance L (a is Townsend coefficient, d is distance between electrodes) ... 

Ionization by collision of pairs of excited helium atoms was first proposed by Schade on the basis of observations of the pressure dependence of additional ionization that was in excess of that due to avalanche electrons in a Townsend discharge. Further studies by Schade " and Buttner confirmed the original observations " and lead to the conclusion that the reaction... 

An electron avalanche occurs in a medium when the drifting electron attains sufficient energy from the electric field to effect collisional ionization. In dilute gases, this process is described by the first Townsend coefficient, a, which is defined as... 

In solids, electronic breakdown is observed in semiconductors (Seeger, 1973). At a sufficiently high electric field strength, electrons from the valence band can tunnel directly into the conduction band. This effect is also called internal field emission and it forms the basic principle of the tunnel diode. A necessary condition for this type of electronic breakdown is a narrow band gap (1 to 2 eV) and a high electron mobility. Avalanche breakdown takes place in the depletion layer of a reversed p-n junction where electric field strengths up to 10 V/cm are obtained. Collisional ionization by electrons (and holes) across the band gap takes place. Here we only consider the effect of the electrons. The relative increase of the number of charge carrier pairs per unit of length is called the ionization rate, a (the first Townsend coefficient of the gas phase), defined as... 

At longer gap distances, the distribution is characterized by two time lags which are summarized in Table 2. Analysis of the data with a single electron avalanche model (see Section 2.8) gave a first Townsend coefficient of a = 6.4 x 10 cm" at 3.5 MV/cm (Arii et al, 1979). This value is one order of magnitude higher than data estimated by Haidara et al., (Haidara and Denat, 1991) for cyclohexane and propane (see Section 8.2). 

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