Townsend ionization coefficient

In order to achieve breakdown, electrons (either from the air or from the body) must be accelerated to a sufficient velocity to ionize the air and breed more electrons by any one of several processes. In an actual gas, however, some of the kinetic energy of the electrons is lost in collisions with air molecules without resulting in ionization. This combined effect has been expressed in terms of the Townsend ionization coefficient. As a body becomes smaller, its curvature increases and the electric field intensity drops off more rapidly with distance from the surface consequently, to accelerate electrons a given amount, the body surface field intensity must be higher than for a flat surface. Actually, because of increased attenuation resulting from the increased distance that an electron must travel through air to achieve a given acceleration, the required surface intensity must increase even faster. 

In Fig. 5, the relationship between the Townsend ionization coefficients (ion pair production) for air at atmospheric pressure at different temperatures and field strengths is given. 

FIG. 25. Simulation results for quasi-Townsend discharges (a) ionization coefficients for SiHa, H2, and Ar (b) attachment coefficients for SiHa and CFa-... 

No attempts to calculate those limiting surface field intensities or gradients for small particles have been reported in the literature. However, point-to-plane corona has been studied in detail. Loeb (L8) has proposed a method for calculating positive corona threshold limits making use of established values of the Townsend first ionization coefficient. Loeb s threshold formula is... 

The disadvantage of the fluid model is that no kinetic information is obtained. Also, transport (diffusion, mobility) and rate coefficients (ionization, attachment) are needed, which can only be obtained from experiments or from kinetic calculations in simpler settings (e.g. Townsend discharges). Experimental data on... 

For Reaction R4 the rate coefficient Pxx+A computed from Formulae 2.1 and 10 agrees closely with the values found from experiment (usually denoted as a/n or a/p) and long known as the Townsend coefficient of ionization (24). For Reaction R5 and for similar reactions generating the radiative species H2 (a 3V) and N2 (C 3nu), a similar concordance is found between experiment and the predictions of theory (16, 58, 59, 61, 74). In discussing possible interpretations of Kirkby s data for positive column reaction, it is assumed that the Maxwellian form for /(E) is a valid approximation to the true distribution. 

Townsend coefficient - In a radiation counter, the number of ionizing collisions by an electron per unit path length in the direction of an applied electric field. 

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... 

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