Basic Processes of Electrical Discharges

Abstract

Eighty years ago Townsend observed that the current flowing between parallel plate electrodes, when a potential difference was applied between them and when a small number of electrons were released from the cathode, varied with the applied electric field E as shown in Fig. 1. He recognized that an important parameter was the ratio E/p of the field strength and gas pressure, p, and introduced the parameter α which became known as the “Townsend primary ionization coefficient.” α describes the production of electron-positive ion pairs by collisions between electrons and molecules in the discharge gap. If this is the only collision process operating, then the electron current in the gap increases with distance ℓ from the cathode according to the equation

$${i_\ell } = {i_0}{\kern 1pt} \exp \left( {a\ell } \right)$$

((1))

In practice, the increase in current is often greater than that predicted in Eq. (1) and is better described by an equation of the form

$${i_\ell } = \frac{{{i_0}{\kern 1pt} \exp \left( {\alpha \ell } \right)}}{{1 - \left( {\omega /\alpha } \right)\,\left\{ {\exp \left( {\alpha \ell } \right) - 1} \right\}}}$$

((2))

where ω/α is the so-called “generalized secondary ionization coefficient.” This secondary coefficient represents the net effect of a number of possible collision processes occurring either in the discharge gap or at the cathode.