First Townsend ionization coefficient

Description

The Townsend discharge is a gas ionization process where free electrons, accelerated by a sufficiently strong electric field, give rise to electrical conduction through a gas by avalanche multiplication caused by the ionization of molecules by ion impact. The avalanche is a cascade reaction involving electrons in a region with a sufficiently high electric field in a gaseous medium that can be ionized, such as air. Following an original ionization event, due to such as ionizing radiation, the positive ion drifts towards the cathode, while the free electron drifts towards the anode of the device. If the electric field is strong enough, the free electron gains sufficient energy to liberate a further electron when it next collides with another molecule. The two free electrons then travel towards the anode and gain sufficient energy from the electric field to cause impact ionization when the next collisions occur; and so on. This is effectively a chain reaction of electron generation, and is dependent on the free electrons gaining sufficient energy between collisions to sustain the avalanche. Townsend’s experiment consists of planar parallel plates forming two sides of a chamber filled with a gas. A direct current high voltage source is connected between the plates, the lower voltage plate being the cathode while the other is the anode. Forcing the cathode to emit electrons using the photoelectric effect, the current flowing through the chamber depends on the electric field between the plates in such a way that gas ions seemed to multiply as they moved between them. A Townsend discharge can be sustained only over a limited range of gas pressure and electric field intensity. The first Townsend ionization coefficient, expresses the number of ion pairs generated per unit length by a negative ion (anion) moving from the cathode to the anode.