Townsend avalanche (Townsend discharge)


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.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.

Related formulas


IThe current flowing in the device (A)
I0The photoelectric current generated at the cathode surface, (A)
anThe first Townsend ionization coefficient (m-1)
dThe distance between the plates of the device (m)