Townsend (unit)

Definition

It is defined by the relation

1 T d = 10 − 21 V ⋅ m 2 = 10 − 17 V ⋅ c m 2 .

For example, an electric field of

E = 2.5 ⋅ 10 4 V / m

in a medium with the density of an ideal gas at 1 atm,

N = 2.5 ⋅ 10 25 m − 3

gives

E / N = 10 − 21 V ⋅ m 2 ,

which corresponds to 1 T d .

Uses

This unit is important in gas discharge physics, where it serves as scaling parameter because the mean energy of electrons (and therefore many other properties of discharge) is typically a function of E / N over broad range of E and N .

The concentration N , which is in ideal gas simply related to pressure and temperature, controls the mean free path and collision frequency. The electric field E governs the energy gained between two successive collisions.

Reduced electric field being a scaling factor effectively means, that increasing the electric field intensity E by some factor q has the same consequences as lowering gas density N by factor q.