Townsend (unit)

The Townsend (symbol Td) is a physical unit of the reduced electric field (ratio E/N), where formula_1 is electric field and formula_2 is concentration of neutral particles.
It is named after John Sealy Townsend, who conducted early research into gas ionisation.
Definition.
It is defined by the relation
formula_3
For example, an electric field of
formula_4
in a medium with the density of an ideal gas at 1 atm, the Loschmidt constant
formula_5
gives
formula_6,
which corresponds to formula_7.
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 formula_8 over broad range of formula_1 and formula_2.
The concentration formula_2, which is in ideal gas simply related to pressure and temperature, controls the mean free path and collision frequency. The electric field formula_1 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".