Townsend (unit)

The Townsend (symbol Td) is a physical unit of the reduced electric field (ratio E/N), where is electric field and 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

For example, an electric field of

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

gives

,

which corresponds to .

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 over broad range of and .

The concentration , which is in ideal gas simply related to pressure and temperature, controls the mean free path and collision frequency. The electric field 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.

See also

References


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