Townsend (unit)

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

${\displaystyle 1\,{\rm {Td}}=10^{-21}\,{\rm {V\cdot m^{2}}}=10^{-17}\,{\rm {V\cdot cm^{2}}}.}$

For example, an electric field of

${\displaystyle E=2.5\cdot 10^{4}\,{\rm {V/m}}}$

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

${\displaystyle N=2.5\cdot 10^{25}\,{\rm {m^{-3}}}}$

gives

${\displaystyle E/N=10^{-21}\,{\rm {V\cdot m^{2}}}}$,

which corresponds to ${\displaystyle 1\,{\rm {Td}}}$.

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 ${\displaystyle E/N}$ over broad range of ${\displaystyle E}$ and ${\displaystyle N}$.

The concentration ${\displaystyle N}$, which is in ideal gas simply related to pressure and temperature, controls the mean free path and collision frequency. The electric field ${\displaystyle E}$ 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.