Townsend (unit)

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The townsend (symbol Td) is a physical unit of the reduced electric field (ratio E/N), where $E$ is electric field and $N$ is concentration of neutral particles.

It is named after John Sealy Townsend, who conducted early research into gas ionisation.

It is defined by the relation $1\ {\text{Td}}=10^{-21}\ {\text{V}}\cdot {\text{m}}^{2}=10^{-17}\ {\text{V}}\cdot {\text{cm}}^{2}.$

For example, an electric field of $E=2.5\cdot 10^{4}\ {\text{V}}/{\text{m}}$ in a medium with the density of an ideal gas at 1 atm and 0 °C, the Loschmidt constant $n_{0}=2.6867811\cdot 10^{25}\ {\text{m}}^{-3}$ gives $E/n_{0}\approx 10^{-21}\ {\text{V}}\cdot {\text{m}}^{2},$ which corresponds to 1 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 $E/N$ 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.

See also

References

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