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 defined by the relation

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

For example, an electric field of E = 2.5 \cdot 10^{4} \, {\rm V/m} in a medium with density N = 2.5 \cdot 10^{25} \, {\rm m^{-3}} gives E/N = 10^{-21} \, {\rm V \cdot m^{2}}, which corresponds to 1 \, {\rm Td}.

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.

It is named after John Sealy Townsend.

See also[edit]