Townsend (unit) explained

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.

Definition

It is defined by the relation

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

For example, an electric field of

E = 2 . 5 ⋅ 10 ^{4} {\rm V/m}

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

n _{0} = 2 . 6867811 ⋅ 10 ^{25} {\rm m ^{-3} }

gives

E/n _{0} ≈ 10 ^{-21} {\rm V ⋅ m ^{2} },

which corresponds to

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

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

• Electric glow discharge
• Vacuum arc

References

• A Bankovic´, S Dujko, R D White, J P Marler, S J Buckman, S Marjanovic´, G Malovic´, G Garc´ıa and Z Lj Petrovic, Positron transport in water vapour. 2012 New J. Phys. 14 035003.