Physics:Townsend (unit)
The townsend (symbol Td) is a physical unit of the reduced electric field (ratio E/N), where [math]\displaystyle{ E }[/math] is electric field and [math]\displaystyle{ N }[/math] 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
[math]\displaystyle{ 1 \, {\rm Td} = 10^{-21} \, {\rm V\cdot m^2} = 10^{-17} \, {\rm V\cdot cm^2}. }[/math]
For example, an electric field of
[math]\displaystyle{ E = 2.5 \cdot 10^{4} \, {\rm V/m} }[/math]
in a medium with the density of an ideal gas at 1 atm, the Loschmidt constant
[math]\displaystyle{ n_0 = 2.6867811 \cdot 10^{25} \, {\rm m^{-3}} }[/math]
gives
[math]\displaystyle{ E/n_0 \approx 10^{-21} \, {\rm V \cdot m^{2}} }[/math],
which corresponds to [math]\displaystyle{ 1 \, {\rm Td} }[/math].
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 [math]\displaystyle{ E/N }[/math] over broad range of [math]\displaystyle{ E }[/math] and [math]\displaystyle{ N }[/math].
The concentration [math]\displaystyle{ N }[/math], which is in ideal gas simply related to pressure and temperature, controls the mean free path and collision frequency. The electric field [math]\displaystyle{ E }[/math] 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.
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