Townsend (unit)

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 $1\mathrm{T}\mathrm{d}=10^{-21}\mathrm{V}\mathrm{\cdot }{\mathrm{m}}^{\mathrm{2}}=10^{-17}\mathrm{V}\mathrm{\cdot }\mathrm{c}{\mathrm{m}}^{\mathrm{2}}.$ For example, an electric field of $E=2.5\cdot 10^4\mathrm{V}/\mathrm{m}$ in a medium with the density of an ideal gas at 1 atm, the Loschmidt constant $n_0=2.6867811\cdot 10^{25}{\mathrm{m}}^{\mathrm{-}\mathrm{3}}$ gives $E/n_0\approx 10^{-21}\mathrm{V}\mathrm{\cdot }{\mathrm{m}}^{\mathrm{2}}$, which corresponds to $1\mathrm{T}\mathrm{d}$.

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.