The townsend (symbol Td) is a physical unit of the reduced electric field ( ratio E/N), where

E

electric field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...

and

N

is concentration of neutral particles. It is named after

John Sealy Townsend Sir John Sealy Edward Townsend, FRS (7 June 1868 – 16 February 1957) was an Irish-British mathematical physicist who conducted various studies concerning the electrical conduction of gases (concerning the kinetics of electrons and ions) and ...

, who conducted early research into gas ionisation.

Definition

It is defined by the relation

1 \,  = 10^ \,  = 10^ \, .

For example, an electric field of

E = 2.5 \cdot 10^ \,

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

Loschmidt constant The ''Loschmidt constant'' or Loschmidt's number (symbol: ''n''0) is the number of particles (atoms or molecules) of an ideal gas in a given volume (the number density), and usually quoted at standard temperature and pressure. The 2014 CODATA recom ...

n_0 = 2.6867811 \cdot 10^ \,

gives

E/n_0 \approx 10^ \,

, which corresponds to

1 \,

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 An ideal gas is a theoretical gas composed of many randomly moving point particles that are not subject to interparticle interactions. The ideal gas concept is useful because it obeys the ideal gas law, a simplified equation of state, and is a ...

simply related to pressure and temperature, controls the

mean free path In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as a ...

and

collision frequency Collision frequency describes the rate of collisions between two atomic or molecular species in a given volume, per unit time. In an ideal gas, assuming that the species behave like hard spheres, the collision frequency between entities of specie ...

. 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''.

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. Electrical breakdown Plasma physics {{plasma-stub

Definition

Uses

See also

References