The Townsend discharge is a gas ionization process where an initially very small amount of free electrons, accelerated by a sufficiently strong electric field, give rise to electrical conduction through a gas by avalanche multiplication: when the number of free charges drops or the electric field weakens, the phenomena ceases. It is a process characterized by very low current densities: in common gas filled tubes, typical magnitude of currents flowing during this process range from about A to about A, while applied voltages are "almost constant". Subsequent transition to ionisation processes of dark discharge, glow discharge, and finally to arc discharge are driven by increasing current densities: in all these discharge regimes, the basic mechanism of conduction is avalanche breakdown. Townsend discharge is named after John Sealy Townsend.
Quantitative description of the phenomenon
The basic setup of the experiments investigating ionization discharges in gases consist of a planar parallel plate capacitor filled with a gas and a continuous current high voltage source connected between its terminals: the terminal at the lower voltage potential is named cathode while the other is named anode. Forcing the cathode to emit electrons (eg. by irradiating it with a X-ray source), Townsend found that the current flowing into the capacitor depends on the electric field between the plates in such a way that gas ions seems to multilply as they moved between them. He observed currents varying over ten or more orders of magnitude while the applied voltage was virtually constant: the experimental data obtained from his (and his school's) first experiments are described by the following formula
:
where
* is the current flowing in the device,
* is the photoelectric current generated at the cathode surface,
* is the
* is the "first Townsend ionisation coefficient", expressing the number of ion pairs generated per unit length (e.g. meter) by a negative ion (anion) moving from cathode to anode,
* is the distance between the plates of the device.The almost constant voltage between the plates is equal to the breakdown voltage needed to create a self-sustaining avalanche: it "decreases" when the current reaches the glow discharge regime. Subsequent experiments revealed that the current rises faster than predicted by the above formula as the distance increases: two different effects were considered in order to explain the physics of the phenomenon and to be able to do a precise quantitative calculation.
Gas ionisation caused by motion of positive ions
Townsend put forward the natural hypothesis that "also positive ions produce ion pairs", introducing a coefficient expressing the number of ion pairs generated per unit length by a positive ion (cation) moving from cathode to anode. The following formula was found
:
since