In electromagnetism, the Townsend discharge or Townsend avalanche is an ionisation process for gases where free electrons are accelerated by an electric field, collide with gas molecules, and consequently free additional electrons. Those electrons are in turn accelerated and free additional electrons. The result is an avalanche multiplication that permits significantly increased electrical conduction through the gas. The discharge requires a source of free electrons and a significant electric field; without both, the phenomenon does not occur.

The Townsend discharge is named after John Sealy Townsend, who discovered the fundamental ionisation mechanism by his work circa 1897 at the Cavendish Laboratory, Cambridge.

The avalanche occurs in a gaseous medium that can be ionised (such as air). The electric field and the mean free path of the electron must allow free electrons to acquire an energy level (velocity) that can cause impact ionisation. If the electric field is too small, then the electrons do not acquire enough energy. If the mean free path is too short, then the electron gives up its acquired energy in a series of non-ionising collisions. If the mean free path is too long, then the electron reaches the anode before colliding with another molecule.

The avalanche mechanism is shown in the accompanying diagram. The electric field is applied across a gaseous medium; initial ions are created with ionising radiation (for example, cosmic rays). An original ionisation event produces an ion pair; the positive ion accelerates towards the cathode while the free electron accelerates towards the anode. If the electric field is strong enough, then the free electron can gain sufficient velocity (energy) to liberate another electron when it next collides with a molecule. The two free electrons then travel towards the anode and gain sufficient energy from the electric field to cause further impact ionisations, and so on. This process is effectively a chain reaction that generates free electrons. Initially, the number of collisions grows exponentially, but eventually, this relationship will break down—the limit to the multiplication in an electron avalanche is known as the Raether limit.

The Townsend avalanche can have a large range of current densities. In common gas-filled tubes, such as those used as gaseous ionisation detectors, magnitudes of currents flowing during this process can range from about 10⁻¹⁸ to 10⁻⁵ amperes.^{[citation needed]}

Townsend's early experimental apparatus consisted of planar parallel plates forming two sides of a chamber filled with a gas. A direct-current high-voltage source was connected between the plates, the lower-voltage plate being the cathode and the upper-voltage the anode. He forced the cathode to emit electrons using the photoelectric effect by irradiating it with x-rays, and he found that the current flowing through the chamber depended on the electric field between the plates. However, this current showed an exponential increase as the plate gaps became small^{[disputed – discuss]}, leading to the conclusion that the gas ions were multiplying as they moved between the plates due to the high electric field.

Townsend observed currents varying exponentially over ten or more orders of magnitude with a constant applied voltage when the distance between the plates was varied. He also discovered that gas pressure influenced conduction: he was able to generate ions in gases at low pressure with a much lower voltage than that required to generate a spark. This observation overturned conventional thinking about the amount of current that an irradiated gas could conduct.

The experimental data obtained from his experiments are described by the formula