A
streamer discharge, also known as
filamentary discharge, is a type of transient
electrical discharge. Streamer discharges can form when an insulating medium (for example air) is exposed to a large
potential difference. When the
electric field created by the applied voltage is sufficiently large, accelerated
electrons strike air
molecules with enough energy to knock other electrons off them,
ionizing them, and the freed electrons go on to strike more molecules in a chain reaction. These
electron avalanches (Townsend discharges) create ionized, electrically conductive regions in the air near the electrode creating the electric field. The
space charge created by the electron avalanches gives rise to an additional electric field. This field can enhance the growth of new avalanches in a particular direction. Then the
ionized region grows quickly in that direction, forming a finger-like discharge called a
streamer.