English: Vacuum breakdown, i.e. vacuum arcing can occur when metal surfaces are subjected to high electric fields in vacuum. When a high electric field is applied between a metal cathode (bottom) and anode (top), electrons can be emitted via field emission. If a nanoprotrusion is present on the cathode surface, the electric field can be significantly enhanced. Emission of electrons causes currents in the cathode that result in resistive heating and an increase in temperature. At a large enough temperature, the atoms on the surface of the cathode can evaporate. These neutral atoms can collide with the emitted electrons via impact ionization. This results in the formation of an ionized conducting plasma between the cathode and the anode.