A
vacuum arc can arise when the surfaces of
metal electrodes in contact with a good
vacuum begin to emit
electrons either through heating (
thermionic emission) or via an
electric field that is sufficient to cause
field electron emission. Once initiated, a vacuum arc can persist since the freed particles gain
kinetic energy from the electric field, heating the metal surfaces through high speed particle collisions. This process can create an incandescent
cathode spot which frees more particles, thereby sustaining the arc. At sufficiently high currents an incandescent
anode spot may also be formed.