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 in an electric field that is sufficient to cause field electron emission.
This process can create an incandescent cathode spot, which frees more particles, thereby sustaining the arc.
TVA discharges can be ignited in high-vacuum conditions between a heated cathode (electron gun) and an anode (tungsten crucible) containing the material.
The accelerated electron beam, incident on the anode, heats the crucible, together with its contents, to a high temperature.
After establishing a steady-state density of the evaporating anode material atoms, and when the voltage applied is high enough, a bright discharge is ignited between the electrodes.