High Quality Content by WIKIPEDIA articles! In vacuum tubes, a hot cathode is a cathode electrode which emits electrons due to thermionic emission. In the accelerator community, these are referred to as thermionic cathodes. (Cf. cold cathodes, where field electron emission is used and which do not require heating.) The heating element is usually an electrical filament. Hot cathodes typically achieve much higher power density than cold cathodes, emitting significantly more electrons from the same surface area.