hotfilament

Hot filament is a thermionic filament used to emit electrons by thermionic emission when heated to extreme temperatures. It is a key component in many electron-emission devices, most notably vacuum tubes, cathode-ray devices, electron microscopes, and some X-ray tubes. In common usage, the term is more often described as a heated filament or thermionic cathode, rather than the filament itself, but "hot filament" is encountered in historical and technical contexts.

Most hot filaments are made from tungsten wire, sometimes doped with thorium or coated with oxides to improve electron emission. Filaments are heated either directly by passing current through the filament (directly heated) or indirectly by a separate heater coil that transfers heat (indirectly heated cathodes). Direct heating reaches high emission at the cost of shorter life due to evaporation and damage; indirect heating yields longer life but lower emission density.

Operation involves maintaining a stable temperature to achieve sufficient electron emission while avoiding rapid evaporation or burnout. Filament life depends on current, temperature, design, and vacuum quality; lifetimes range from several hundred to several thousand hours in typical equipment. Evaporation of tungsten and oxide emission material gradually darkens the filament, reducing performance.

With the rise of solid-state electronics, the use of hot filaments in consumer devices declined, but they remain essential in some high-power radiography tubes, electron microscopes, and specialized RF tubes. In modern contexts, the term "hot filament" is less common than simply "thermionic filament" or "hot cathode." In 3D printing, the term is not standard; the plastic filament is heated in the hot end to melt and extrude, rather than the filament itself being heated to emit electrons.