Principles
Hot cathodes may be either directly heated, where the filament itself is the source of electrons, or indirectly heated, where the filament is electrically insulated from the cathode; this configuration minimizes the introduction of hum when the filament is energized with alternating current. The filament is most often made of tungsten. With indirectly heated cathodes, the filament is usually called the heater instead. The cathode for indirect heating is usually realized as a nickel tube which surrounds the heater.
The first cathodes consisted simply of a tungsten filament heated to white incandescence (known as bright emitters). Later cathodes are typically covered with an emissive layer, made of a material with lower work function, which emits electrons more easily than bare tungsten metal, reducing the necessary temperature and lowering the emission of metal ions. Cathodes can be made of pure sintered tungsten as well; tungsten cathodes in the shape of a parabolic mirror are used in electron beam furnaces. Thorium can be added to tungsten to increase its emissivity, due to its lower work function. Some cathodes are made of tantalum.
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