CATHODES

Tungsten Cathodes

Directly heated tungsten-filament cathodes are made from the pure metal. Because tungsten filaments must operate at a high temperature (a dazzling white) to emit electrons in useful quantity, tungsten requires a relatively large amount of filament heating power; in other words; its emission efficiency is low. The large filament power dissipation requires a relatively large bulb for a fixed plate dissipation. or reduces the permissible plate dissipation for a bulb of fixed size.

Advantages of tungsten as a cathode material are its ruggedness and its ability to withstand relatively heavy positive-ion bombardment in high-voltage tubes. This bombardment, resulting from the presence of minute amounts of residual gas, is naturally more severe at higher plate voltages. Cathode materials which depend on a thin, active surface layer for their emission may quietly have this layer sputtered away by positive ions, with a resulting loss in emissivity. In the case of tungsten, the emission is an inherent property of the metal itself, so that there is no loss in emission even if some of the surface is sputtered away by positive-ion bombardment.