The heaters are used for getting required operating temperatures of cathodes and getters. The heaters performing heat transfer by radiation are most widespread. They consist of a wire made of refractory metal protected from a co-touch with the cathode body or a getter by ceramic insulation. The power emitted by passing an electric current through the wire is transferred by radiation to the cathode or getter. The operating temperature of the heater can reach 1600°C.

More info here: