Spiral cathodes for microwave heating magnetrons

Cathodes have been developed for magnetrons for microwave heating using thermally stable alloys of an activator metal and a platinum group metal as a surface layer on the cathode. Metal alloy cathodes (MAC) utilizing Ir5La or Re2Th were tested and shown to be capable of operating at a lower temperature than carburized thoriated tungsten or pure tungsten cathodes. An alternative cathode configuration was developed and tested in which a spiral wound cathode had an additional source of activator metal installed as a bushing on a rod within the hollow interior of the winding and heated by radiation. When the spiral is heated the active metal evaporating from the source initially is adsorbed on the inner surface of the spiral and then migrates to the external surface via surface diffusion. Three variations of this cathode configuration were tested. At T = 1000°Cbrightness and with a collector current of 2 to 3 A/cm2 the cathode life without replenishment is 3000 h and the provision of an activator replenishment source was found to permit a potential severalfold increase in life. A 6000 h operating life has been achieved to date and the life tests are presently still being continued.