Thermionic cathode heated by electron bombardment

A structure wherein a cathode, which is made of a material, such as lanthanum hexa-borides, prone to react with metals at high temperatures and having a high electron emissivity, is held by a supporter which is made of an electrically insulating material. Concentric metal cylinders are arranged at the outer circumference of the supporter to surround a part of the cathode and a heating coil is arranged in the interstice between the metal cylinders. An electron-emissive metal oxide layer is formed on the inside surface of the inner metal cylinder. Thermions created from the oxide layer strike the cathode, and the cathode is heated by the heat of the electron bombardment.