To reduce the amount of heat generated by a heater so as to improve durability by providing a specular metallic thin film in a surface opposite the cathode of a first electrode adjacent to a negative electrode so as to be larger than the electron radiation surface of the negative electrode.
A specular metallic thin film is composed of a vacuum-deposited film of metal of at least one kind selected from aluminum, gold, silver, copper and rhodium. The specular metallic thin film 13 is provided in a surface opposite the cathode K of a first electrode G1 positioned adjacent to the cathode K. Even if heat emitted from the cathode K heated by the heater H or from a heater H is received, this heat is reflected, a change in a position relation between the cathode K and the first electrode G1 is reduced and the amount of power consumed by the heater H necessary for holding the cathode H in a given temperature is reduced. Since the metallic thin film 13 is formed larger in area than the electron radiation surface of the cathode K, heat emitted from the cathode K or the heater H is effectively reflected and the amount of heat generated by the heater H necessary for holding the cathode K in the given temperature is reduced.
ONO OSAMU
SATO KAZUNORI
