PURPOSE: To reduce the diameter of an anode frame body while induced reactance is being kept in a preset value by providing recesses and protrusions that are long in the axial direction and are wave-formed in the circumferential direction on the inner surface of an anode cylinder and additionally providing anode veins protruding toward cathodes at the protrusions of these recesses and protrusions.

CONSTITUTION: An anode cylinder 5 made of pure copper has recesses and protrusions 6a, 6b, 6c, 6d, ... that are long in the axial direction and are wave- formed in the circumferential direction on the inner surface and anode veins 3a, 3b, ... are additionally provided at the protrusions 6b, 6d, ... of these recesses and protrusions. Since the recesses 6a, 6c, ... are made circular and their radius is considerably smaller than the inner surface radius of the anode cylinder 5, the substantial inner surface radius r2 of the anode cylinder 5 becomes larger than the inner surface radius of the conventional anode cylinder and the substantial protruded length l3 of the anode veins 3a, 3b, ... becomes larger than the mechanical length. As a result, while induced reactance is being kept in a present value, the inner and outer surface radiuses r and R of the anode cylinder 5 and the mechanical length of the anode veins 3a, 3b, ... can be reduced.