PURPOSE: To specify precisely the distance between cathode and anode portions by providing an exhaust pipe over or parallel to a rotating shaft in such a way that the anode portion can be seen through it from outside an envelope.

CONSTITUTION: A cathode 1 is opposed to an anode 2, is supported against a holder 5 (2), and secured to a stem 7 via a hood 6. The anode 2 and a rotating shaft 8 are secured to each other via a nut 9, and the shaft 8 is supported against a bearing 10 and rotates at high speed during operation. The stem 7 is provided with an exhaust pipe 11, and the pipe 11 is provided in such a way that the nut 9 of an anode portion B can be seen from outside through the hood 6 and the holder 5 (2). The anode portion B and a cathode portion A held on respective holders 14, 15 are designed so that when their mechanical dimensions are known in advance, the distance between anode and cathode can easily be adjusted by the sliding of the holders on a base. Also, when the mechanical dimensions of the anode and cathode portions and the positions of the holders on the base can be measured by the application of laser beams from a laser optometry portion 16, the anode-to-cathode distance can easily be set.