PURPOSE:To stabilize the cathode temperature and achieve the long pulsation of the captioned unit or its continuouswave operation by providing a hole at the tip of an electron gun electrode and using a base section as an ion collector with a radiating blade. CONSTITUTION:In an electron gun assembly body 2, an electron gun electrode 10 containing an annular cathode 8, and heater 9, and an annular anode 11 are assembled together coaxially with a tube axis 8. Besides, a hole 12 is provided at the tip of the electron gun electrode 10. Further, the base section of the electron gun electrode 10 is used as an ion collector 13 and a radiating blade 14 is provided at its outside. Thus, the ion reaching the tip of the electron gun electrode 10 passes through the hole 12 and caught by the ion collector 13. Since heat of this caught ion bombardment is diffused externally through the radiating blade 14, cathode temperature can be stabilized.
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