PURPOSE: To catch all electron beam traveling at the collector section reliably by bending in the direction of collector electrode wall while to prevent concentric overheating of collector wall by providing a rotary field generator for forming rotary field crossing with axis while rotatable in circumferential direction at the collector section.

CONSTITUTION: A rotary field generator 19 is arranged at the outercircumference at a portion of collector section 14 to produce field H in the interior of collector in the direction perpendicular with the central axis while rotatable in the innercircumferential direction as shown by arrow. It is preferable to have a solenoid coil to be driven by A.C. power source 20 and controller 21 to produce alternative fields in X and Y directions having phase difference by 90° thus to produce rotary field. It is arranged near to the cavity section 13 in such range that the rotary field will scarcely influence on the resonance cavity section 13. In operation in case of pulse operation gyrotron, it must be considered that pulse frequency and rotary frequency of field will be synchronous to concentrate at a portion of collector wall in circumferential direction to prevent impingement of electron.