Universal Electron Gun Design for a CW Third Harmonic Gyrotron with an Operating Frequency over 1 THz

The design of a magnetron-injection gun (MIG) suitable for using in the CW 3-rd harmonic gyrotron with an output frequency of 1.185 THz and an output power about 100 W, intended for DNP/NMR spectroscopy applications is described. To improve the mode selection, several electron optics schemes are investigated. The MIG allows using both single-beam and double-beam schemes of the gyrotron operation with two different operating modes. Requirements for the electron beam parameters were found. The design of the triode MIG forming two generating beams with a quality suitable for 3-rd harmonic operation is suggested and optimized. The specific feature of the MIG is the possibility to form either two beams or one beam using the same electrode geometry and different number of emitting rings. The suggested MIG is not sensitive to small misalignment and reasonable manufacturing errors. A key feature of the design is that to improve the mode selection an electron beam with increased velocity spread is proposed. It is shown that it is possible to form two beams with practically equal and quite high pitch factors in all range of operating currents and at the same time small enough coefficient of reflection from the magnetic mirror in spite of large enough value of velocity spread.