Design of a high voltage multi-cavity 35 GHz phase-locked gyrotron oscillator

This paper describes the design for an experimental high power, phase locked gyrotron oscillator. The electron beam is generated by a 1 MV pulseline accelerator, and the reference signal is provided by a 35 GHz, 20 kW magnetron. The expected output power is in the range of 1 to 10 MW. The design is based on a solid 1 MeV, 100 Amp, 4 mm electron beam with a momentum pitch ratio of 0.75. The locking signal from the magnetron is introduced via a prebunching cavity. A second (passive) bunching cavity is used to increase the locking frequency bandwidth obtainable with a given locking power. The bunching cavities are designed to operate in the fundamental TE 111 cylindrical cavity mode. Some competition from the TE 112 higher order axial mode could not be avoided due to the constraint on the minimum drift tube diameter set by the requirement to propagate the electron beam. The bunching cavities include two axial slots to control the cavity Q factor and suppress competing modes. Additional slots and apertures are used to suppress oscillation in the drift spaces. The output cavity operates in the TE sub 121 mode and is also slotted to reduce competing mode excitation. The maximum phase-locking bandwidth is estimated to be 0.1 percent and the time to achieve phase locked operation is about 20 nsec which is consistent with the pulselength of the NRL VEBA accelerator.