140-GHZ GYROTRON EXPERIMENTS BASED ON A CONFOCAL CAVITY

We have designed and experimentally demonstrated the operation of a no vel quasioptical gyrotron oscillator based on an overmoded confocal wa veguide cavity, This cavity effectively suppresses undesired modes, an d therefore has extremely low mode density. Stable single-mode, single -frequency operation was achieved in the TE06 mode at 136 GHz, A peak RF output power of 66 kW, corresponding to an efficiency of 18%, was m easured. By varying the cavity magnetic field, high-power generation w as observed at 136 GHz in the TE06 mode and at 114 GHz in the TE05 mod e, These frequencies correspond to the high Q modes of the confocal re sonator, The low Q modes were either weak or not observed. In this pap er, we will review the design procedure for this cavity and present ex perimental data verifying its effectiveness in reducing the number of modes that can be excited. The confocal waveguide could also be used i n high-power, gyro-TWT amplifiers to provide greater operating stabili ty and bandwidth, especially in an overmoded waveguide structure.