THEORY OF A 2-CAVITY GYROTRON FREQUENCY-MULTIPLIER

A frequency multiplying device is analyzed that provides power at selected cyclotron harmonics in a quasioptical gyrotron. The device utilizes a prebunching cavity and an uptapered magnetic field that impose a double-resonance condition on the radiation generated. This selects among cyclotron harmonics in the output cavity even though the cavity structure may support all harmonics. The prebunching is accomplished by modulation of electron orbital magnetic moments by a low-frequency-input rf field. Two design examples are given for quasioptical gyrotron frequency multiplier operation at the third cyclotron harmonic (four or seven times the input frequency) with a 10-15 % electronic efficiency and greater than 33% overall rf conversion efficiency. The drawback of this method of prebunching is that high-input electric-field intensities are required for efficient operation. However, it is shown that the double-resonance method allows complete harmonic selection in the linear regime for zero detuning between the cyclotron and field frequency. When this detuning is nonzero, the selected harmonic still has a higher linear efficiency than all others.