Advanced Overmoded Circuits for Gyro-Amplifiers

Abstract

To solve the narrow-bandwidth problem associated with cavity-related gyro-amplifiers, a new interaction circuit, containing clustered cavities is considered. In particular, the use of a cluster of cavities in frequency multiplying gyro-amplifiers is described. An analytical theory of a simple frequency multiplying device has been developed, and compared with numerical simulations using the Maryland Gyrotron Code (MAGY). The analytical results and MAGY code simulations are in good agreement. In the small signal regime, the bandwidth of a cluster-cavity device (with two cavities in the cluster) is twice that of a single cavity device, while both have the same peak bunching. We also investigate the effect of coupling between the cavities of a cluster, and the performance of a three-cavity cluster. A four-cavity cluster has been employed as a second harmonic buncher in a new type of Ka band, three-stage, harmonic-multiplying gyro-amplifier, which consists of a fundamental gyro-TWT input and second harmonic gyro-TWT output sections. This amplifier achieved 80 kW output power centered at 33.6 GHz with a bandwidth of 0.3 %, efficiency of 16 % and gain of 36 dB in the high order TE04 mode. MAGY simulations have been carried out and compared with the experimental data. Mode competition is a principal issue in high-power gyrotron research and development. A vaned TE0n mode converter has been proven to be effective in converting one designated TE0n mode into another designated TE0m mode while suppressing unwanted modes. A quasi-analytical theory has been developed to describe the electromagnetic field in the mode converter, and different modes have been calculated. By using a mode matching technique, the nonsymmetric field was incorporated in the MAGY code. This modification is a significant extension of MAGY capabilities. The results of scattering calculations for a vaned mode converter from the modified MAGY agree with the High Frequency Structure Simulator (HFSS) simulation to with 2%. This thesis consists of the following parts: 1. a review of gyrotron oscillator and amplifier research and development; 2. the concept, theory and experimental study of cluster cavities; and, 3. a study of a vaned TE0n mode converter. It is hoped that this research will improve the understanding of gyro-amplifiers using clustered cavities and/or TE0n mode converter structures, and advance research on gyro-amplifiers.