Investigation Of High-efficiency Quasi-optical Mode Converter For Gyrotron Oscillator

Gyrotrons are one kind of the electron cyclotron maser(ECM) devices. Gyrotrons fill the sub-millimeter gap between the infrared lasers and the conventional microwave tubes, thus providing EM beams with high average output power(1 MW and more) and long operation duration. Gyrotron technology is driven by fusion plasma heating, millimeter wave directed energy weapon, advanced material processing and biomedicine, etc. The mode converter design is a primary research area, particularly for application in gyrotrons operating at higher order waveguide modes. Internal quasi-optical(QO) mode converters transform the high-order waveguide mode of a gyrotron cavity into a quasi-Gaussian beam suitable for free-space propagation. In terms of the QO mode converter design,high efficiency is important not only to minimize the prime power required for the operation of high power gyrotrons, such as those used for ITER, but also to guarantee satisfactory operation over the lifespan of the device. Given the said advantages, Denisov-type QO mode converter is helpful for improving the overall gyrotron efficiency. Foreign research institutions have made great achievements in the study of quasi-optical mode converter, but the research level is still relatively backward in China. This dissertation devotes to systematically study the basic principles and synthesis of Denisov-type QO mode converter for gyrotron oscillator. The main works of the dissertation are as follows:1. The principle of Denisov-type quasi-optical mode converter is investigated in detail. The process of Gaussian bundle's generation is analyzed through expanding the Gaussian function. The concept of closed mode is introduced. Then, the method for selecting the satellite modes is discussed in detail, together with the design method of perturbation structure.2. A simulation code for Denisov-type quasi-optical mode converter GQOMC-D is devoloped based on coupled mode theory, vector diffraction integration and physical optics, which is compared to the design parameters and experimental results reported in literature for its validity.3. According to the simulation code and the design method provided in this dissertation, a Denisov-type quasi-optical mode converter used in 95 GHz TE6,2 mode gyrotron oscillators is designed. A phase correction code based on K-S algorithm is investigated and used to modify the shape of last mirror. Simulation results indicate that a Gaussian-like beam is obtained at the output window with the scalar content of 99.12% and the conversion efficiency of 94.08%.4. A Denisov-type quasi-optical mode converter dedicated to the 110 GHz TE22,6 mode gyrotron oscillator is designed. The newly-designed converter mainly consists of a Denisov-type launcher and four reflectors. The simulation results show that the fundamental Gaussian mode scalar and vector content of output beam reaches 98% and 94%, the diffraction loss of the system is below 3%.