Research On High Power Millimeter Wave Quasi-optical Devices

The high power millimeter wave has a variety of application in electron cyclotron resonance heating of plasma, directed-energy weapon, and material processing. The traditional guiding wave technology is difficult to meet the requirement of these application systems due to its limimted power capacity, transimission efficiency, and mode purity. The quasi-optical technology is used to characterize methods and tools, devised for handing electromagnetic wave propagating in the form of directive beam, whose width is greater than the wavelength, which provides a new approach for the transmission of the high-power millimeter wave. Therefore, research on high power millimeter wave quasi-optical transmission devieces contribute to take full advantage of high power source.Based on several scientific research items, this dissertation presents thorough and careful monographic studies on the related technical issues with the high power transmission line such as the quasi-optical mode converters, miter bend, and resonant diplexer. The main contents and results as follows:1. Combined with the geometric optical description and the microwave engineering technology, the traditional method for synthesis the Denisov launcher is analysised and improved by introducing the phase-rematching technology, which enhance the ability to generate the satellite modes of the relative low-order modes. Besides, a new equivalent current integration method is proposed to analysis the radiation performance of the launcher efficiently. A 94 GHz TE5,3 quais-optical mode launcher based on the technology mentioned above is synthesized. The numerical calculation shows a well-focused field at the aperture with a scalar Gaussian mode content of more than 98% is obtained.2. The mirror system of the quasi-optical mode converter is investigated thoroughly using the vector diffraction theory. An optimization algorithm based on the Katsenelenbaum-Semenov algorithm for the phase-corrected mirrors is developed. Then a mirror system is designed to match 94 GHz TE5,3 quasi-optical mode launcher. Finally, a well-focused field at the output window of gyrotron with the waist radius of 12 mm is obtained. The scalar Gaussian mode content of the beam is 99.52%,and the vector Gaussian mode content of the beam is 95.93%. For low power measurements on the quasi-optical mode converter, the mode generators for the working modes with high purity are studied. A circularly polarized TE5,3 mode generator for 94 GHz quasi-optical mode converter are designed, fabricated, and measured. The performance of the quasi-optical mode converter is investigated using the proposed mode generator, and a salar Gaussian mode content in the window plane more than 98% has been achieved.3. The propagation characteristics of Gaussian beam and mixture waveguide modes are studied systematically, and a design method of miter bend for arbitrary TE0m( m ?1) mode is presented. According to the proposed method, the TE0m(m ?1) mode is converted into a mixture modes, show a similar propagation characteristics with the Gaussian beam, which provides properly focusing to locally decreasing the diffraction losses at the edges of the mirror that inevitably exists in any miter bend design. An adaptive design method based on the mode matching technique and the Genetic Algorithm is introduced to synthesize the mode converter to obtain the required mixture modes. A miter bend for the TE0,1 mode capable of transmitting hundreds kilowatts power levels at Ka-band is designed, fabricated, and measured. The results show that, from 29 GHz to 31.5 GHz, the miter bend can transmit the TE0,1 mode with high efficiency without arcing when the input beam with a peak power 130 kw, and a average power 10 kw.4. The transmission characteristics of the resonant diplexer, determined by the round-trip length and the coupling efficiency, are deduced using the travelling wave method, which offer a direct and novel approach to research the resonant diplexer. The mode purification characteristics, Q-factors, and the internal loss are also discussed in theory. A 170 GHz quasi-optical resonant diplexer consisting of two phase inversion mirrors and two planar phase gratings, which is compatible with electron cyclotron resonance heating for the international experimental tokamak reactor, is designed and fabricated. In order to make the structure of the diplexer simple, the LP01 beam is used directly without being converted into a Gaussian-beam. The beam propagation inside of the diplexer is presented and the working principles and final results of each component(the matching/resonant mirror, the diffraction grating, and the grating coupler) are discussed in details.5. The performance of the 170 GHz quasi-optical resonant diplexer and its potential application are investigated experimentally. Three measurement methods(S-parameter measurement, calorimetry measurement, and resonator method) are applied to obtain the transmission characteristics of the resonant diplexer, and the results show a transmission efficiency of the resonant channel and the non-resonant channel of 0.95 and 0.97, respectively. The mode output and mode filtering of the resonant channel is investigated by changing the input modes, and the average wrong mode suppression for low-order modes is 8 dB, and for the very-high-order modes reach to 17.5 dB. The isolation of the diplexer is tested by measuring the power of the LP01 mode in the isolated port for different loads at the outputs, and the results shows that the port isolation is better than 40 dB for the multi-mode case, and is at least reach 60 dB when the output port is matched The obtained results demonstrate the possibility of reducing the complexity of large ECRH systems, filtering the wrong modes in the transmission line system, as well as receiving electron cyclotron emission radiation from the plasma by sharing the launcher with the transmitted electron cyclotron resonance heating.