Study Of A Quasi Optical Mode Converter For Tereaherz Cyrotron

The terahertz band is sandwiched between the millimeter wave and infrared wave spectrums. The generation and application of the wave in such frequency range have drawn spectacular attention all around the world. Meanwhile, the gyrotrons have been proven to be one of the most promising devices to deliver high power terahertz signals. To enlarge the size of the beam-wave interaction cavity in the terahertz band where the wavelength is quite short, the devices are usually designed and operated with high-order modes. To enable efficient transmission and application of the wave, enlarge the size of the collector and enhance the output power, the high-order modes should be transformed by quasi-optical mode converter and extracted in the transverse direction. The adoption of the quasi-optical mode converter can reduce the reflection at the output window which may otherwise harm the stability of the beam-wave interaction. Thus the study of the quasi-optical mode converter for a THz gyrotron is demanding.This thesis deals with the theoretical study and experimental verification of a quasi-optical mode converter for a W-band gyrotron firstly, and then studies another quasi-optical mode converter for a 0.42 THz gyrotron operating with TE17,4 mode.The main content and contribution of this thesis can be summarized as below:1、A terahertz Denisov launcher is designed by replacing the traditional polygon perturbation with sinusoid perturbation。The Denisov launcher is the improved version of the Vlasov type, where the input wave is pre-bunched into a quasi-Gaussian beam by a section of perturbed waveguide. The traditional perturbation is usually in the form of helical corrugation where the depth of the perturbation has a polygon distribution along the propagation direction. The existence of single point at the turning point of the polygon brings difficulties to the derivation and numerical calculation of the coupling coefficients. The sinusoid perturbation adopted in this thesis exerts continuous partial derivation, which enables accurate calculation of the power distribution of modes in the waveguide and reduces the parasitic modes.2、The radiation field distribution from the launcher is calculated by Vector Diffraction Integral which agrees reasonably well with the experimental results. In a launcher designed for high order mode, the wave is radiated from the aperture formed by the helical cut. The power and efficiency of the radiation field are the key factors that affect the performance of the whole quasi-optical mode converter. Thus the field distribution at the output aperture should be accurately calculated. Although the 3D simulation software can be used, the required computational resource and time become unacceptable as the operation modes as well as frequency goes higher and higher. Vector Diffraction Integral can fulfill the requirement of the accuracy of calculation as well as reduce the occupied computational resources and time.3、The Vector Diffraction Integral and Scalar Diffraction Integral are used jointly to calculation the field distribution on the mirrors. The Scalar Diffraction Integral requires less computation and exerts high speed. But it has low accuracy as the field distribution at the edge of the mirror is neglected. The Vector requires more computation and provides high accuracy. In this thesis, the advantages of the two are combined. The Scalar Diffraction Integral is adopted to calculate the positions and sizes of the mirrors, while the Vector Diffraction Integral is used to calculate accurately the field distribution and adjust the geometric parameters.4、A W-band quasi-optical mode converter is designed for TE6,2 mode at 94 GHz. More perturbation factors are added to the traditional perturbation equation to enhance the flexibility of the structure. The axial power distribution along the section of perturbation is calculated by the mode coupling theory. The Gaussian beam parameters at the output aperture are analyzed. The influence of the rotation direction of the output wave on the efficiency and power of the quasi-optical mode converter is studied. Four mirrors are used to adapt the phase of the output field, according to the corresponding research project.A THz quasi-optical mode converter is designed for TE17,4 mode at 420 GHz. Since there will be a number of parasitic modes for a high order mode operation, these modes should be analyzed and suppressed accordingly. As the cut-off radius is very small in the terahertz band, the collector operates highly over-moded and is unsuitable to be connected directly with the Denisov launcher. Thus a section of transitional waveguide should be adopted to shrink the radius of the collector before being connected to the quasi-optical mode converter. The hot test of the quasi-optical mode converter is conducted.