220 Ghz High Power Quasi-optical Mode Conversion Technology Research

The gyrotron has been received widespread attention as a very important high power millimeter and sub-millimeter wave sources. The gyrotron devices operate in low-losses high-order cavity modes, which needs to be converted into a linearly polarized fundamental Gaussian wave beam(TEM00mode) in order to free space transmission for application. In high frequency band, the inside radius of the conventional mode converters is too small to fabrication, so, that is a very effective way to employ a quasi-optical mode converter. A quasi-optical mode converter in this paper has been designed for the needs of a gyrotron with the center frequency of220GHz and the operating mode of the TE03mode, consisting of a Vlasov launcher, a quasi-elliptical cylindrical mirror and a parabolic mirror.A quasi-optical mode converter studied by the method of combining the geometrical optics theory and vector diffraction theory. Numerical calculation programs designed by MATLAB software analyze the quasi-optical mode converter. Firstly, the studying of Vlasov launcher:the stepped-cut section length of the stepped-cut Vlasov launcher is optimized by the aperture power of the stepped-cut section surface; the influence of the radiation characteristics of the bevel-cut Vlasov launcher with different bevel-cut angles are studied; comparative analyzed of the radiation characteristics between stepped-cut Vlasov launcher and bevel-cut Vlasov launcher; Verified the numerical results by the results of CST simulation software.For the mirror system of the quasi-optical mode converter, received relevant model parameters of the a quasi-elliptical cylindrical mirror and a parabolic mirror by using the method of geometrical optics theory, and then completed the numerical analysis program on the basis of the vector diffraction theory to analyze the quasi-optical mode converter (including stepped-cut Vlasov launcher, quasi-elliptical cylindrical mirror and a parabolic mirror). In the output window, transformed mode is compared with the standard Gaussian wave beam (TEM00mode) to calculate that Gaussian content with95.23%, the efficiency of transmission power with70.09%and the waist radius with 7mm. Calculated and analyzed showed that when the designed quasi-optical mode converter operates in the two frequency of215GHz and225GHz, their electric field distribution characteristics of the Gaussian beam in the output window are in agreement with electric field distribution characteristics of the Gaussian beam of the quasi-optical mode converter operated in220GHz. So that the designed quasi-optical mode converter with the center frequency of220GHz has stable capability of the mode transformation in the band of215GHz-225GHz.