The high-order mode output of high power millimeter wave gyrotron should he converted due to its disadvantages in space transport, which can he hardly solved by waveguide mode converter. Thus the built-in quasi-optical mode converter is studied. The converter consists of Vlasov launcher, quasi-parabolic mirror and phase correcting mirros. The simulation program is developed according to scalar diffraction theory and KSA algorithm, and phase correcting mirrors are designed. The calculation analysis of field distribution at the reference plane shows that, the converter's scalar conversion efficiency is larger than 98% , and the vector conversion efficiency is larger than 93%. The results indicate that the method can guide the design of quasi-optical mode converter efficiently.%高功率毫米波回旋管输出的高阶模式不利于空间传输,必须进行模式转换.由于不便采用波导模式转换器,因此对内置准光模式转换器进行了研究.内置准光模式转换器由Vlasov辐射器、准抛物柱面镜和相位修正镜组成.基于标量衍射理论和KSA算法,编制了数值模拟程序,设计了相位修正镜.计算分析了参考平面处的场分布,表明该准光模式转换器的标量转换效率大于98％,矢量转换效率大于93％.计算结果表明,该方法可以高效地指导准光模式转换器的设计.
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