Mode conversion and high-efficient transmission of submillimeter wave gyrotron output for plasma diagnostics

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In Fukui University, we have developed ultra-high frequency, medium-power, step-tunable, submillimeter wave gyrotrons (Gyrotron FU Series), operating at cyclotron fundamental and second harmonic resonances. The gyrotrons cover the frequency range up to 890 GI-lz and the power levels are between 100W and 1 kW.In this study, we developed a transmission line system to conduct the output of our submillimeter wave gyrotrons for application to scattering measurement of density fluctuations excited spontaneously in thermonuclear fusion devices.(1) We have designed and manufactured components of the transmission line operating in a wide frequency range from 156 GHz (TE12 cavity mode) to 812 GHz (TE19 cavity mode). Namely, an antenna system for conversion to Gaussian mode, polarizer consisting of two metal mirrors with grooves, and so on. The complete transmission line was constructed using these components and tested by using the output power of Gyrotron FU IV.Radiation patterns at the point of scattering in a plasma were observed and the transmission efficiency was determined by the result.(2) The characteristics of the polarizer were tested by using the gyrotron output power. The direction of polarization can be controlled by adjusting the relative angle between grooves of the two mirrors.(3) The observed results are in good agreement with those expected from the design parameters of the transmission line, We have planned to apply the line to submillimeter wave scattering measurements of thermonuclear fusion devices.