Transmission test of 300 GHz band gyrotron power by corrugated waveguides

A low loss transmission line is one of key elements for realization of sub THz band collective Thomson scattering (CTS) diagnostics in large fusion devices. This paper reports transmission tests of corrugated waveguides with 300 GHz band gyrotron power. Two types of corrugated waveguide are considered. One is 1.25 inch corrugated waveguide. The other is 3.5 inch corrugated waveguide already installed in the Large Helical Device (LHD) for electron heating with lower frequency gyrotrons. At first, transmission test with existing 1.25 inch waveguides was carried out. This waveguide is not optimized for 300 GHz band and substantial attenuation along waveguides was observed. A theoretical analysis explaining the large attenuation is performed.