Measurement of RF transmission mode conversion due to waveguide thermal deformation using 170GHz gyrotron

The mode conversion by thermal deformation of the transmission line (TL) for electron cyclotron heating and current drive (EC H&CD) system caused by high-power long-pulse operation was studied. Mode contents in 40 m length TL test stand were measured and compared for before and after the 600 kW/169 s long-pulse operation. The mode content indicated that HE11 mode decreased by 10% and LP11 mode increased. The miter bend after the 12 m long horizontal straight section in TL was moved for 1.5 mm which deformed the short vertical waveguide section. The increased LP11 mode was converted from HE₁₁ mode in the deformed waveguide. Although the TL kept high transmission efficiency under high-power long-range TL, mode conversion loss was significantly increased by thermal deformation of TL.