DEVELOPMENT OF MODE CONVERTERS FOR 28 GHZ ELECTRON-CYCLOTRON HEATING-SYSTEM

Small circular waveguide mode converters used in the 28 GHz, 200 kW ECH system for the Hitachi tokamak HT-1 have been developed. In order to convert from the TE02 to the TE11 mode, TE02-to-TE01 and TE01-to-TE11 mode converters are used. Both converters use periodic waveguide wall perturbations, the case of the former, circularly symmetric perturbations, and in the case of the latter, curvature perturbations. The lengths of the mode converters were successfully shortened with high mode conversion efficiencies by decreasing the number of periods and then optimizing the periods using quite a small diameter for the converters. The 30-phi TE02-to-TE01 mode converter is 103.34 mm long and its mode conversion efficiency measured by a scalar network analyser is 96% at 28 GHz. The TE01-to-TE11 mode converter is 427.38 mm long and has an efficiency of 79%, which is 10% lower than the design value. This discrepancy resulted mainly from mode conversion from the TE11 to the TE21 mode, caused by the smaller number of periods.