Adapting ICRF Transmission Line Components For Use at ~0.5 GHz for Helicon Experiments on DIII-D

A system to test fast wave current drive in the lower hybrid range of frequencies (“helicon”) is operating on the DIII-D tokamak, at rf power levels of up to 1 MW at 476 MHz available from a single klystron. The power is transmitted from the source over a long transmission line to the input of a 30-element traveling-wave antenna of the comb-line type mounted in the DIII-D vacuum vessel. The transmission line is largely constructed of 9”-diameter 50 Ohm coaxial line, with the portion closest to the tokamak built of 6” 25 Ohm coax. The vacuum feedthrough used at the vessel wall is adapted from ICRF feedthroughs originally intended for operation at ~50 MHz. Modifications of the coaxial components proved necessary to obtain acceptably low reflections in the lines at 476 MHz. Characterization of the components as a function of frequency up to 0.5 GHz are shown and the modifications are discussed. The linear (1 mW - 100 W) properties and the properties at up to 0.5 MW of the in-vacuum portion of the transmission lines are discussed, including the important effects of multipactoring at the operating frequency of the helicon system. Operation into plasmas at coupled power levels up to 0.3 MW was achieved in June 2021; initial results are reported.