Performance Measurements for the ECH Transmission Lines on DIII-D

The power measurements in the waveguide transmission lines of the ECH system in DIII-D for the six 110~GHz gyrotrons showed higher transmission coefficient after eliminating 14~miter bends, rerouting the lines, and improving the angular alignment of the rf beam at the waveguide input. The transmission in the waveguide lines increased by between 2\% and 7\% per line, translating into more than 120~kW power gain when the gyrotrons are tuned up for high power. The linearity of the power injected in the tokamak with the gyrotron cavity loading was verified for all 6 systems. After the realignment of the rf beam at the waveguide input, the measured HE$_{11}$ mode content was between 85\% and 94\%, contributing to the increase in the transmission coefficient. A higher HE$_{11}$ mode content is expected to improve transmission in future conversion miter bends. A 4-port rf monitor was used with a directional coupler to measure the HE$_{11}$, HE$_{21}$, and TE$_{01}$ mode amplitudes. The 4-port monitor and dummy load measurements for one gyrotron show that a maximum in the HE$_{11}$ mode corresponds to the best angular alignment.