Improved Efficiency of the ECH Transmission Lines on DIII-D

The transmission in the waveguide lines of the electron cyclotron heating (ECH) system on DIII-D was measured for the six 110~GHz, 1~MW class gyrotrons. After the angular alignment of the rf beam was improved, the measured transmission loss for 4 of the lines was less than -1.1~dB, close to the -1~dB theoretical value. Measurements showed HE$_{11}$ mode content over 85\% for all the lines, indicating reduced losses from mode conversion. The higher transmission resulted in an 87~kW increase in the power measured at the DIII-D end of the line for the waveguide system with the best improvement. A newly developed 4-port rf monitor was used together with a directional coupler to measure the HE$_{11}$, HE$_{21}$, and TE$_{01}$ mode amplitudes for diagnosis of waveguide performance. The HE$_{11}$ signal was proportional to the transmitted power. The results show that a minimum in the undesired HE$_{21}$ and TE$_{01}$ mode amplitudes corresponds to the best angular alignment. Future plans include reducing the number of miter bends in each line by rerouting the lines and using lower loss miter bends.