ECH Waveguide Power Loss Measurement at DIII-D

The DIII-D EC system uses circular corrugated waveguides to convey MW-level millimeter-wave power from the five 110 GHz gyrotron sources to the DIII-D tokamak, for electron heating and current drive. A new system has been implemented to measure ohmic loss in these waveguides. Building on previous testing, Resistance Temperature Detectors (RTDs) have been permanently installed along one of the waveguide runs to measure transmission loss during plasma pulses. Signals from the RTDs are digitized by a multiplexed LabJack data acquisition system, and Python code has been developed to produce a graphical user interface (GUI) for the system. Miter bends in waveguides can excite lossy higher-order modes, which cause enhanced dissipation and heating of adjacent waveguide sections. In the ~100 m-long waveguides used at DIII-D, this results in significant power loss (20 – 30 %). This RTD system can help monitor and more accurately quantify these losses. We describe the development of this system and display data acquired through its use.