Satoru Yajima

Accurate reflectivity data of millimeter wave absorber materials are essential for predicting heat-flux distributions and optimizing the design of dummy loads (DLs) used in electron cyclotron resonance heating (ECRH) systems. However, conventional millimeter wave reflectivity measurements typically exhibit uncertainties of approximately ±5%, which are insufficient for reliable design. In this study, a high-accuracy reflectivity measurement was developed and applied to thermally sprayed TiO2 coatings. Measurements were performed at 110 GHz, corresponding to the operating frequency of the JT-60SA ECRH system. To improve measurement accuracy, an HE11-mode launcher was employed to suppress beam divergence during free-space propagation, and a two-dimensional scanning measurement was performed to capture the full reflected-wave intensity distribution. In addition, a reference-based calibration method using a copper reflector eliminated the need for direct estimation of the incident wave intensity and reduced the influence of source-power fluctuations. Reflectivities were measured for incident angles between 30° and 65° under both S- and P-polarized conditions, achieving an uncertainty of less than ±0.8%. The measured data were successfully approximated using a Fresnel-based model with a single fitting parameter. The resulting reflectivities were implemented in ray-tracing and thermal analyses and validated through comparisons with infrared-camera and calorimetric measurements obtained during 500 kW high-power experiments. Good agreement was obtained for temperature-rise distributions, temporal temperature evolution, and absorbed-power fractions. These results demonstrate that the proposed method provides reliable reflectivity data for predicting heat-flux distributions and power-absorption characteristics and serves as a powerful tool for the design and optimization of low-reflection DLs.