Speaker
Description
In the commissioning of JT-60SA, two ECRH systems were operated at 110 GHz with a small-diameter waveguide of 31.75 mm (Unit1) and 82/110/138 GHz with a large-diameter waveguide of 60.3 mm (Unit2) [1]. The large-diameter waveguide potentially has a larger mode conversion loss due to misalignment than the small-diameter waveguide, and its feasibility in a 100 m-class transmission line (TL) is considered to be a large concern. Quantitative evaluation of installation accuracy is quite important, but its measurement could not be performed during the installation of TL due to spatial limitations. The results of the transmission efficiency evaluation carried out before the plasma experiment showed that the transmission efficiencies in each system are 66% (Unit1) and 79-85% (Unit2), and the differences between the measured and calculated values are about 8% for small-diameter waveguides and about 2% for large-diameter waveguides. The transmission efficiency of the large-diameter waveguide was in good agreement with the calculation, and the estimated misalignment averaged over the entire TL was 0.9 $\pm $ 0.3 mrad [2]. Furthermore, the output polarization at the end of the 100 m class TL was measured and compared with the predicted value, and the difference was found to be less than 6%, which is equivalent to 0.7% degradation of mode purity in the plasma. In the plasma experiment, ECRH systems were commanded in almost all discharge sequences (326 shots, in total, including un-successful sequences), and microwaves were injected reliability. It should be mentioned that the highest injected power to the tokamak vessel from only one TL was ~1.3 MW. This result demonstrated the feasibility of the system operated with the gyrotron output power of 1.5 MW in the actual tokamak condition.
Based on the results of the integrated commissioning phase, four transmission systems were designed for the next initial research phase. Assuming the previously demonstrated installation accuracy of 0.9 $\pm $ 0.3 mrad, transmission efficiency calculations showed that transmission efficiencies of 85-90% could be expected for all four designed systems, exceeding the design targets. In addition, polarizer placement was optimized and mode purity of greater than 99.9% for both 110/138 GHz frequencies and greater than 99% for 82 GHz was shown to be achievable.
References
[1] T. Kobayashi et al., Nucl. Fusion, 62, p.026039 (2022)
[2] H. Yamazaki et al., Fusion Eng. Des., 196, p.114015 (2023)
