Description
During the integrated commissioning phase 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) [1,2]. Although the large-diameter waveguide contributes to reduce transmission loss, it potentially has a larger mode conversion loss due to misalignment compared to the small-diameter waveguide. Feasibility in a ~100 m transmission line (TL) is considered to be a large concern but quantitative measurement of TL installation accuracy could not be performed during the installation of TL due to spatial limitations. In order to measure the TL installation accuracy, the transmission efficiency was measured by comparing the power measurement by a dummy load nearby gyrotron and launcher. The results showed that the transmission efficiencies in each system are 66% (Unit1, small-diameter TL) and 79, 84% and 85% (Unit2, large-diameter TL at 82 GHz, 110 GHz and 138 GHz, respectively). The transmission efficiency of the large-diameter waveguide was in good agreement with the calculation assuming 0.9 mrad waveguide tilt connection of averaged over the entire TL. It is small enough not to lose the advantage of large-diameter waveguides. For the evaluation of the transmission characteristic, the output polarization at the end of the TL was measured and compared with the predicted value. The difference is less than 6%, which is equivalent to 0.7% degradation of mode purity in the plasma for three operational frequencies. These results demonstrate the feasibility of large-diameter waveguides at multi-frequencies.
In the plasma experiment during the integrated commissioning phase, ECRH systems were operated in almost all 326 discharge sequences, which suggests the high reliability of the system. During the integrated commissioning, the output power of the gyrotron is increased from 1 MW to 1.5 MW. The availability of the 1.5 MW gyrotron operation for 5 s was confirmed by a dummy load. However, a transmission power of more than 1 MW, which is the design power of the transmission line, had not yet been demonstrated. After the several shots of conditioning, the 1.5 MW (1.3 MW injection power) shots with a duration of 1 s are successful. This result demonstrates the possibility of further increasing the total heating power.
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 mrad, the transmission efficiencies are calculated as 85-90%, which satisfy the design targets. According to the output polarization prediction, the mode purity is expected to be higher than 99% for three operating frequencies of 82/110/138 GHz. Based on this design, procurement of transmission components was initiated, and acceptance testing of the gyrotron power supply and diamond window unit was conducted.
References
[1]. T. Kobayashi et al., Nucl. Fusion, 62, p.026039 (2022)
[2]. H. Yamazaki et al., Fusion Eng. Des., 196, p.114015 (2023)
