Performance of Impedance Transformer for High-Power ICRF Heating in LHD

There are two types of ion cyclotron range of frequencies antennas in the Large Helical Device. The handshake form (HAS) antenna has high heating efficiency. However, its loading resistance is small, and injection power is limited by the voltage of the transmission line. On the other hand, the field-aligned-impedance-transforming (FAIT) antenna has higher loading resistance than the HAS antenna despite having a smaller antenna head. However, the high voltage on the transmission line is again the bottleneck for high-power injection, as with the HAS antenna. We developed an ex-vessel impedance transformer for the HAS and FAIT antennas to decrease the voltage on the transmission lines by increasing loading resistance. The estimated enhancement factors of loading resistance were 1.65 and 2.50 for the HAS and the FAIT antennas, respectively, and the experimental result for the HAS antenna was consistent with the estimation. Therefore, higher power injection will be possible.