Upgrade of ICRF Antennas by Utilizing Impedance Transformers in LHD

For the high-power and long-pulse ion cyclotron range of frequencies (ICRF) heating of plasma in the Large Helical Device (LHD), two types of ICRF antennas are used. One is the Field-Aligned-Impedance-Transforming (FAIT) antenna. It has an In-Vessel Impedance Transformer (IVIT) in the vacuum region of the antenna and shows the possibility of high-power injection despite the short antenna head. To enhance the performance more, an Ex-Vessel Impedance Transformer (EVIT) was attached outside the LHD vacuum vessel. As a result, the injectable power increased. The other is the Handshake form (HAS) antenna. Plasma can be efficiently heated by adjusting the phase difference between currents in straps. However, the injectable power from the HAS antenna was originally small. Therefore, later an EVIT was attached to it. Moreover, the transmission line in the vacuum region was remodeled to form an IVIT. By utilizing these impedance transformers, the performance of the HAS antenna was drastically improved. (C) 2022 The Japan Society of Plasma Science and Nuclear Fusion Research