Control of a steady-state operation using ICRF heating in the large helical device

In the large helical device (LHD), a steady-state operation (SSO) is carried out using ion cyclotron range of frequencies (ICRF) heating. Feedback control systems aiming for stable RF power injection, constant plasma density and real-time antenna impedance matching (RTIM) are required for long-duration discharges. When feedback control systems are employed, injected RF power and plasma density are successfully kept constant. Furthermore, magnetic axis sweeping is important for reducing the temperature of hot spots on ICRF antenna protectors and graphite divertor plates; the produced heat flux around the ICRF antennas is locally concentrated on these. By moving the magnetic axis outward, the hot spot temperatures on the divertor plates are decreased, and the plasma duration is extended to more than 300 s.