Real-time impedance matching system for ICRF heating in LHD

In the Large Helical Device (LHD), long-pulse plasma experiments have been conducted using an ion cyclotron range of frequencies (ICRFs) heating system. Real-time impedance matching is necessary to keep injecting ICRF heating power into the plasma against the variation of the antenna impedance that changes gradually during a long-pulse plasma discharge. A feedback control system for the real-time impedance matching was constructed and utilized for long-pulse plasma discharges. As a component of the impedance matching device, liquid stub tuners were used. The reflected power ratio was reduced and kept sufficiently low by controlling the liquid heights during long-pulse plasma discharges, and the problem of the gradual increase in the reflected power was solved.