Characteristics of a complex-conjugate impedance antenna system for ion cyclotron resonance frequency (ICRF) heating and current drive (H & CD) are discussed in this paper. Large RF power is reflected in such transitions as that of ELMy H–L mode due to the large change in plasma resistance during the ICRF H & CD. In such a case the RF power injection to the plasma must be ceased in order to protect the tetrode vacuum tubes. The idea of a complex-conjugate impedance antenna system to mitigate the large reflected RF power, referred to as ELM tolerance, has recently been proposed. It has been proved that the reflected RF power fraction can be reduced even during a large change in plasma resistance. But it is not reduced to the level allowable for the tetrode tubes. Therefore, an improved system adding a single stub tuner is proposed. Using it, the reflected RF power fraction can be reduced to the allowable level. Numerical calculations and experiments are carried out for this system. It is found that the experimental data agree with numerical calculations.
Coupler impedance calculations for ICRF heating of large tokamaks