Anomalous absorption in ECRH experiments due to the parametric low-threshold excitation of localized UH waves

Abstract

We describe the theoretical model which interprets the anomalous phenomena, i.e. the generation of backscattering signal observed in the ECRH experiments at TEXTOR, TCV, TJ-II, ASDEX-UG, LHD and FTU, as a consequence of the excitation of the parametric decay instability (PDI) leading to anomalous damping of the pump wave. The PDI power-threshold is shown to be extremely low due to the localization of both or one daughter upper hybrid (UH) waves in presence of a nonmonotonic (hollow) density profile, which is often observed in the ECRH experiments due to the magnetic island or the density pump-out effect. In the case of the extraordinary wave pump the model predicts substantial (up to 25%) anomalous absorption in the electron channel and explains the anomalous ion acceleration by the generation of secondary low frequency waves which directly transfer the pump power to the ion component. The possibility of anomalous absorption of the O-mode pump in the ECRH experiment due to the parametric excitation of trapped UH wave is also discussed and the anomalous absorption rate at the 10% level is predicted.