Methods for Reducing Anomalous Losses in ECRH Experiments at Second Resonance Harmonic

An approach is analyzed that makes it possible to reduce anomalous absorption in experiments on electron cyclotron resonance heating (ECRH) at the second harmonic of the electron cyclotron resonance. The anomalous absorption is associated with excitation of the low-threshold parametric decay instability of the extraordinary pump wave at the local maximum of the nonmonotonic density profile. The general case is considered, which corresponds to nonlinear excitation of only one localized daughter upper-hybrid wave occurring as a result of the primary decay process. It is shown that, due to rather low instability threshold, it could hardly be completely suppressed in ECRH experiments using megawatt microwave beams. However, an increase in the radius of the pump wave cross section can considerably reduce the corresponding anomalous absorption.