Saturation of the low-threshold parametric decay of the O-mode pump in the fundamental harmonic ECRH experiment

Abstract

The saturation regime of the low-power-threshold ordinary (O) wave parametric decay instability leading to the excitation of a localized upper hybrid (UH) wave and a low-hybrid wave is analysed. The cascade of secondary decays of a daughter UH wave to a localized UH wave and ion Bernstein wave is considered as presumably the most important nonlinear mechanism responsible for the transition to a saturation regime. A set of equations describing this nonlinear frequency down-conversion is derived and then solved both analytically and numerically under the conditions typical of the on-axis fundamental harmonic ordinary mode (O1-mode) electron cyclotron resonance heating experiments at the FTU tokamak.