Power threshold and saturation of parametric decay instabilities near the upper hybrid resonance in plasmas

Abstract

Parametric decay instabilities (PDIs) occur for large-amplitude waves in quadratically nonlinear media, where they provide a limit of validity of linear theories and allow efficient coupling between different, well-defined wave modes. We investigate PDIs near the upper hybrid resonance in plasmas by injection of high-power electron cyclotron (EC) waves at the ASDEX Upgrade tokamak. Our measurements of PDIs have an unprecedented frequency resolution, far below the ion cyclotron frequency, allowing the first observations of secondary and tertiary PDIs during the saturation phase in a controlled laboratory setting. Furthermore, we are for the first time able to systematically compare theoretical predictions of the EC wave power thresholds, which must be exceeded to excite such PDIs, with experimental observations, validating the theory. Our findings are relevant for EC wave heating and current drive in tokamaks and stellarators, including future fusion power plants, as well as in low-temperature laboratory and industrial plasmas, inertial confinement fusion, and ionospheric modification experiments.