Reduced threshold parametric instabilities during X2 heating in ASDEX Upgrade*

Second harmonic X-mode (X2) heating is popular in several fusion devices such as the ASDEX Upgrade (AUG) tokamak. X2 heating benefits from the availability of efficient microwave sources, favorable beam accesibility and strong absorption. Furthermore, X2 heating has generally been considered free of undesired nonlinear wave effects known as parametric decay instabilities (PDIs). However, new observations from AUG show scattering due to PDIs during X2 heating at a greatly reduced gyrotron power threshold which is correlated with the passing of a rotating island. The generated scattering presents a danger to microwave sensitive equipment and the instability may result in substantial power absorption into unintented regions of the plasma. We present a theoretical model on why the island reduces the PDI power threshold. The island can act as a cavity, trapping PDI daughter waves and giving rise to an absolute- instead of a convective instability. The scattering is not visble when the gyrotron beam intersects the O- or X-point of the island but rich spectra emmerge between these points. This is because only an intermediate density in the decay region can excite two trapped PDI daughter waves necessary to overcome the power threshold. Dedicated particle-in-cell simulations confirm this.