Experimental investigation of parametric decay instability in Wendelstein 7-X

Stellarators such as Wendelstein 7-X (W7-X) rely on microwave heating (ECRH) to reach high performance scenarios. However, if the injected power exceeds a critical threshold, non-linear interactions, like Parametric Decay Instability (PDI), may take place, where the injected “pump” microwave decays into a pair of daughter waves. Besides reducing the efficiency of the ECRH system, daughter waves may cause severe damage to microwave diagnostics and to plasma-facing probes. Here, we investigate the properties of the anomalous signal detected via the Collective Thomson Scattering diagnostic during the last W7-X experimental campaign, OP1.2(b), and explore the hypothesis of excitation due to PDI along the ECRH beams. The signal may occur as sideband peaks, symmetrically arranged around the pump frequency, or as a broadband structure, continuously stretched 500 MHz below the pump frequency. We describe the main physical quantities affecting the occurrence and the structure of the signal, and identify an experimental power threshold for the instability. We further discuss similarities with anomalous signals detected in ASDEX Upgrade that can be explained by PDI.