Nonlinear decay of high-power microwaves into trapped modes in inhomogeneous plasma

Abstract We present novel experimental evidence of parametric decay instability microwave beams in the plasma edge Wendelstein 7-X stellarator. propose that is sustained by trapping only one daughter wave non-monotonic density profile measured with high spatial resolution within a stationary magnetic island. The power levels and spectral shapes detected signal are reproduced numerical modelling theoretical threshold predicted around 300 kW, comparable observations. predict fraction drained waves 4% experiments, potentially increasing above 50% for more hollow profiles. Such absorption could significantly reduce efficiency heating current-drive system tokamaks stellarators.