Nonlinear decay of high-power microwaves into trapped modes in inhomogeneous plasma. (1st July 2022)

• Record Type: Journal Article
• Authors: Tancetti, A.; Nielsen, S.K.; Rasmussen, J.; Gusakov, E.Z.; Popov, A.Yu.; Moseev, D.; Stange, T.; Senstius, M.G.; Killer, C.; Vecséi, M.; Jensen, T.; Zanini, M.; Abramovic, I.; Stejner, M.; Anda, G.; Dunai, D.; Zoletnik, S.; Laqua, H.P.; the W7-X Team,
• Is Part Of: Nuclear fusion. Volume 62:Number 7(2022)
• Publication Date: 2022-07-01
• Subjects: parametric decay instability -- upper hybrid wave -- ion Bernstein waves -- electron cyclotron resonance heating -- collective Thomson scattering -- Wendelstein 7-X stellarator; Nuclear fusion -- Periodicals; 621.48405
• DOI: 10.1088/1741-4326/ac5d61
• Languages: English
• ISSNs: 0029-5515

Abstract

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