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

A. Tancetti*, S. K. Nielsen, J. Rasmussen, E. Z. Gusakov, A. Yu Popov, D. Moseev, T. Stange, M. G. Senstius, C. Killer, M. Vecséi, T. Jensen, M. Zanini, I. Abramovic, M. Stejner, G. Anda, D. Dunai, S. Zoletnik, H. P. Laqua

*Corresponding author for this work

Research output: Contribution to journalJournal articleResearchpeer-review


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.

Original languageEnglish
Article number074003
JournalNuclear Fusion
Issue number7
Number of pages6
Publication statusPublished - 2022


  • Collective Thomson scattering
  • Electron cyclotron resonance heating
  • Ion Bernstein waves
  • Parametric decay instability
  • Upper hybrid wave
  • Wendelstein 7-X stellarator


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