On the interaction between the ion cyclotron resonance heating and scrape-off layer turbulence via coherent waves

It was reported on the ASDEX-Upgrade tokamak and recently confirmed on the Tore Supra tokamak (Antar 2010 et al Phys. Rev. Lett. 105 165001, Antar 2012 Nucl. Fusion 52 103005) that ion cyclotron resonance heating (ICRH) reduces turbulence in the scrape-off layer (SOL) by strongly affecting the large intermittent structures. In this paper, and for the first time, we show that ICRH, with a frequency at 57 MHz, excites waves at frequencies about 1.7 MHz as a consequence of parametric decay instability. This is reflected in the power spectra of the ion saturation signals, measured by fixed Langmuir probes acquired at 200 MHz, as well as in the cross-correlation between two toroidally distanced probes and in the bicoherence spectra. The velocity and the wavenumber of this perturbation is determined indicating that they may be ion sound waves. Furthermore, it is shown that they are nonlinearly coupled to the turbulent fluctuations leading to the strong modification of the power spectrum and could provide explanations about the decrease in the level of turbulence in the SOL during ICRH. This aspect is reflected in the enhanced three-wave coupling between these waves and turbulence studied by applying the bispectral analysis. Turbulence appears thus to be affected when ICRH is applied by the intermediate of ion sound waves. (paper)