"Controlling Turbulence by RF Waves," by Prof. Ghassan Antar (American University of Beirut)

Thursday, May 7, 2015 - 1:00pm to 2:00pm
Plasma Seminars

Plasma Seminar

Physics & Astronomy Building (PAB) Room 4-330
Thursday, May 7, 2015
1-2 PM
Refreshments at 12:30 PM
Guest Speaker: Prof. Ghassan Antar (American University of Beirut)

Talk Title: "Controlling Turbulence by RF Waves"


Turbulence properties in the scrape off layer (SOL) in the presence of ion cyclotron frequency heating (ICRH) are compared to instance where it is absent on the ASDEX-Upgrade tokamak. The discharges are in H-mode maintained by neutral beam injection. During ICRH, the SOL plasma density increases whereas turbulence large-scale and convective structures are shown to be suppressed. Consequently, the level of turbulent fluctuations decreases significantly. Turbulence suppression is also reported during edge localized modes (ELMs) where both the ELMs induced transport and duration are strongly affected. These results were confirmed on the Tore Supra tokamak in L-mode. Moreover, it is shown 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 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 of 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. We deduce that ICRH may be used as an effective tool to control turbulence transport and to reduce ELMs amplitude.

PAB 4-330