A real-time tracking for optimal wave injection in overdense plasma heating experiments at 140 GHz in FTU

Abstract

Experiments on overdense plasma heating through the mode-coupling scheme known as “O-X-B Double Mode Conversion”, obtained launching a narrow beam of millimeter-waves at 140 GHz frequency and 400 kW power, are scheduled for the next experimental campaigns of the FTU tokamak. Such a scheme, not yet demonstrated at electron density higher than the critical one (2.4·1020m−3) for the 140 GHz ordinary mode, and consequently at such a high frequency, exploits the conversion of an ordinary polarized wave (O) into the extraordinary (X) one, followed by a subsequent conversion to Bernstein (B) waves, which are then absorbed by the plasma. In the specific case of FTU the overall efficiency of this scheme is mainly determined by the coupling efficiency between the O- and the X-wave, which can occur only for ordinary polarized radiation propagating in a very narrow angular range at the cutoff region.