Characterisation of SOL density fluctuations in front of the LHCD PAM launcher in Tore

Abstract

The density fluctuations, modified by Lower Hybrid Wave (LHW), is analyzed in Tore Supra with reference to the injected LHW power, density and the gap between LCFS (Last Closed Flux Surface) and the PAM (passive‐active‐multijunction) launcher. The density fluctuations are measured with RF probes installed at the PAM launcher front. A density scan at nominal toroidal field (3.8 T) shows that the fluctuations rate stays nearly constant (∼50%) for 〈ne〉〈3.5×1019m−3 and with LHW power up to 2MW. However, when increasing the density above 〈ne〉〈∼4.2×1019m−3, using strong gas puffing, the fluctuation rate increases to >70% and is characterized by strong negative spikes, with typical frequency >100kHz. These are most likely originating from acceleration of electrons in the LHW near field due to parasitic absorption, as evidenced on the IR images, showing hot spots on the side limiters.