Density modification in the scrape-off-layer during LHW coupling at EAST

The results presented in this paper are an extension of our recent studies (Kong et al 2013 Plasma Phys. Control. Fusion 55 065007, 065008) on lower hybrid wave (LHW) coupling. The effects of LHW power, edge recycling, edge-localized-mode (ELM) activity and gas puffing on the scrape-off-layer (SOL) density modifications will be further studied in this paper. The SOL density profile usually depends on the competition between an increment by ionization of neutrals and a depletion by the ponderomotive force (PMF) in L-mode plasma. Experimental results indicate that the SOL density with a lower edge recycling intensity could be reduced by a higher LHW power because ionization of a few neutrals cannot provide sufficient electrons to increase the SOL density and thus degrade the LHW coupling. In addition, in H-mode plasma, it is observed that the significant reduction of RC is associated with higher LHW power compared with the oscillation of RC during ELM activity, implying the priority of neutral gas ionization by LHW power. Studies also indicate that gas puffing in the GIM_e side could be more efficient in improving LHW coupling due to local ionization, inferred by the luminosities on the surface of the poloidal limiter (PL) and confirmed by SOL density measurement. The evolution of edge density with various puffing rates exhibits increments of far SOL density focused on the narrower layer several centimeters in front of the PL, possibly suggesting that the ionization of neutral gas mainly occurs in this region. (paper)