Experimental investigation on PDI bifurcation of lower hybrid wave during electron density ramp-up in EAST

Effect of parametric decay instability (PDI) on current drive efficiency of 4.6 GHz lower hybrid wave in EAST is investigated experimentally, showing the PDI channel bifurcation of lower hybrid (LH) wave for the first time in EAST. Firstly, experiments with 3 platforms of LH power were performed, achieving the LH power required for the PDI occurrence. Secondly, PDI bifurcation experiments were further carried out by ramping up plasma electron density. Loop voltage increases with density increase, implying the decrease of current driven by LH wave. PDI bifurcation during electron density ramp-up was studied by analyzing the parallel refractive index (n_∥) and the frequency spectrum broadening, which is measured with a radio frequency (RF) magnetic probe array recently installed close to the LH antenna. It is observed for the first time that both of them firstly increase with density, then there is not much variation and a clear sideband in the frequency spectrum is also observed when the density is up to 4×10^19 m^(-3), suggesting the change of the PDI channel. Calculation of the mode growth rate driven by PDI shows that when the edge electron density is up to 1.9×10^18 m^(-3), the growth rate of ion cyclotron quasi-mode (ICQM) will exceed that of ion sound quasi-mode (ISQM), quantitively explaining that with a density increase, PDI channel partly transits from ISQM to ICQM channel. Studies provide a possible way to reduce the power deposition in the edge region and improve LHCD capability by means of mitigating PDI behavior.