Characterization of density scanning experiments with NBI and LHW on EAST

This paper presents the results of the density scanning experiment on the 2018 EAST campaign to study the effect of gas fueling on energy confinement of the ELMy H-mode. The experiment is carried out in a USN configuration, with neutral beam and lower hybrid wave heating and gas fueling, with the upper triangularity δup ∼ 0.47. The total stored energy, H98, and βN decrease with normalized density. Compared to the variations in temperature at the pedestal, the core temperature decreases more significantly for both Te and Ti, leading to a large reduction in core pressure and an increase in the pedestal electron collisionality ν*e,ped. The increase in ν*e,ped could reduce the pedestal current and result in a decrease in the value of q in the core region. It was observed that the frequency of type I ELMs increases with density and the edge localized mode size becomes smaller at high density plasma. An m/n = 2/2 tearing mode was observed at the core of the plasma and can coexist with a sawtooth at low density plasma while this tearing mode disappeared at high gas fueling plasma. The reversal radius of the sawtooth (where q = 1) moves toward the magnetic axis as density increases. The degradation in performance with density may be due to two reasons: the more monotonic shear q profile and the weakening of the stabilizing effect of fast ions on ion temperature gradient modes at high density by D2 gas fueling. It seems that there is a strong link between core transport and pedestal parameters which are influenced by gas fueling, resulting in a significant degradation of energy confinement.