Characteristics of edge fluctuation and transport in the edge localized mode suppressed plasmas by the resonant magnetic perturbation field

The stochastic layer at the pedestal top by the resonant magnetic perturbation (RMP) field penetration has been known as a key mechanism in the RMP edge localized mode (ELM) suppression in tokamak plasmas. The local pressure profile flattening and broadband fluctuation increase characterize the pedestal top during the RMP ELM suppression. Despite the importance of turbulence to understand the pedestal transport, the origin of the increased fluctuation and its characteristics were not fully understood. Here, we suggest that the partially stochastic island at the pedestal top can play a significant role. The nonlinear mode coupling between an island and fluctuations can result in the fluctuation growth, and the stochastic fields around the island can make the spatial structure of turbulence less predictable. We used the bicoherence and the rescaled Jensen-Shannon complexity to analyze the three-wave coupling and the statistical characteristics of fluctuations, respectively. We show that (1) the weak but significant nonlinear mode coupling exists in the narrow region at the pedestal top and (2) the spatial structure of turbulence becomes less predictable around the pedestal top. These observations are not inconsistent with the expectation of the partially stochastic island. The latter can be used to estimate the effective width of the stochastic layer over which the characteristics of fluctuations and so transport are statistically distinguished from those over the normal field geometry. In addition, we found that the divertor particle flux near the striking point becomes less predictable with the more penetration of the RMP field into the plasma.