Increased fluctuation in the edge tokamak plasmas with the resonant magnetic perturbation field.

The resonant magnetic perturbation (RMP) field has been utilized to suppress the edge localized mode (ELM) in tokamak plasmas. The pedestal top during the RMP ELM suppression is characterized by the increased broadband fluctuation, i.e. plasma turbulence. The origin of the increased fluctuation and its characteristics were not fully understood. Here, we show that the increased fluctuation involves the weak but significant nonlinear coupling in a narrow region at the pedestal top and the spatial structure of turbulence becomes less predictable around that region as expected for the stochastic fields around magnetic islands. We analyzed the bicoherence and the rescaled Jensen-Shannon complexity of the electron temperature fluctuation measured in KSTAR experiment to identify the nonlinear coupling and the change of statistical turbulence characteristics, respectively. Our analyses suggest that the nonlinear coupling through a partially stochastic island layer at the pedestal top contributes to the fluctuation increase during the RMP ELM suppression.