Turbulence analysis by estimating Kubo number in L-mode, H-mode, and ELMs-suppressed H-mode plasmas

Abstract

The characteristic of turbulences in the L-mode, H-mode, and the ELMs-suppressed H-mode plasmas are investigated by estimating the Kubo number. The Kubo number is a normalized autocorrelation time which shows whether the nonlinear interactions are strong or not in the system. The turbulence with strong interactions is called the strong turbulence. In the opposite case, it is called the weak turbulence. If the Kubo number is larger than one, the nonlinear interactions are significant and the quasilinear approximation fails. If the Kubo number is smaller than one, the turbulence is weak and the quasilinear approximation holds. The KSTAR BES (Beam Emission Spectroscopy) system provides the density fluctuation information. By the correlation analysis of the measured density fluctuation, the characteristic length and the time, i.e., the correlation length and the correlation time, of the plasma turbulence are obtained. Electron temperatures are measured by the ECE (Electron Cyclotron Emission) diagnostic system. When the plasma is dense enough and the electron temperature is of the order of 1keV, the Kubo numbers estimated in the L-mode and H-mode plasmas are well above the unity, suggesting that the turbulence is strong and the quasilinear approximation fails in the L-mode and H-mode plasmas. In addition, the estimated Kubo number in the L-mode plasma is much higher than the estimated Kubo number in the H-mode plasma. There is a clear change of the profiles of the Kubo number before and after the ELMs suppression, suggesting that the RMP (Resonant Magnetic Perturbation) influences the turbulence of the SOL plasmas. When the plasma is thin, i.e., low density, and the temperature is low, the Kubo number is estimated to be below the unity, indicating that the turbulence is weak.