H. Zushi, N. Nishino, K. Hanada H. Honma, H.Q. Liu, Y. Higashizono M. Sakamoto,Y. Nakashima, M. Ishiguro, T. Ryoukai, S. Tashima, K. Nakamura, H. Idei, M. Hasegawa, A. Fujisawa, O. Mitarai, A. Fukuyama, Takeiri, Y. Takase8), T. Maekawa, Y. Kishimoto, K. Toi, M. Kikuchi9) and the QUEST group
Study of Edge Turbulence from the Open to Closed Magnetic Field Configuration during the Current Ramp-up Phase in QUEST
Date of publication:
22 IAEA Fusion Energy Conference, EXS/P2-22
Statistical features of fluctuations including blobs are investigated using the CCD imaging technique in open and closed magnetic configurations. In a simple open magnetic configuration with vertical (B_z) and toroidal fields (B_t) slab plasma produced by electron cyclotron waves is studied as a function of B_z/ B_t. It is found that fluctuations become dominated by blobs depending on the ratio B_z/ B_t. In the plasma source region R_s helix-sinusoidal perturbations are exited, whose helix angle and vertical wavelength are consistent with pitch angle and turn distance of the magnetic field lines there. Steep gradient in the intermediate region R_im outside R_s triggeR_s the plasma to expand radially as a blob ejection. Along the ridge line of the maximum inveR_se scale length a wave front of the helix perturbation moves outward. Acceleration of a blob is found in the source-free region R_sf. It is found that the probability density function pdf of fluctuations in all B_z/ B_t expeR_iments is close to a gamma distribution. Ohmic plasma is ramped-up from the slab plasma and closed surface LCFS is formed. Fluctuations inboard side and outer scrape off layer SOL are studied in this process. Former one is characterized by small amplitude fluctuation level ?eltaI/I~ a few %, but latter one shows intermittent large amplitude. Two dimensional structures of the higher moments (skewness S and kurtosis k) representing the shape of pdf are studied. In the former it is observed that these structures consistent with the LCFS, and the pdf is close to a Gaussian distribution. The numerical coefficients characterizing the Pearson system are also derived. In the outboard SOL, it is found that S and k are not only a function of the magnetic flux, but also the magnetic field lines. The pdf is consistent with the Gaussian one for 0.1m from the LCFS, and becomes beta and gamma ones far from the LCFS. Based on the observed numerical coefficients in the Pearson system a relation between pdfs and model of density fluctuation is discussed.