Space/time statistics of the turbulence in a tokamak edge plasma

The results of a study of the statistical space/time properties of the turbulence observed in the edge region of the Pretext tokamak are reported. A local estimate of the wavenumber frequency spectra S(?,?) for poloidal (ky)an d toroidal (kz) wavenumbers is determined for the range 0.5 cm?1 < k < 7.5 cm?1, for both potential and density fluctuations obtained from spatially fixed Langmuir probe pairs. The S(ky, ?) spectra aredominated by low frequencies ( < 200 kHz) and small wavenumbers (< 3 cm?1) and appear broadened about an approximately linear statistical dispersion relation, (?). The broadening is characterized by a spectral width ?k (?) (root-mean-square deviation about (?)) which is of the order of (?). The turbulence appears to propagate poloidally with an apparent mean phase velocity of (1?2) ? 105 cm?s?1 in the ion diamagnetic drift direction. Measurements of the fluctuation-induced particle transport reveal the particle flux to be outward and resulting primarily from the low-frequency, long-wavelength components of the turbulence. A particle diffusion coefficient Dx, estimated from the measured particle flux and density profiles, is of the order of the coefficient estimated for Bohm diffusion, but appears to increase with density.

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