Density-Fluctuations in the Tortur Tokamak

Quantitative measurements of the electron density fluctuation spectra, S(omega, k), are presented as a result of collective scattering measurements with 2 mm waves at various radial positions in the equatorial plane of the TORTUR Tokamak plasma. The spectra are determined in a wide frequency range up to 100 MHz and in a k-range between 400 and 4400 m-1 corresponding to scattering angles between 8-degrees and 90-degrees. It was found by performing experiments for various directions of k, that the fluctuations are isotropic in the poloidal plane, i.e. k0 almost-equal-to k(r). The total fluctuation level rises from 2% in the plasma centre to more than 10% near the plasma periphery. The theoretically expected frequency ranges for various models, like density gradient-driven modes, trapped electron modes and the high-frequency high m-number magnetic tearing modes are much smaller than the experimental values. This can be explained by a frequency shift due to toroidal rotation. Fluctuations at frequencies above 10 MHz in the ion cyclotron frequency range, for which the set-up was especially built, turned out to be absent. The decay coefficients of the k-spectra are found to be - 3.6 +/- 0.6 which is larger than the Kolmogorov value (8/3). Predictions for the radial dependence of by the filamentation hypothesis of Taylor are found to be consistent with experimental values for r/a less-than-or-equal-to 0.5. Values for the electron thermal diffusivity calculated by the heuristic expression chi(e) = gamma/2, with gamma and k(perpendicular-to) derived from the scattering data, were found to be in agreement with values derived from power balance and perturbation measurements.