Density fluctuations in the TORTUR tokamak

Abstract

Quantitative measurements of the electron density fluctuation spectra, 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 approximately=kr. 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.