Observations of low frequency density fluctuations in a tokamak edge plasma

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Abstract
Density fluctuations in the outer edge of the plasma, behind the limiter, have been investigated in the TEXTOR tokamak by a new technique using a probing beam of thermal neutral lithium. With this method, radial profiles of the electron density and its fluctuations are obtained and thus also the relationship between turbulence and plasma parameters, especially the density gradient scale lengths. The turbulences are found to consist of coherent fluctuations in the frequency range of sawtooth oscillations (<100 Hz) and fluctuations having a broadband spectrum of frequencies up to ~1 MHz. The frequency spectra vary, depending on radial position and the density gradient scale length. It is also shown that the fluctuation level is well correlated with the density gradient scale length.