Space resolved fluctuations of electron density measured by means of two thermal Li-beams in TEXTOR-94

The turbulent fluctuations is the most probable origin of the anomalous transport of energy and particles in tokamaks. But the instability mechanism is not yet understood. The improved knowledge of the temporal-spatial structure of fluctuations is a prerequisite for deep comprehension of their nature. None of the existent diagnostics up to beginning of this work provided the simultaneous measurements of the radial and poloidal fluctuations structure with sufficient spatial and temporal resolution. In this work a new technique using spectroscopical observations of atomic beams for the measurement of electron density fluctuation in the scrape-off layer was developed, tested and applied on TEXTOR-94. This new diagnostic allows simultaneous studies of radial and poloidal fluctuation parameters, as the correlation lengths, as the wavenumber and frequency spectra and as the propagation velocities and lifetimes. The advantage of this method is, as compared with traditional measurements with Langmuir probes, that the good spatial and temporal resolution can be achieved without disturbance of the plasma. Moreover, the application of continuous beams, permits the measurements over the whole discharge. (orig.)