Two-Point Correlation Measurements of Density Fluctuations in the W7-AS Stellarator

1.Introduction Measurements of uctuations in a wide selection of plasma parameters are essential in order to continue the progress towards an understanding of anomalous transport caused by turbulence. At the W7-AS stellarator, uctuations in the plasma density are measured by e.g. collective scattering of a CO2 laser beam on the plasma. We will present a preliminary analysis of experiments performed using this system. The results show evidence of correlated density uctuations between two separate measurement volumes. 2. Experimental Con guration A heterodyne collective scattering system (see Fig. 1) using a CO2 laser has been installed on the W7AS stellarator with the purpose of studying core density uctuations [1]. Two vertically line integrated measurement volumes (waist radius w of 4 mm) sensitive to identical wavenumbers k? were created in the plasma. The wave vectors were always directed at an angle of 5 with respect to the major radius R in order to be perpendicular to the magnetic eld lines in the center of the plasma (z = 0) where the beams were focused. The length of the volumes L can be approximated by 4w s .