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Shear effect on the radial profile of fluctuations measured by a reciprocating Langmuir probe in Tore Supra

A fast-scanning Langmuir probe is used to study fluctuations of the ion saturation current in the scrape-off layer of Tore Supra. The radial profile of the root mean square level of the fluctuations is measured. Additionally, the innermost fraction of the fluctuation profile is recovered by a fast acquisition system, allowing us to make correlation studies. The radial profile of the ion saturation current fluctuations displays a dip with a width of about 1 cm and a 50% decrease of j /j at a probe position that coincides with the radial position of the mid-plane limiter on which the plasma is leaning. This effect is observed although no magnetic connection exists between the probe and this limiter. A correlation analysis technique is used to study the radial behaviour of the fluctuations during the probe plunge. It reveals that the dip in the fluctuation level is associated with a dip in the poloidal velocity of the fluctuations and that at the same location, the poloidal correlation length of the fluctuations is reduced. As the poloidal velocity does not reverse, this shear region is not the radial electric field inversion radius. These observations suggest that the fluctuations are decorrelated and reduced by a shear of the poloidal velocity connected with the limiter position and which extends poloidally to the reciprocating probe. In order to test the shear hypothesis, the experimental shearing rate of the fluctuating structures is calculated and compared to the experimental life time of the turbulence. The shearing rate is found to be sufficient to explain the fluctuation decrease.