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 sat uration current in the scrape-off layer of Tore Supra. The radial profile o f the root mean square level of the fluctuations is measured. Additionally, the innermost fraction of the fluctuation profile is recovered by a fast a cquisition system, allowing us to make correlation studies. The radial prof ile of the ion saturation current fluctuations displays a dip with a width of about 1 cm and a 50% decrease of delta j/j at a probe position that coin cides 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 us ed to study the radial behaviour of the fluctuations during the probe plung e. It reveals that the dip in the fluctuation level is associated with a di p in the poloidal velocity of the fluctuations and that at the same locatio n, the poloidal correlation length of the fluctuations is reduced. As the p oloidal velocity does not reverse, this shear region is not the radial elec tric field inversion radius. These observations suggest that the fluctuatio ns are decorrelated and reduced by a shear of the poloidal velocity connect ed with the limiter position and which extends poloidally to the reciprocat ing probe. In order to est the shear hypothesis, the experimental shearing rate of the fluctuating structures is calculated and compared to the experi mental life time of the turbulence. The shearing rate is found to be suffic ient to explain the fluctuation decrease.