Filamentary velocity scaling validation in the TCV tokamak

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A large database of reciprocating probe data from the edge plasma of TCV (Tokamak à Configuration Variable) is used to test the radial velocity scalings of filaments from analytical theory [Myra et al., Phys. Plasmas 13, 112502 (2006)]. The measured velocities are mainly scattered between zero and a maximum velocity which varies as a function of size and collisionality in agreement with the analytical scalings. The scatter is consistent with mechanisms that tend to slow the velocity of individual filaments. While the radial velocities were mainly clustered between 0.5 and 2 km/s, a minority reached outward velocities as high as 5 km/s or inward velocities as high as −4 km/s. Inward moving filaments are only observed in regions of high poloidal velocity shear in discharges with B × ∇B away from the X-point, a new finding. The filaments have diameters clustered between 3 and 11 mm, and normalized sizes â clustered between 0.3 and 1.1, such that most filaments populate the resistive-ballooning regime; therefore, most of the filaments in TCV have radial velocities with little or no dependence on collisionality. Improvements in cross-correlation techniques and conditional averaging techniques are discussed which reduce the sizes determined for the largest filaments, including those larger than the scrape-off layer.