Emergence and decay rate of the edge plasma flow shear near a critical transition

16 pages, 14 figures.-- PACS nrs.: 52.35.Ra, 52.30.-q, 52.70.-m, 52.25.Gj, 52.25.Fi.-- Special section: Papers from the 10th International Workshop on Fast Ignition of Fusion Targets (Hersonissos, Crete, Greece, Jun 12-18, 2008).Recently, the experimental results for the emergence of the plasma shear flow layer in TJ-II have been explained as a second-order phase transition like process by using a simple model of envelope equations for the fluctuation level, the averaged poloidal velocity shear and the pressure gradient (2006 Phys. Plasmas 13 122509). Here, we extend this model by incorporating radial coupling. The model is applied to the study of the turbulence-shear flow interaction when the energy flux is low. Transition dynamics and their concomitant thresholds are examined within the context of this model. The effect of an external torque induced by electrode biasing has also been considered. In particular, we analyze the decay rate of the shear flow after switching off the biasing.This research was sponsored in part by the DGI (Dirección General de Investigación) of Spain under project No ENE2006-15244-C03-01 and by CM-UC3M (Comunidad de Madrid - Universidad Carlos III) Project No CCG07-UC3M/ESP-3407.Publicad