First experimental observation of a two-step process in the development of the edge velocity shear layer in a fusion plasma

The radial origin and the process of formation of the velocity shear layer (VSL) in the stellarator TJ-II have been investigated. Results show that the VSL starts its formation in the region of maximum density gradient and then expands radially outwards, until it is fully established and shows the universal properties of edge sheared flows observed in fusion plasmas. A two-step process in the edge shear formation is proposed: 1) a seeding mechanism linked to plasma gradients and 2) an amplification process in which edge shearing rates and fluctuations are self-organized near marginal stability. (authors)