Reynolds stress dynamics and turbulent energy transfer during stimulated H-Mode transition in RFX-mod tokamak

The present thesis has been devoted to the investigation of the turbulence properties in the edge region of RFX-mod operated as a tokamak during the transition from Low confinement (L-mode) to High confinement (H-mode) mode aided by the use of a biasing electrode inserted into the plasma. The mechanism of L-H transition is not fully understood in the framework of thermonuclear plasma science. Among the hypothesis, it has been suggested that turbulence itself may contributed to the formation of the sheared E X B flow in the external region which represents a distinct feature of the H-Mode. The proposed model foresees a transfer of momentum from fluctuations to the mean plasma flow through the Reynolds stress mechanism. This has been observed in various devices but no detailed analysis has been done so far on L-H transition induced by the application of an electrode capable of providing additional radial electric field and thus E X B flow. The thesis has been done on RFX-mod, run as a tokamak in Upper Single Null (USN) configuration, and the analysis has focused on data of the external region, close to the separatrix by means of an insertable probe. The Reynold stress contribution to the transition from L to H mode have been determined with its relation to the increase of flow velocity. A robust change is seen in velocity shear across the transition. An analysis of the dynamics of the flow is presented reporting that additive terms to the model have to be considered for a complete description of the phenomenon in case of induced biased H-Mode and some theoretical hypothesis have been proposed in accordance with experimental results. Eventually contribute of turbulences to viscosity and diffusive momentum have been studied confirming the results previously obtained in the thesis.