ITBs dynamics in helical RFX-mod states by high time resolution Te measurements

Electron Internal Transport Barriers (eITBs) in the core of RFX-mod plasmas with 3D helical magnetic topology are characterized by a complex dy namics which has been recently observed in detail thanks to the high time resolution T e measurements obtained by double filter technique wi th a multichords soft-x-ray (SXR) diagnostic 1 . The evolution of temperature profiles during heli cal states are used as input for heat transport analysi s in order to determine the electron thermal diffusivity evolution across the helical magnetic s urfaces for several cases of eITBs dynamics, with a special attention to those phases where the therm al structures are growing/disappearing or when the helical topology is being lost. In fact, thermal structures with important T e gradients do not persist for the entire duration of the magnetic helical states but show an intermit tent behaviour. A statistical approach has been implemented in order to investigate what causes the partial or definitive end of eITBs. Such a study has revealed different time scales involved for mag netic and thermal plasma properties; in particular, the increase of the magnetic chaos and the partial loss of the helical magnetic topology usually occur in a phase where the electron tempera ture gradient is already decreasing. For a deeper understanding of this phenomenology, more mechanism s having a role in the deterioration of thermal structures are taken into consideration: fo r instance the microtearing perturbations, measured by pick up coils at the edge of RFX-mod, o r the pressure driven instabilities, analyzed by using the stability COBRA code 2 . References [1] P.Franz et al., Nucl. Fusion 53 (2013) 053011 [2] R. Sanchez et al., Computer Physics Communicati ons 141 (2001) 55-65