Coherent structures in the simple magnetised Thorello plasma

Turbulence induced anomalous transport mechanism in magnetised plasmas is a major unsolved problem affecting both our basic plasma physics understanding and the performances of fusion aimed devices. Experimental investigation of turbulent regime is actively pursued in fusion aimed as well as in basic plasma physics toroidal devices. Experiments reported here have been performed in the Thorello device, operating at the University of Milano-Bicocca. Here a low temperature, high density steady turbulent plasma state can be produced and mantained in a hydrogen low pressure discharge. Turbulence has been studied by means of multiple pin electrostatic probes: fairly long time series of fluctuating plasma parameters have been obtained and correlated. A conditional sampling analysis has been performed in order to detect and study the spatiotemporal evolution of structures in a 2D section of the device in the turbulent state. Results show the presence of spatially small density fluctuations propagating mainly under the overall ExB drift. Such structures are associated to a local modification of the electric field. The overall effects is a net ExB drift which produce particle fluxes inducing a transport of plasma across the magnetic field towards the limiter. This advection mechanism could contribute substantially to the anomalous transport besides the diffusive one. Advanced statistical analysis tools has been employed too, in order to extract significant signatures of the turbulent state.