Coherent structures in the plasma edge turbulence of the RFX and CASTOR experiments

A fundamental role in fusion plasmas transport processes is played by electrostatic turbulence. In the edge region of toroidal experiments, the use of Langmuir probes allows a detailed investigation of the temporal and spatial structure of this turbulence. In recent years, the issue of self-similarity and its violation, termed intermittency, has received substantial attention due to its implications in terms of universality of turbulence behaviour. In particular, a general trend has been identified, by which the probability distribution function (PDF) of the fluctuations is almost gaussian at large scales, and develops strong tails at small scales. this behaviour, which looks independent from the magnetic configuration of the plasma, is due to strong events which occur at small scales more frequently than in the gaussian case. In the RFX reversed field pinch experiment, by the use of specific detection algorithms and of conditional averaging, it has been possible to associate the intermittent events to coherent structures present in the turbulence. It has also been demonstrated that these structures play an important role in determining the overall turbulence-driven transport. In the CASTOR tokamak, a similar kind of behaviour is observed. Furthermore, the use of one and two-dimensional arrays of probes has allowed to perform an imaging of the turbulence. This approach has revealed the presence of large scale coherent structures with a very long lifetime.