Edge turbulence in RFX-mod virtual-shell discharges

In the Reversed Field Pinch eXperiment RFX-mod a Gas Puffing Imaging Diagnostic (GPID) is used to investigate the dynamical structure of plasma edge turbulence. The GPID measures the radiation emitted from He gas puffed in H discharges and allows the investigation of edge plasma properties with high time and space resolution at high plasma current during the entire discharge. The characterization of edge turbulence has been carried out in terms of power spectrum, cross-correlation and toroidal velocity of structures. The scaling of such velocity with the density corresponds to different values of the dimensionless ratio νci/νei (ion cyclotron frequency/electron–ion collision frequency) ranging from below to above unity. The toroidal characteristic length of intermittent structures at different time scales has been determined by applying the conditional average technique on the structures identified by the continuous wavelet transform. These lengths are in the range 15–45 mm, while the number of structures increases with the electron density, the propagation velocity, opposite to the plasma current, decreases with the density and saturates at −20 km/s.