Avalanche statistics of fluctuation-induced fluxes from the SLPM and the W7-AS stellarator

Abstract Measurements of fluctuating floating potentials and ion saturation currents at different radial locations in the Santander Linear Plasma Machine [Castellanos et al., Plasma Phys. Control. Fusion 47, 2067 (2005)] and at the edge of the W7-AS stellarator by means of radially movable Langmuir probes allow to infer the corresponding fluctuation-induced radial flux temporal series. Avalanche-like transport events are identified in the time series and statistically characterized in terms of avalanche size/duration/quiet-time distributions and size-duration scaling relations. Transport is diffusive in the inner and intermediate radial region of the SLPM $r < r_{\rm tr} \approx 2.6$ cm, undergoing a transition at $r_{\rm tr}$, becoming non-diffusive in the outermost region of the device, $r > r_{\rm tr}$. The results obtained at the edge of the W7-AS stellarator are similar to those found in SLPM for $r > r_{\rm tr}$, i.e., consistent with what would be expected for scale-free, self-similar plasma transport dynamics near a critical state.