Filament structures, intermittent fluctuations and broad average profiles at the boundary of magnetically confined plasma

A stochastic model has been developed in order to describe the dynamics of intermittent fluctuations due to radial motion of blob-like structures in the scrape-off layer (SOL) of magnetically confined plasma. Uncorrelated pulses move radially outwards with a random distribution of amplitudes, sizes and velocities. The pulses have a fixed shape and an exponentially decaying amplitude due to parallel drainage towards the divertor plates. In its simplest form, the model leads to exponentially decaying average radial profiles.

More generally, we investigate the implications of correlations between filament parameters on the mean profiles and higher-order statistical moments. A broad distribution of pulse velocities leads to non-exponential profiles and strongly increased intermittency in the far-SOL. It is demonstrated that this explains many features from experimental measurements. Three mechanisms are identified for increased intermittency with radial distance into the SOL: (i) a change in amplitude distribution due to parallel drainage, (ii) correlation between pulse amplitudes and duration times, and (iii) stagnation of pulses when the velocity depends on the instantaneous amplitude.

Model predictions will be presented for theoretically predicted relations between pulse amplitudes, sizes and velocities in inertial and sheath dissipative regimes. This demonstrates the formation of a shoulder structure in the average density profile with broad far-SOL profiles, high relative fluctuation levels throughout the SOL, and strong intermittency with positively skewed and flattened probability density functions in the far-SOL. The results will be discussed in the context of first-principles-based turbulence simulations of the SOL as well as experimental measurements of the Alcator C-Mod device. The relevance for plasma-wall interactions like sputtering will be pointed out.