Blob motion and flattened far-SOL profiles in Alcator C-Mod

Fluctuations in the boundary of the Alcator C-Mod device are investigated with probe and gas puff imaging measurements in a series of experiments with a scan of core plasma density. A time delay estimation method is applied to deduce blob velocities within the boundary region. In discharges characterized by low density, the dynamics of blobs manifest in the far scrape-off layer, exhibiting radial velocities reaching up to 0.5 km/s. As the core plasma density approaches the Greenwald empirical discharge density limit, blob dynamics dominate the entire scrape-off layer, extending even inside the separatrix with radial velocities exceeding 1 km/s. The fluctuations display pronounced intermittency in the region primarily governed by blob structures. The time-average particle density profiles are found to be well described by transport due to blob-like structures, with e-folding lengths of more than 10 cm for the highest core densities. Both the radial profiles and the fluctuations are well described by the stochastic process modelling the blobs as a superposition of uncorrelated pulses moving radially outwards. The probability density functions and the frequency power spectral densities of the fluctuations are in excellent agreement with the predictions of the stochastic model. These results demonstrate the primary role that the large-amplitude fluctuations associated with radial blob motion can play in the interaction of magnetically confined plasma with limiter structures and the main chamber walls.