Fluctuations in the near and far scrape-off layer of Alcator C-Mod

Profiles and fluctuations in the boundary region of the Alcator C-Mod device is investigated with gas puff imaging (GPI) and Langmuir probe measurements. Exceptionally long measurement time series are obtained under stationary plasma conditions by dwelling the probe at a fixed radial position in the scrape-off layer (SOL) through an entire discharge. A detailed analysis of measurement data from both the near and the far SOL regions is presented, comparing GPI and Langmuir probe measurements. The results are interpreted in a statistical framework describing the fluctuations as a super-position of uncorrelated pulses, corresponding to large-amplitude blobs moving through the SOL.

The probe measurements close to the separatrix reveal that the time scale of the duration of the fluctuations is significantly shorter than that measured in the far SOL. These near-SOL fluctuations are close to normally distributed resulting in small relative fluctuations. In the far SOL there is an order unity relative fluctuation level and the probability density for the fluctuations is a strongly skewed Gamma distribution. It is demonstrated that the fluctuations become increasingly intermittent as the line-averaged density increases. A novel time delay estimation technique applied to GPI data reveals that the large-amplitude far SOL fluctuations are associated with blob structures moving radially outwards. The radial blob velocity increases significantly with line-averaged density, leading to profile broadening and flattening.



The results presented here emphasizes the role of large-amplitude fluctuations for plasma-wall interactions, which is particularly strong as the empirical discharge density limit is approached.