Plasma fluctuations in the scrape-off layer and at the divertor target in Alcator C-Mod and their relationship to divertor collisionality and density shoulder formation

Highlights: • A combination of divertor rail Langmuir probes and midplane scanning probes were used to interrogate scrape-off layer fluctuations. • Increasing core averaged Greenwald fraction results in a clear density shoulder formation in the midplane profiles as per previous measurements. • In addition, fluctuations become more intermittent in the far scrape-off layer of both the divertor and the midplane. • Using nitrogen seeding in the divertor, the divertor plasma temperature is reduced and collisionality increases over two orders of magnitude at the strike point. • However, there appears to be little to no effect on midplane density profiles and far scrape-off layer fluctuation statistics. -- Abstract: Experiments indicate that the formation of density shoulders in the far scrape-off layer (SOL) is associated with an increase in cross-field transport and the propagation of coherent structures of high density known as blobs. It has been suggested that far SOL divertor collisionality (Λ_div) may be a controlling parameter in this physics, which could be challenging for heat flux mitigation schemes. At Alcator C-Mod, a new high spatial resolution divertor Langmuir probe array (rail probes) combined with a scanning Mirror Langmuir probe has been used to investigate this phenomenon in detail. Experiments were performed to mitigate divertor heat flux and thus increase Λ_div by two different methods and observe the divertor and scrape-off layer plasma response: (1) a 3-point density scan is found to increase the fluctuation intermittency at both the midplane and divertor, and produce a density shoulder at elevated Greenwald fraction; (2) a 3-point N₂ seeding scan is found to increase the divertor strike point collisionality, by over two orders of magnitude yet no noticeable trend is observed in either the density profile or the fluctuation statistics. It is concluded that Λ_div near the strike point is not by itself a controlling parameter for SOL fluctuations and shoulder formation which is important for localized heat flux mitigation schemes targeting the strike point where heat flux is the greatest.