Structure and properties of the electrostatic fluctuations in the far scrape-off layer region of Alcator C-Mod

Abstract

The temporal statistics of density and potential fluctuations measured in both the main chamber and divertor far scrape-off layer (SOL) regions (>5 mm outside the magnetic separatrix) of the Alcator C-Mod tokamak [I. M. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] are found to change markedly with plasma density. As the plasma density is increased, the fluctuations become correlated over long and short time scales and exhibit a higher level of persistence and intermittency. Correspondingly, fluctuation-induced cross-field particle fluxes are found to increase by a factor of ∼5 when the plasma density is doubled. These observations are consistent with results from SOL particle balance, which indicate an increased level of cross-field transport as the density is raised. These observations also support the idea that SOL transport dynamics behave as a self-organized system, particularly at high plasma densities.