The role of polarization currents in disconnecting blobs from the divertor target in Alcator C-Mod

Cross-field transport in the scrape-off layer (SOL) is dominated by coherent field-aligned structures of high density and temperature known as blobs. Theoretical work on blob dynamics indicates that cross-field ν_ExB propagation is driven by the level of blob polarization which is a function of the current closure mechanism within the blob: they move slower when electrically connected to the sheath in the divertor than when the current closure is dominated by current paths above the X-point. Multi-machine experiments have suggested that increased radial particle transport and the formation of a SOL density shoulder is due to an increased divertor collisionality. However, magnetic shear enhanced polarization currents that affect the current closure above the X-point were not considered in these analyses. At Alcator C-Mod we find that the SOL profiles and fluctuation statistics are independent of changes in divertor collisionality, suggesting that blobs are always disconnected from the divertor target sheath and that the current must close by some other mechanism. C-Mod data indicates that the shear enhanced polarization current is a significant contributor and is sufficient to fulfill this role.