Effects of neutral particles on edge dynamics in

1 Neutral particle densities and energy losses have been measured in the Alcator C-Mod tokamak [Hutchinson et al. Their effect on the formation and evolution of the edge barrier which accompanies the enhanced confinement regime are discussed. The neutrals can enter the edge dynamics through the particle, momentum and energy balance. Neutral densities of up to 5x10 16 m −3 have been measured in the edge barrier region. Neutrals enter the local dynamics around most of the periphery , not just at the X-point. High resolution measurements of the ionization profile have been obtained for the region near the separatrix. The profile shifts inside the separatrix as the plasma is making a transition from Low to High-mode confinement regimes, partly accounting for the dramatic rise in edge density. The measured neutral density is large enough to affect the bulk ion momentum by charge-exchange, and thereby introducing a negative radial electric field at the edge. At the same time, significant edge heat flux, carried by the neutrals, contributes to the measured power loss. At very high edge densities, this loss mechanism could contribute to quenching H-modes.