[en] The Alcator tokamak routinely operates in a regime in which the plasma is opaque to neutrals, the ratio of mean free path l/sub n/ to radius a being l/sub n//a approximately 0.1. There are two important consequences for high density tokamak operation. First, the plasma fueling, i.e., the buildup and maintenance of high density (in excess of 10¹⁵ cm^-3 at the center) cannot be the result of neutral ionization, and second, the plasma shields the wall from the harmful effects of energetic neutrals. In the case of the fueling problem, it appears that an inward convection of plasma is required in order to increase the density to the observed levels. Simulations show that the Ware pinch coupled with anomalous particle transport near the edge is capable of mocking up the density rise; however, the time scale in the present model is about a factor of 2 longer than the experimental time scale. The plasma shield is the result of multiple ionization and charge exchange of neutrals which occurs when l/sub n/ less than a. Not only do energetic neutrals become reabsorbed before hitting the wall, but the background neutral density falls rapidly as l/sub n/ decreases; hence the intensity of the source of energetic neutrals diminishes. The effect has been clearly observed experimentally and is in qualitative agreement with computations of the neutral gas dynamics