On particle transport in toroidal plasmas

This paper draws attention to some mechanisms which can contribute to the understanding of particle transport in Tokamaks and similar systems. First, it is demonstrated that, even when there are highly anomalous heat losses due to electron heat conduction, the particle losses do not necessarily have to become strongly anomalous but can remain nearly neoclassical. Second, plasma-neutral gas interaction is shown to have a number of important effects on plasma equilibrium, transport and profile shaping, even in Tokamaks where the boundary layer only contains a small fraction of neutral particles. Thus, under rather general conditions of strong recycling, the characteristic plasma pressure gradient in the boundary layer becomes independent of the rate of particle transport. On the other hand, the ratio between the average electron density and the neutral density at the plasma edge provides a measure of the plasma particle loss rate. Finally, in Tokamaks near the density limit, the heat losses due to plasma-neutral gas interaction affect the energy confinement time. The present analysis appears to be consistent with experiments, as far as orders of magnitude are concerned.