Effect of edge neutrals on the low-to-high confinement transition threshold in the DIII-D tokamak

To study the effect of edge neutrals on the low-to-high confinement transition threshold, a broad range of plasma discharges has been analyzed. These discharges vary by gas puffing and pumping rates, position of the X point, and line-averaged density. It is shown that the determination of the neutral density (or neutral pressure) in the scrape-off layer (SOL) can give a misleading indication of the neutral population inside the separatrix. An increase of neutral density in the SOL creates an increase of plasma density that, in turn, increases the opacity to the neutrals and results in reduced neutral penetration. At a constant magnetic field, the transition power divided by the density appears to be a function of a single parameter measuring the neutrals effect. From this analysis, this parameter cannot be uniquely identified. For instance, it may be the radial decay length of the neutral profile or the charge-exchange damping rate at about r/a approximate to 0.95. A similar correlation exists between these neutral parameters and local plasma parameters such as electron and ion temperature. This indicates that a missing parameter linked to the neutrals exists in the power threshold scaling laws. (C) 1998 American Institute of Physics. [S1070-664X(98)01007-6]