The ITER reference plasma solution[1] has a 1% Ne concentration added to the divertor to enhance radiation, thus lowering the heat flux to the divertor walls below 5 MW/m^2. The ITER reference geometry includes a louvered-wall in the private flux region and a vertical target which intercepts the strike-point for better power and particle control. This paper examines the detailed neutral gas behavior of this system using the newly enhanced DEGAS+ computer code.

The DEGAS+ code which already included elastic collisions between ions and neutral atoms, and elastic collisions between neutral molecules and neutral atoms, was modified to include elastic collisions between ions and neutral molecules. These collisions are particularly important in two regimes: (1) the back flow of neutral molecules from the louvered portion of the "gas box" into the divertor plasma , and (2) the low-temperature plasma near the walls which recycle a significant number of molecules due to low-energy ions recombining at the saturated surface and being released. In both of these regimes the returning molecules can scatter off low-temperature ions with high ion flow velocities. This can lead to dissociation of the molecule or can impart a significant momentum to the molecules along the flow velocity vector. These molecules find themselves in a plasma with a temperature too low for ionization and end up striking walls or louvers with non-thermal velocities --- some as high as 3 to 5 eV.

The magnitude of the molecular effect compared to the angular and energy distributions of the atoms and ions along the divertor surfaces will be presented. Implications for erosion, tritium inventory, particle control, and gas pumping will be discussed.

[1] private communication, Dr. Andre Kukushkin, ITER JCT, April 1997.