ANALYSIS OF NOBLE-GAS RECYCLING AT A FUSION PLASMA DIVERTOR

Near-surface recycling of neon and argon atoms and ions at a divertor has been studied using impurity transport and surface interaction code s. A fixed background deuterium-tritium plasma model is used correspon ding to the International Thermonuclear Experimental Reactor (ITER) [I TER EDA Agreement and Protocol 2, ITER EDA Documentation Series No. 5 (International Atomic Energy Agency, Vienna, 1994)] radiative plasma c onditions (T-e less than or equal to 10 eV). The noble gas transport d epends critically on the divertor surface material. For low-Z material s (Be and C) both neon and argon recycle many (e.g., similar to 100) t imes before leaving the near-surface region. This is also true for an argon on tungsten combination. For neon on tungsten, however, there is low recycling. These variations are due to differences in particle an d energy reflection coefficients, mass, and ionization rates. In some cases a high flux of recycling atoms is ionized within the magnetic sh eath and this can change local sheath parameters. Due to inhibited bac kflow, high recycling, and possibly high sputtering, noble gas seeding (for purposes of enhancing radiation) may be incompatible with Be or C surfaces, for fusion reactor conditions. On the other hand, neon use appears compatible with tungsten. (C) 1996 American Institute of Phys ics.