Modeling of neutral transport and impurity generation in the tore supra pump limiter

A system of modular pump limiters is employed in Tore Supra for particle control. It includes one large outboard module which is instrumented with an optical multi-channel analyzer viewing the neutralizer plate, in addition to pressure, temperature and particle diagnostics. The transport of neutrals in both the particle scoop mode and in the pump limiter mode with active pumping is modeled with the DEGAS code. Equilibrium neutral pressure and density, H_α-intensity at the neutralizer plate, and particle back-flow into the scrape-off layer are calculated for typical projected plasma edge conditions. Effects on neutral particle transport of varying the length of the "shelf" separating the throat region from the pumping chamber are investigated. The generation and transport of carbon impurities arising from ion impact sputtering at the neutralizer plate are estimated. The short mean-free-path for ionization of sputtered carbon atoms suggests the possibility of spectroscopically measuring the effects of poloidal flux variations arising from shadowing of the modular limiter system.