Pump-limiter experiments and analysis in the TEXTOR tokamak

The ALT-1 pump limiter was used effectively for particle control in the TEXTOR tokamak. Two distinctively different pump-limiter modules gave markedly different performance. Operation both as the main limiter and in the shadow of the main TEXTOR limiters allowed evaluation of the pump limiter over a wide range of incident plasma parameters. Incident ion fluxes of up to 9.5 X 1018 cm+2s-1 are measured with Langmuir probes mounted on ALT-1. Pressures of up to 3 mtorr are measured in the large (700 l) ALT-1 vacuum chamber with no active pumping. With active pumping (7000 l/s) particle-removal rates of up to 7 torrl/s are measured. The pressure rise in the pump limiter is strongly dependent on the interaction of neutrals with the edge plasma. The experimental results are found to agree closely with a 3-D Monte-Carlo neutral-transport code that includes the atomic physics of the plasma-neutral interaction. Langmuir probes mounted directly on the ALT limiter, and a scanning probe located 900 toroidally from the limiter are used to characterize the edge plasma