GEOMETRY-EFFECTS ON PARTICLE REMOVAL WITH THE PUMP LIMITER ALT-I

In the course of the ALT-1 pump limiter experiments on TEXTOR, limiter heads of different shapes, exhaust channels and materials have been applied and their performance has been compared. Particle removal rates up to 10 Torr 1/s have been observed in the case of TiC surfaces. Values with the graphite head at the same radial position are typically a factor of 2-3 lower. The removal rate for a 'detached plasma' discharge is 30-50% reduced as compared to a normal one. The removal efficiency reaches values in excess of 50%. The results suggest a guiding rule for the design of pump limiters with high removal efficiency: The effective pumping speed at the neutralizer plate should be large compared to the conductance for neutral particles backstreaming from the neutralizer plate to the main plasma.