Initial alt-i pump limiter studies on TEXTOR

Abstract

The ALT-I pump limiter has been used to control the fueling and recycling characteristics of TEXTOR during stable, reproducible tokamak discharges. The module has been operated in three modes: (1) normal limiter, with no particle collection, (2) particle scoop, with a maximum ~ 2×10−3 Torr pressure rise in the 700 1 unpumped collection chamber, and (3) pump limiter, with up to ~10000 1/s pumping speed and particle removal rates of up to 6×1020/s. In a comparison of operation in modes (1) and (3) using identical gas fueling programs, the total core electron number decreased by as much as 50%. The effective TEXTOR particle confinement time, τ∗p = τp/(l - R), was decreased by a similar ratio. Within the throat region, during typical operation, electron densities and temperatures were 6×1011−2×1012/cm3 and 15–30 eV. These conditions are representative of an operating regime in which there is high reionization of neutrals, but no change in incoming plasma parameters in the throat region. Energetic particles near the deflector plate were observed. During a gradual insertion of TiC-coated ALT-I beyond the stainless steel TEXTOR main limiters, the density of Ti in the plasma increased to a level similar to those of Cr and Fe. The gas injection fueling efficiency while puffing hydrogen directly into the plasma at the pump limiter tangency point was measured to be > 0.9. These results are discussed in conjuction with measurements of particle flow within ALT-I and other plasma diagnostics to characterize pump limiter operation on TEXTOR.