ORIGIN OF LOW-Z IMPURITIES IN TEXTOR WITH AN ALL CARBON SURROUNDING

Carbonization was first applied in TEXTOR on stainless steel main limiters, antenna protection limiters and on the Inconel liner. We deduced - assuming equal amounts of carbon and oxygen in the plasma core - the concentration ratios C/H approximately equals O/H approximately equals 0. 6% after standard carbonizations and approximately equals 0. 3% under optimum conditions (during a few tens of consecutive discharges). The plasma dilution was small. When graphite limiters were installed mid-1985, the low Z impurity concentration was higher by factors of five initially and two later compared to the earlier standard case. The authors tried to answer the following questions: what are the lowest concentrations of C and O which can be achieved in the presence of large area graphite elements (case of e. g. JET and TFTR)? and, why did the oxygen concentration increase when the graphite limiters were installed? )