Studies of impurity migration in TEXTOR by local tracer injection

Abstract Tracer experiments have been carried out by injection 13 C marked methane through test limiters exposed to the scrape-off-layer in TEXTOR. The influence of impact energy and flux on depositing 13 C species has been studied. One experiment has been performed with biased test limiter (−300 V) in order to increase energy of positively charged ions and the other one with 10 times reduced 13 CH 4 injection rate compared to previously used injection rate. Biasing of the test limiter increases the resulting 13 C deposition by a factor of ∼6 – post-mortem analysis yields a 13 C deposition efficiency of ∼1.7% compared to ∼0.3% without biasing. Reducing the injection rate increases 13 C deposition efficiency to ∼0.7%, which is more than two times larger compared to experiments with previously used injection rate. ERO modelling shows that enhanced re-erosion of redeposits is still necessary to reproduce measured 13 C deposition efficiencies.