Studies of impurity migration in TEXTOR by local tracer injection

Tracer experiments have been carried out by injection 13C marked methane through test limiters exposed to the scrape-off-layer in TEXTOR. The influence of impact energy and flux on depositing 13C 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 13CH4 injection rate compared to previously used injection rate. Biasing of the test limiter increases the resulting 13C deposition by a factor of ∼6 – post-mortem analysis yields a 13C deposition efficiency of ∼1.7% compared to ∼0.3% without biasing. Reducing the injection rate increases 13C 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 13C deposition efficiencies