Injection of active species (scavengers) in divertor plasmas has been proposed as a possible technique to inhibit the formation of tritium-rich carbon films in hidden regions under carbon-based ITER scenarios. To date, molecular nitrogen injection is the best-documented example of this concept. In this paper, the results recently obtained in ASDEX-Upgrade of nitrogen injection in the divertor are presented. The deposition rates at the sub-divertor region are reduced by a factor of up to five in nitrogen-seeded plasmas, as measured by the quartz microbalance diagnostic. No perturbation in the divertor plasma parameters for the chosen injection conditions is observed. Mass spectra at several toroidal locations indicate that conversion of C1 into C2 hydrocarbons takes place near the injection port in the presence of nitrogen, in agreement with earlier observations made in laboratory plasmas.
Recent results from divertor and scrape-off layer studies at JET