JET has operated with divertors of differing geometries since 1994. Impurities accumulated in the inner leg of all the divertors, and operation of the first (Mk I) divertor with beryllium tiles demonstrated that most are eroded from the main chamber walls and swept along the scrape-off layer to the inner divertor. Carbon deposited at the inner divertor is then locally transported to shadowed regions such as the inner louvres, where, for example, most of the tritium was trapped during the deuterium-tritium experiment (DTE1). Factors affecting these transport processes (e.g. temperature) are important for ITER, but are not well understood.
Coad, J. P., Rubel, M., Bekris, N., Brennan, D., Hole, D., Likonen, J., Vainonen-Ahlgren, E., & JET-EFDA Contributors (2005). Distribution of hydrogen isotopes, carbon and beryllium on in-vessel surfaces in the various JET divertors. Fusion Science and Technology, 48(1), 551 - 556. https://doi.org/10.13182/FST05-A985