Carbon deposition and hydrogen isotope retention in JET

Hydrogen retention in tokamaks is dominated by two mechanisms: implantation into plasma-facing surfaces and trapping in deposited layers. The amount of hydrogen implantation saturates at typically ∼1021 atoms m-2, giving ∼2 x 1023 atoms in the JET first wall (which is ∼200 m2), whilst codeposition depends firstly on the quantity of carbon deposited, and secondly on the temperature history of the deposits. Generally, codeposition has dominated the retained H inventory in JET, which was typically ∼1024 atoms. The installation of a divertor in JET has necessitated the presence of water-cooled components to protect the divertor field coils, whereas previously all plasma-facing components were normally at least 300 deg. C. Thick carbon-based films are deposited on surfaces in the vicinity of the inner corner of the divertor on surfaces shadowed from the plasma. Because of the divertor cooling, these deposits are at low temperature and their H:C ratio is at least 0.5:1; as a result their contribution to the overall in-vessel inventory is increased. No comparable deposition is found at the outer divertor. In the Mk IIA divertor the majority of the deposition occurs on cool surfaces many centimetres from the plasma, although with a line-of-sight to the vicinity of the inner strike point, and the average amount deposited per pulse is much greater than previously observed. The number of carbon atoms forming the basis of the deposits amounts to several percent of the ion flux to the inner strike point. The build-up of material leads to spalling (probably on venting to air). The mechanism for the generation and transport of the carbon to form the films is unknown, but ELMs may have a role: it is important to fully understand the phenomenon because of the similarities between the JET and ITER divertor geometries. (author)