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Wall retention of deuterium and gaseous impurities in all tungsten ASDEX Upgrade

The tritium inventory is one critical issue of the present ITER design. Gas balance measurements in all tungsten ASDEX Upgrade show different phases of a semi-detached H-mode discharge. After the limiter start-up, the wall is loaded during a high retention phase. After this a steady-state retention is reached. During this phase only 1.5 ± 3.2% of the puffed D atoms are retained in the vessel. During plasma ramp-down outgassing of the wall starts, resulting in a remaining inventory of about 1.6% 10 s after the discharge. The low retention is confirmed by postmortem analysis of tiles. To reach steady-state a wall loading of typical 1.9 × 1022 D atoms is needed. After applying boronization only 1.4 × 1022 D atoms are sufficient. The mechanism of this transient wall loading is still unclear, but from the amount of gas needed it seems to be plausible that not only the divertor but all in-vessel components are involved. Residual gas analysis is hampered by uncertainties of the H/(H + D) ratio for the different water and hydrocarbon molecules. Especially water seems to be produced by outgassing of in-vessel components. Nevertheless a first evaluation yields a D content of hydrocarbons of about 1%, which has only a minor effect for the gas balances.