Dynamic hydrogen retention in the walls of the tokamaks ASDEX Upgrade and DIII-D

The large dynamic particle inventory on the wall is investigated in ASDEX Upgrade and DIII-D with a fast local particle balance. Certain events (e.g. disruptions or ELMs) lead to an inhomogeneous neutral pressure distribution in the vessel, e.g. between main chamber and divertor region. We measure the pressure equilibration of this inhomogeneous distribution and estimate wall outgassing or pumping rates necessary to satisfy the particle balance. Detailed particle balances for the divertor of ASDEX Upgrade after disruptions reveal large additional source and sink terms which decay with time constants of 10-20 ms over up to three orders of magnitude. Taking into account the finite pump time constants of the divertor chambers, ad- and desorption times of the order of 1 ms can be derived. Corresponding studies on DIII-D confirm these results, but with a larger spread of the decay times (10-80 ms). The divertor pressure evolution after ELMs in both tokamaks also confirms the existence of such a fast reservoir.