Modelling of the retention dynamics at the transition from High-confinement phase to Low-confinement phase in a full-tungsten tokamak

The dynamic retention of deuterium in a full-tungsten JET-like tokamak is investigated thanks to the Dynamics of Wall Elements (DWE) code. DWE is a wall model coupled to the SolEdge-EIRENE edge-plasma transport code. It is composed of two internal codes: (i) WE-temp, which determines the wall temperature, and (ii) MHIMS, which determines the transport, trapping and desorption of hydrogen isotopes in the wall material. In this work, four consecutive discharges are modeled and analysed with DWE. These discharges are identical and present a transition from a H-mode phase to a L-mode phase, which induces strongly different plasma-wall interactions. The main results are as follows. (i) A release of deuterium is observed at the vicinity of both strike-points at the end of the H-mode phase. This dynamics is explained by an increase of the wall temperature at both strike-points due to the strong plasma heat flux received there, leading to detrapping from trap with low detrapping energy. On the contrary, during the subsequent L-mode phase, the wall temperature at the strike-points decreases, leading to retention by filling the trap emptied during the H-mode. (ii) A release of deuterium from the first-wall is unexpectedly observed at the transition between the H-mode and the L-mode. This release is a consequence of a decrease in implantation energy between the two phases, as it entails a shallower implantation of deuterium. Despite the different material considered in the simulation, the latter result offers a possible explanation of the strong transient release of deuterium experimentally observed in the JET-ILW at the NBI cutoff during high density H-mode discharges.