Dynamic fuel retention in tokamak wall materials: An in situ laboratory study of deuterium release from polycrystalline tungsten at room temperature

Retention of deuterium ion implanted in polycrystalline tungsten samples is studied in situ in an ultra-high vacuum apparatus equipped with a low-flux ion source and a high sensitivity thermo-desorption setup. Retention as a function of ion fluence was measured in the 10¹⁷–10²¹ D⁺·m⁻² range. By combining this new fluence range with the literature in situ experimental data, we evidence the existence of a retention ∝ fluence^0.645±0.025 relationship which describes deuterium retention behavior on polycrystalline tungsten on 8 orders of magnitude of fluence. Evolution of deuterium retention as a function of the sample storage time in vacuum at room temperature was followed. A loss of 50% of the retained deuterium is observed when the storage time is increased from 2 h to 135 h. The role of the surface and of natural bulk defects on the deuterium retention/release in polycrystalline tungsten is discussed in light of the behavior of the single desorption peak obtained with Temperature Programmed Desorption.