Influence of surface melting on the deuterium retention in pure and lanthanum oxide doped tungsten

The issue of material melting under transient heat fluxes is one of the key concerns associated with the use of tungsten (W) divertor targets in ITER as the next step fusion device. This study aims to closely examine the impact of surface melting on fuel retention with respect to the performance in subsequent operations. Besides pure W, W - 1 wt. % La2O3 (WL10), which is considered as an advanced plasma facing material, is tested as a comparative study. The two tungsten grades are successively exposed to high heat flux loads and low-energy deuterium (D) plasma. The thermal desorption spectroscopy (TDS) measurements indicate that D retention reduces by similar to 40% after shallow surface melting for both of the two W grades. On the other hand, D retention in the WL10 is almost double that in the pure W regardless of whether surface melting occurs or not. The TDS spectra exhibit two main desorption peaks at around 550 K and 700 K, which largely corresponds to grain boundaries and voids respectively. However, an additional peak appears unexpectedly around 1070 K for the melted WL10. We attribute this additional high-temperature peak to the formation of the hydrogen absorbing metal La in the resolidified layer, which is confirmed by transmission electron microscopy and energy dispersive x-ray spectrometry results.