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Plasma facing components in fusion devices are subjected to very high heat fluxes, leading to the potential for material degradation and melting. In the case where PFCs are made out of high Z materials like tungsten, the subsequent contamination of the plasma from wall degradation can become a major problem leading to a shutdown of the fusion reactions. To prevent the heat fluxes from damaging the divertor tiles, it may be necessary to seed gases in the edge plasma, radiating away the heat and cooling down the plasma before it makes contact with walls.
While nitrogen has good radiation characteristics in the edge plasma temperature range, it has the added complication that it will interact chemically with both the hydrogenic fuel and any metal surfaces. The production of tritiated ammonia will complicate the tritium fuel cycle, while interactions with the tungsten surface may lead to enhanced erosion and changes in how hydrogen is trapped in the surface. With limitations on how much tritium can be produced in a fusion reactor, along with regulations governing tritium handling and inventory, it is then crucial to understand how much of it is retained in the PFCs and how much could be released during thermal cycling.
It has previously been observed that simultaneous nitrogen and deuterium co-bombardment can result in a higher retention of deuterium in tungsten [1]. In the current study, we have broadened the parameter range of the previous study and have extended the scope of our surface analysis. As in [1], we observed an increase in deuterium retention in tungsten with the amount retained increased by an order of magnitude in the temperature range ~ 500 to 600K. This increase occurs for N+/D+ ratios as low as 1% and is accompanied by large-scale blistering of the surface.
XPS analysis of the co-bombarded surfaces indicate the formation of a tungsten-nitride layer, both at room temperature and at 575K. However, we believe that a change in the nature of the tungsten-nitride layer from predominantly WN at lower temperature to W2N at higher temperature [2], affects the ability of deuterium to diffuse to the surface.
[1] M.E. Goodland, J.W. Davis, "Influence of nitrogen on deuterium retention in tungsten under sequential and simultaneous irradiation", Nuclear Materials and Energy 26 (2021): 100928
[2] K. Schmid, et al., “Interaction of nitrogen plasmas with tungsten”, Nuclear Fusion, 50 (2010) 025006.