Plasma–wall interactions with nitrogen seeding in all-metal fusion devices: Formation of nitrides and ammonia

Nitrogen is routinely used to control the power load to the divertor targets of tokamak fusion reactors. However, its chemical reactivity can have implications on design and operation of a fusion device. In this contribution experimental results from recent years on three topics are briefly presented. These are the formation of nitrides, the sputtering of beryllium in the presence of nitrogen and the production of ammonia. Laboratory experiments have shown that surface nitrides are formed both on beryllium and tungsten upon exposure to energetic nitrogen ions. Erosion rates of Be by energetic N ions are in good quantitative agreement with modeling. Erosion upon exposure of Be to a mixed N/D plasma is reduced with respect to a pure D plasma. Finally, the appearance of ammonia has been observed in mixed N/D plasmas as well as in the exhaust gas of AUG and JET. The production rate in AUG reached 5% of the injected N atoms in a series of three subsequent N2-seeded discharges.