Long discharges in a steady state with D2 and N2 on the actively cooled tungsten upper divertor in WEST

Nitrogen (N-2) will be used in ITER to enhance the radiative fraction to similar to 90%, thereby cooling the edge plasma and preventing damage to the plasma-facing components. However, the reactivity of N-2 with hydrogen isotopes can lead to the formation of tritiated ammonia (NT3). This should be considered in terms of the in-vessel tritium inventory, the regeneration of the cryo pumps, and the processes in the ITER de-tritiation plant. In the 'W' Environment in Steady-state Tokamak (WEST), a series of long L-mode discharges (similar to 50 s), with a constant N-2 seeding from the outer strike point region has been performed on the upper actively cooled divertor. In the absence of active pumping, the N-2 balance shows steady-state retention during plasma discharge, and is partially (similar to 35%) released in between discharges. Although a significant amount of N-2( 18.65 Pa m(3)) has been injected, the wall still exhibited N-2 pumping capabilities. Under these conditions, as long as this N-2 reservoir is not saturated, there is not enough N available for the detectable threshold of ND3 formation to be reached. In these WEST experiments, no ammonia is detected during the pulse or after the pulse in the outgassing phase. These results are consistent with and complementary to the N-2 seeded experiments performed in the Joint European Torus (JET) with its ITER-like wall and in the Axially Symmetric Divertor Experiment (ASDEX) upgrade.