Influence of He and Ar injection on ammonia production in N-2/D-2 plasma in the medium flux GyM device

Nitrogen used to reduce the power load onto the divertor in tokamak fusion devices (JET, AUG) has the drawback of ammonia formation. Non-negligible quantity of tritiated ammonia could be a serious concern for ITER, since it cannot be reduced through the presently designed fuel cycle loop. In this paper we report a study of the production of ND 3 as a function of the electron temperature (T e ) and neutral pressure in a N 2 /D 2 plasma mixture in the linear machine GyM. The nitrogenized compounds were monitored by Optical Emission Spectroscopy (OES) and Mass Spectrometry (MS). Measurements were performed at different values of T e ranging from 3 eV to 6 eV by varying the microwave power (2.45 GHz, up to 0.6 kW cw) that sustains the plasma, and for different neutral pressure at a constant ratio of nitrogen and deuterium partial pressures. The effect of introduction of He or Ar in the N 2 /D 2 mixtures has been also investigated. The ND 3 produced during plasma experiments has been quantified with a dedicated setup based on an in-line LN 2 trap and Liquid Ion Chromatography (LIC). Mass-spectrometry results showed that ND 3 is formed only during the plasma phase of the experi- ment while LIC showed that ammonia production increases with T e and with the total neutral pressure. Optical Emission Spectroscopy confirms the presence of ND species in all the cases studied. The addition of He and Ar in the N 2 /D 2 plasma, not modifing T e and n e , causes a reduction of ammonia formation, that is negligible in the case of Ar addition, while in the case of He reaches 80%. This suggests that He modifies the physical chemical process occurring at the wall, where the adsorbed He inhibits the reac- tions leading to ammonia formation at the metallic surface of the vessel. These results confirm the active role of a metallic surface in ammonia production and indicate He injection as a promising solution to limit the formation of the tritiated ammonia in the N seeded plasma of ITER. ©2017 The Authors. Published by Elsevier Ltd.