Influence of residence time and helium addition in the ammonia formation on tungsten walls in N2 - H2 glow discharge plasmas

The influence of the residence time for the active species and of the helium addition to the plasma in the ammonia formation in N-2 - H-2-(He) Glow Discharge plasmas has been studied using differential pumped mass spectrometry. Three different residence times for the N-2 molecule were studied: 25, 50 and 100 ms. Other experiments scanning the helium plasma content (0-8%) were performed with a residence time of 100 ms. While no difference in the ammonia formation yields was found between the cases of 25 ms and 50 ms, the ammonia yields were increased by a 25% in the case of 100 ms. Additionally, an increasing helium plasma content enhances the ammonia formation yields up to 45%. Three different effects induced by the presence of helium in the plasma were analyzed: effects in mass spectrometry measurements, changes in the electron temperature and modification of the surface chemistry. The analyzes pointed out to an improved N-H recombination on tungsten walls induced by helium presence as the key factor that could explain the higher ammonia yields experimentally found for the highest helium content in the plasma. (C) 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license.