Experimental evidence of enhanced recombination of a hydrogen plasma induced by nitrogen seeding in linear device Magnum-PSI. (May 2019)

• Record Type: Journal Article
• Authors: Perillo, R.; Akkermans, G.R.A.; Classen, I.G.J.; Vijvers, W.A.J.; Chandra, R.; Jesko, K.; Korving, S.; Vernimmen, J.W.M.; de Baar, M.R.
• Is Part Of: Nuclear materials and energy. Volume 19(2019)
• Page Start: 87
• Page End: 93
• Publication Date: 2019-05
• Subjects: Detachment -- Linear plasma device -- Impurity seeding -- Nitrogen -- Plasma chemistry -- N-MAR; Nuclear energy -- Periodicals; Nuclear fuels -- Periodicals; Nuclear reactors -- Materials -- Periodicals; Radioactive substances -- Periodicals; 621.4833
• DOI: 10.1016/j.nme.2019.02.018
• Languages: English
• ISSNs: 2352-1791

Abstract

Abstract: In this work we investigate the effects induced by the presence of nitrogen in a detached-like hydrogen plasmas in linear plasma machine Magnum-PSI. Detachment has been achieved by increasing the background neutral pressure in the target chamber by means of H2 /N2 puffing and two cases of study have been set up, i.e. at 2 and 4 Pa. Achieved ne are ITER-relevant i.e. above 10 20 m −3 and electron temperatures are in the range 0.8–2 eV. A scan among five different N2 /H2 +N2 flux ratios seeded have been carried out, at values of 0, 5, 10, 15 and 20%. A ne decrease while increasing the fraction of N2 has been observed for both background pressures, resulting in a plasma pressure drop of ̴ 30%. Te remains constant among all scans. The peak intensity of NH*(A 3 ∏->X 3 ∑ −, ∆v = 0) at 336 nm measured with optical emission spectroscopy increases linearly with the N2 content, together with the NH3 signal in the RGA. A further dedicated experiment has been carried out by puffing separately H2 /N2 and H2 /He mixtures, being helium a poorly-reactive atomic species, hence excluding a priori nitrogen-induced molecular assisted recombination. Interestingly, plasma pressure and heat loads to the surface are enhanced when increasing the content of He in the injected gas mixture. In the case of N2, we observe an opposite behavior, indicating that NH species actively contribute to convert ions to neutrals. Recombination is enhanced by the presence of nitrogen. Numerical simulations with two different codes, a global plasma-chemical model and a spatially-resolved Monte Carlo code, address the role of NHx species behaving as electron donor in the ion conversion with H + by means of what we define here to be N-MAR i.e. NHx + H + → NHx + + H, followed by NHx + + e − → NHx- 1 + H. Considering the experimental findings and the qualitative results obtained by modelling, N-MAR process is considered to be a possible plasma-chemical mechanism responsible for the observed plasma pressure drop and heat flux reduction. Further studies with a coupled code B2.5-Eunomia are currently ongoing and may provide quantitative insights on the scenarios examined in this paper.