Experimental evidence of enhanced recombination of a hydrogen plasma induced by nitrogen seeding in linear device Magnum-PSI

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 H 2 /N 2 puffing and two cases of study have been set up, i.e. at 2 and 4 Pa. Achieved n e 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 N 2 /H 2 +N 2 flux ratios seeded have been carried out, at values of 0, 5, 10, 15 and 20%. A n e decrease while increasing the fraction of N 2 has been observed for both background pressures, resulting in a plasma pressure drop of 30%. T e 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 N 2 content, together with the NH 3 signal in the RGA. A further dedicated experiment has been carried out by puffing separately H 2 /N 2 and H 2 /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 N 2 , we observe an opposite behavior, indicating that N H 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 NH x species behaving as electron donor in the ion conversion with H + by means of what we define here to be N-MAR i.e. NH x + H +  → NH x + + H, followed by NH x + + e −  → NH x- 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.