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

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 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 N-2/H-2+ N-2 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)Pi->X-3 Sigma(-), Delta v= 0) at 336 nm measured with optical emission spectroscopy increases linearly with the N-2 content, together with the NH3 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 NeH 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.