• Energy Research

Plasma emission of hydrogen and impurities is measured with use of filtered camera imaging in TCV divertor. The measurement technique is discussed in detail including: selection choice of filters, the absolute calibration procedure and the tomographic inversion. The process is applied to an unseeded L-mode density ramp discharge in an unfavorable for H-mode access magnetic field direction with divertor baffles. Obtained poloidal emission profiles are used to split Balmer emission into excitation and recombination parts revealing a recombination emission maximum on the high field side. The optical emission is further studied by selecting radial profiles in cross sections of interest finding an opposite radial shift direction for high and low n Balmer line emission maxima in detached conditions.

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.