The hydrogen plasma-chemical processes responsible for tokamak divertor detachment are studied experimentally in the linear device Magnum-PSI, with a focus on molecular activated recombination (MAR) in hydrogen plasmas. Hydrogen plasmas with electron densities up to 6×1020 m−3 were created in Magnum-PSI, and hydrogen gas puffing was used to locally enhance plasma–neutral interaction. Thomson scattering and Balmer line spectroscopy measurements show that as neutral pressure is increased, the plasma passes through regimes dominated by ionization, MAR, and electron–ion recombination in turn. Heat and particle fluxes decrease monotonically with pressure. Fulcher band measurements show that in our plasma conditions, a simple model based on Franck–Condon excitation of a thermal vibrational distribution fails to describe the vibrational distribution of the upper state. These results serve as a benchmark for modeling suites that aim to simulate the ITER divertor and motivates their accurate treatment of the discussed processes, particularly MAR.

We have investigated the synergistic effect of a combination of various impurity gases and hydrogen gas on plasma detachment of high temperature plasma, equivalent to scrape-off layer plasma of tokamaks in the GAMMA 10/PDX end region, utilizing an open magnetic field configuration. A small puff of an impurity gas (N 2 , Ne, Ar, Kr, Xe) in combination with a puff of H 2 gas is examined to evaluate their synergistic effect on the formation of detached plasma; the following results are obtained. (i) A combination of N 2 and H 2 puffs showed a clear decrease of electron density and ion flux; (ii) N 2 and H 2 puffs form a strong density gradient along the axial direction; and (iii) other noble impurity gases showed an insufficient synergistic effect. The new results indicate the possibility of achieving a reliable divertor operation scheme and the importance of a deeper understanding of the H 2 and N 2 assisted recombination process.

The leading candidate for impurity seeding in ITER is currently nitrogen. To date, there are only a few studies on the plasma chemistry driven by N 2 /H 2 seeding and its effect on the molecular-activated recombination of incoming atomic hydrogen ions in a detached-like scenario. Numerical simulations are needed to provide insights into such mechanisms. The numerous amount of plasma chemical reactions that may occur in such an environment cannot be entirely included in a 2 or 3-dimensional code such as Eirene. A complete global plasma model, implemented with more than 100 plasma chemical equations and 20 species, has been set up on the basis of Plasimo code. This study shows two main nitrogen-included recombination reaction paths resulted to be dominant, i.e. the ion conversion of NH followed by dissociative recombination and a proton transfer between H 2 + and N 2 , producing N 2 H +. These two processes are referred to as N-MAR (nitrogen-molecular activated recombination) and have subsequently been implemented into Eunomia, a spatially-resolved Monte Carlo code, designed to simulate the neutrals inventory in linear plasma machines such as Pilot-PSI and Magnum-PSI. To study the effect of N 2 on the overall recombination, three cases of study have been set up: from a defined puffing location with a constant total seeding rate of H 2 + N 2 , three N 2 ratios have been simulated, i.e. 0, 5 and 10%. The parameter monitored is the density of atomic hydrogen, being the final hydrogenic product of any recombination mechanism in the scenario considered. The difference in H density between the 0% case and the 10% case is about a factor 3. The importance of NH as electron donor is highlighted and N-MARs confirmed as reaction routes enhancing the conversion of ions to neutrals, making the heat loads to the divertor plate more tolerable. This work is a further step towards the full understanding of the role of N 2-H 2 molecules in a detached divertor plasma.

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