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Plasma Chemistry in Divertor Relevant Plasmas

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One of the crucial issues to produce energy by nuclear fusion is the power exhaust problem. A significant fraction of the power generated in the plasma core is channeled through the scrape-off-layer and is delivered to the plasma-facing-components in the so-called divertor region. Divertor targets, which will be actively-cooled tungsten monoblocks in ITER, are nominally subjected to extremely high heat and particle loads, but can only tolerate up to 10 MW/m2 from a technological point of view. To achieve acceptable heat and particle flux onto the divertor targets, so-called plasma detachment is essential to be set up and controlled. Detachment is an operational regime where particular plasma conditions lead to ion recombination in the volume phase, resulting in lower particle flux, and is characterized by a plasma pressure drop along the magnetic field lines towards the target. It has been extensively proven, both theoretically and experimentally, that impurity seeding facilitates achieving detachment. However, little is known on the impurity-induced plasma chemical processes occurring in the divertor region during detachment operation. To address that, dedicated experiments in the linear plasma device Magnum-PSI have been carried out. Linear plasma machines, often referred to as divertor simulators, allow the generation of steady-state divertor-relevant plasma scenarios with great diagnostic accessibility. Plasma detachment has been firstly achieved by H2 gas injection in the target chamber, with neutral background pressure ranging from 0.3 to 16 Pa. In this work, the influence of three different impurities i.e. N2, Ar and He on detachment performance of a hydrogen plasma is evaluated. Those species have been actively puffed in the target chamber, together with H2, at flux ratios of 0, 5, 10, 15 and 20 %. The background neutral pressures were kept fixed at 2 and 4 Pa i.e. divertor-relevant conditions. Results highlight the beneficial role of N2+H2 seeding, decreasing the plasma pressure in front of the target and reducing the heat flux delivered to it. Interestingly, when looking at plasma density and temperature profiles, we observe that electron temperature remains constant among the scans while the electron density decreases with increasing the content of N2 in the seeded mixture. This is a further indication of enhanced recombination taking place, where ions are converted to neutrals in the plasma volume phase. An opposite trend is found concerning He and Ar. In fact, injection of H2+He and H2+Ar gas mixtures led to an increased heat flux in both cases compared to only H2. A Residual-Gas-Analyzer (RGA) has been used to study the conversion efficiency of N2 to ammonia, showing conversions between 3 and 5 %. The molecular emission band at 336 nm, corresponding to the electronic transition of NH*(A3Π→ X3Σ, Δv = 0), has been observed with optical emission spectroscopy and its intensity increases linearly with the flux ratio of N2 in the seeded mixture. Moreover, plasma radiation has been monitored by using a bolometry system. No significant trends as a function of injected nitrogen have been observed, hence excluding any power limitation effects. Similar experiments have been carried out with GAMMA10/PDX, a linear plasma machine located at the University of Tsukuba. The uniqueness of that machine lays in the capability of achieving high electron temperatures, albeit electron densities lower by two orders of magnitude compared to Magnum-PSI. Those experiments allowed us to investigate the effect of impurities on detached-like plasmas in a wider range of parameters. Results are in line with what has been achieved in Magnum-PSI, highlighting H2+N2 gas as the most favorable mixture to reduce the particle flux to the target. Numerical simulations are needed to provide a comprehensive understanding of the fundamental atomic and molecular processes occurring in divertor-like environment. A three-step approach has been adopted as follows: at first, global plasma models have been set-up on the basis of Plasimo code. Global models are spatially-averaged simulations allowing one to implement a large set of plasma chemical equations and to highlight the most relevant processes among them. Extended models have been built up for N2-H2, Ar-H2 and He-H2 plasma scenarios. 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 H2+ and N2, producing N2H+. These two processes are referred to as N-MAR (nitrogen-molecular activated recombination). Concerning the remaining two cases, no significant ion recombination process seems to be driven by the presence of either Ar and He. The resulting reduced scheme for H2-N2 chemistry has been implemented in Eunomia, a spatially-resolved Monte Carlo code suited for the transport of neutrals in linear plasma devices. Finally, Eunomia has been coupled with B2.5, a fluid code solving plasma equations. Simulation results of all the three cases of study (H2-He, H2-N2, H2-Ar) qualitatively reproduce the favorable effect of N2, while confirming the deteriorating effect of He and Ar on detachment performance. The importance of NH as electron donor is highlighted and N-MAR confirmed as reaction route enhancing the conversion of ions to neutrals, making the heat loads to the divertor plate more tolerable. This work represents a further step towards the full understanding of the role of plasma chemical volume processes in a detached divertor plasma.
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Article
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