Numerical modelling of detached plasma experiments with differential pumping in Magnum-PSI

Sufficient decrease of plasma pressure, ion and heat flux along the scrape-off layer of a tokamak fusion reactor are imperative to ensure the survival of the divertor tiles. This specific condition, defined as detachment, occurs within low temperature, high density, highly recycling\u3cbr/\u3eplasma that can also be realized in linear plasma devices such as Magnum-PSI [1]. In MagnumPSI, high recycling is achieved by utilizing differential pumping across three vacuum chambers. Experiments have been conducted to mimic detachment in the Magnum-PSI linear device by\u3cbr/\u3evarying the neutral background pressure at the chamber of the recycling target via H2 gas puffing [2], while the hydrogen plasma source parameters are kept constant across the experiments. Numerical modelling is carried out to help gain further insights regarding the physics behind detachment. The experiments are benchmarked with a coupled fluid-kinetic approach using the B2.5- EUNOMIA code package [3]. EUNOMIA is a Monte Carlo neutral simulation optimized for linear\u3cbr/\u3egeometry. Thomson scattering measurements without gas puffing are used as a plasma boundary condition at the source in the simulation, and the gas pressure in the target chamber will be varied. The resulting electron density and temperature simulated profiles near the target are\u3cbr/\u3ecompared with profiles measured in experiments. This paper presents the result of the benchmark tests, and identifies the collisional processes and other physical effects relevant to the detached plasma stat

When and why are streamers attracted to dielectric surfaces?

Summary form only given. Solid insulation surfaces in gas insulated high voltage (HV) equipment can be advantageous or dangerous with respect to dielectric breakdown by a discharge in the gas insulation, depending on whether the surface blocks the discharge (perpendicular, or dielectric barrier, configuration) or allows the discharge to creep along it (tangential configuration). Although discharge propagation along a surface is an old problem, there is still room for improved understanding of the fundamental physics and for the development of knowledge-based design rules for HV equipment. We thus investigate experimentally the initial (streamer) phase interacting with a dielectric surface. We studied streamers inside a gas-filled vessel using ICCD imaging, both stroboscopically as well as with single-shots. Inside the vessel, HV was applied to a needle located 10-15 cm above a grounded cathode. A dielectric sample was placed in the discharge gap. We varied several experimental parameters, such as pressure, gas composition, relative permittivity, pulse voltage and various geometrical parameters to study their effect on the discharge's affinity to prefer the dielectric surface instead of propagating through the bulk gas. Our experimental results provide us with the necessary information to start an in-depth discussion about the important mechanisms governing discharge propagation on surfaces. We show that the local availability of free electrons and the local electric field together determine the behavior of the discharge and explain how several parameters influence this behavior

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

\u3cp\u3e 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 \u3csub\u3e2\u3c/sub\u3e /N \u3csub\u3e2\u3c/sub\u3e puffing and two cases of study have been set up, i.e. at 2 and 4 Pa. Achieved n \u3csub\u3ee\u3c/sub\u3e are ITER-relevant i.e. above 10 \u3csup\u3e20\u3c/sup\u3e m \u3csup\u3e−3\u3c/sup\u3e and electron temperatures are in the range 0.8–2 eV. A scan among five different N \u3csub\u3e2\u3c/sub\u3e /H \u3csub\u3e2\u3c/sub\u3e +N \u3csub\u3e2\u3c/sub\u3e flux ratios seeded have been carried out, at values of 0, 5, 10, 15 and 20%. A n \u3csub\u3ee\u3c/sub\u3e decrease while increasing the fraction of N \u3csub\u3e2\u3c/sub\u3e has been observed for both background pressures, resulting in a plasma pressure drop of ̴ 30%. T \u3csub\u3ee\u3c/sub\u3e remains constant among all scans. The peak intensity of NH*(A \u3csup\u3e3\u3c/sup\u3e ∏->X \u3csup\u3e3\u3c/sup\u3e ∑ \u3csup\u3e−\u3c/sup\u3e , ∆v = 0) at 336 nm measured with optical emission spectroscopy increases linearly with the N \u3csub\u3e2\u3c/sub\u3e content, together with the NH \u3csub\u3e3\u3c/sub\u3e signal in the RGA. A further dedicated experiment has been carried out by puffing separately H \u3csub\u3e2\u3c/sub\u3e /N \u3csub\u3e2\u3c/sub\u3e and H \u3csub\u3e2\u3c/sub\u3e /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 \u3csub\u3e2\u3c/sub\u3e , 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 \u3csub\u3ex\u3c/sub\u3e species behaving as electron donor in the ion conversion with H \u3csup\u3e+\u3c/sup\u3e by means of what we define here to be N-MAR i.e. NH \u3csub\u3ex\u3c/sub\u3e + H \u3csup\u3e+\u3c/sup\u3e → NH \u3csub\u3ex\u3c/sub\u3e \u3csup\u3e+\u3c/sup\u3e + H, followed by NH \u3csub\u3ex\u3c/sub\u3e \u3csup\u3e+\u3c/sup\u3e + e \u3csup\u3e−\u3c/sup\u3e → NH \u3csub\u3ex-\u3c/sub\u3e \u3csub\u3e1\u3c/sub\u3e + 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. \u3c/p\u3

Studying the influence of nitrogen seeding in a detached-like hydrogen plasma by means of numerical simulations

\u3cp\u3eThe leading candidate for impurity seeding in ITER is currently nitrogen. To date, there have only been a few studies on the plasma chemistry driven by N\u3csub\u3e2\u3c/sub\u3e/H\u3csub\u3e2\u3c/sub\u3e 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 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 the Plasimo code. This study shows that two main nitrogen-including recombination reaction paths are dominant, i.e. the ion conversion of NH followed by dissociative recombination, and the proton transfer between and N\u3csub\u3e2\u3c/sub\u3e, producing N\u3csub\u3e2\u3c/sub\u3eH\u3csup\u3e+\u3c/sup\u3e. These two processes are referred to as N-MAR (nitrogen molecular-activated recombination) and have subsequently been implemented in Eunomia, which is a spatially resolved Monte Carlo code designed to simulate the neutral inventory in linear plasma machines such as Pilot-PSI and Magnum-PSI. To study the effect of N\u3csub\u3e2\u3c/sub\u3e on the overall recombination, three studies have been set up, and from a defined puffing location with a constant total seeding rate of H\u3csub\u3e2\u3c/sub\u3e + N\u3csub\u3e2\u3c/sub\u3e, three N\u3csub\u3e2\u3c/sub\u3e ratios were 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 of three. The importance of NH as an electron donor is highlighted, and the N-MAR reaction routes are confirmed to enhance the conversion of ions to neutrals, making the heat loads to the divertor plate more tolerable. This work is a further step towards a full understanding of the role of N\u3csub\u3e2\u3c/sub\u3e-H\u3csub\u3e2\u3c/sub\u3e molecules in a detached divertor plasma.\u3c/p\u3

High-fluence and high-flux performance characteristics of the superconducting Magnum-PSI linear plasma facility

\u3cp\u3e The Magnum-PSI facility is unique in its ability to produce and even exceed the heat and particle fluxes expected in the divertor of a fusion reactor, combined with good access to the plasma-material interaction region for diagnostics and relatively easy sample manipulation. In addition, it is possible to study the effects of transient heat loads on a plasma-facing surface, similar to those expected during so called Edge Localized Modes. By virtue of a newly installed superconducting magnet, Magnum-PSI can now maintain these conditions for hours on end for truly long term tests of candidate plasma facing materials. The electron density and temperature in the plasma beam center as a function of different magnetic fields up to 1.6 T, gas flow and source current are determined: particle fluxes greater than 10 \u3csup\u3e25\u3c/sup\u3e m \u3csup\u3e−2\u3c/sup\u3e s \u3csup\u3e−1\u3c/sup\u3e and heat fluxes of up to 50 MW m \u3csup\u3e−2\u3c/sup\u3e are obtained. Linear regression and artificial neural network analysis have been used to gain insight in the general behavior of plasma conditions as a function of these machine settings. The plasma conditions during transient plasma heat loading have also been determined. These capabilities are now being exploited to reach fluence of up to 10 \u3csup\u3e30\u3c/sup\u3e particles m \u3csup\u3e−2\u3c/sup\u3e at ITER-relevant conditions, equivalent to a significant fraction of the divertor service lifetime for the first time. \u3c/p\u3