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| Hauptseite > Publikationsdatenbank > Assessment of erosion in recessed areas of fusion devices using multi-scale computer simulations |
| Hauptseite > Publikationsdatenbank > Assessment of erosion in recessed areas of fusion devices using multi-scale computer simulations |
| Book/Dissertation / PhD Thesis | FZJ-2025-04774 |
2025
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-95806-867-4
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Please use a persistent id in citations: doi:10.34734/FZJ-2025-04774
Abstract: The development of fusion reactors is a promising research field that leads to a greenenergy source with virtually infinite fuel. By confining a hot plasma in a magnetic field, the massive energy stored in hydrogen isotopes’ nuclei can be released and made available for human use. The current fusion reactor experiments successfully manage to heat a plasma to the extremely high temperatures needed for nuclear fusion, but the machines still serve as stepping stones to a fusion power plant with electricity generation. The confinement of a fusion plasma requires technologies with high precision, power input and durability, which currently prevent economic efficiency that a power plant must provide. Two of the most important devices in this field are the Joint European Torus (JET) and the International Thermonuclear Experimental Reactor (ITER), currently under construction. In an experimental fusion reactor, a large range of diagnostics is needed on the one hand to provide input to active control methods of the plasma, but on the other hand to learn more about the physics of a magnetically confined fusion plasma. Optical diagnostics with mirror systems adopt a key role in the operation of fusion experiments and are placed in the outer regions of the fusion device. The location of the optical diagnostics is recessed behind the main wall so that incoming heat and particle fluxes from the main plasma are minimized and damage to the diagnostic systems is prevented as good as possible. However, erosion of the mirror and deposition of impurity particles are still likely and could negatively impact the performance of the diagnostics and thus lead to critical issues for the tokamak operation. Additionally, in ITER the diagnostics likely cannot be manually accessed once fusion power operation starts due to activation of the materials caused by the high-energy fusion neutron flux, so long-term operability under the expected plasma conditions needs to be secured. Predictions of thelife-time of the optical diagnostics with validated tools are needed. The ERO2.0 impurity transport and plasma-wall interaction code can provide such numeric simulations of materials exposed to plasmas in fusion devices. However, in the recessed areas of the diagnostic mirrors, the Monte-Carlo code is approaching its limits due to the limited amount of statistics: only a minuscule fraction of simiulated test particles reaches the areas relevant for diagnostics. To correctly resolve these areas, an unreasonably large amount of test particles and thus computation time would be required if ERO2.0 is used in its standard setup. In this thesis, the erosion and deposition on diagnostic first mirrors in JET and ITER have been numerically analysed, by the development and application of a workflow for ERO2.0 simulations of recessed areas in fusion devices with adequate resolution. Firstly, code updates focussing on runtime optimizations of the recently developed ERO2.0 Guiding-Centre Approximation (GCA) tracing methods are implemented, bringing large improvements in code efficiency so that a larger amount of particles can be simulated in a reasonable computation time. As this is not sufficient to solve the statistics problems in the Monte-Carlo code, a three-stage simulation approach is introduced, in which the simulation volume is successively focussed more and more to the volume around the mirrors. This multi-stage workflow is first applied to JET, where an ITER-like mirror test assembly (ILMTA) was exposed in an experimental campaign operating with beryllium (Be) first wall and tungsten (W) divertor PFCs. The results of the simulation are compared to the experimental findings. The deposition of impurities on three mirrors located in the ILMTA matches to a satisfactory degree between numeric simulation and experimental measurement, therefore the validity of the approach is confirmed. Afterwards, the workflow is applied to ITER, where mirror systems are planned in the diagnostic first wall (DFW) in the Equatorial and Upper Port Plug (EPP/UPP). This predictive modelling is used to assess the impinging fluxes onto the molybdenum (Mo) First Mirrors (FMs) located in both ports, assuming a Be first wall, a W divertor and a steel DFW, for which pure iron (Fe) is used as a proxy in the modelling. The full workflow is evaluated in three plasma scenarios over the complete expected ITER experimental operation time, two H-mode scenarios and one L-mode scenario assuming constant plasma conditions in divertor configuration over the full simulation time, respectively. The main finding is that even after more than 2000 h of operation in a high-power H-mode plasma case, the centre of both FMs accumulates less than 0.5nm impurity materials, while erosion of the Mo mirrors is not expected to exceed 2.5nm in all scenarios. A strong geometric influence of the cone-shaped aperture located in the front of the mirrors is found, leading to increased impurity deposition on the edges of the FMs. Multiple additional case studies with different material assumptions are performed in this work to assess the credibility of the results and give further outlook of the impact of different wall material combinations on the first mirror erosion and deposition.
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