Plasma-Material Interactions The Effect of Mixed and Impurity Materials on the Performance and Reliability of Plasma-Facing Components T. Sizyuk* and A. Hassanein School of Nuclear Engineering, Purdue University, West Lafayette, IN 47907, USA * email: [email protected] Background The ability of plasma-facing materials (PFMs) in fusion environment to withstand subsequent erosion and maintain clean interface as well as deuterium and tritium behavior depends on material properties and composition, which will continuously be changed during reactor operation due to both collisional and thermal processes. Analysis of composite/mixed materials as plasma-facing components in ITER-like and future successors requires extensive experimental and modeling research, including models development, validation and integration, self-consistent simulations and benchmarking to understand the integrated effects and the complexity of all the physical processes involved. Current essential areas of PMI research include hydrogen isotopes penetration, permeation, and recycling, retention and related material degradation, e.g., in tungsten1; nanostructures formed by helium on the surfaces of metallic walls, e.g., fuzz2; chemistry of isotopes uptake, e.g., in liquid PFMs3; neutrons irradiation on resulting isotopes retention, e.g., in tungsten4; transient plasma effects on material degradation5, erosion6 and irradiation of surrounding surfaces, e.g., in ITER design7. All above processes and reactions affect long-term PFMs performance and can change dynamics of each other. Self-consistent simulations, numerical and experimental, integrating as much as possible effects will lead to more realistic picture of plasma material interactions in reactor environment. Dynamic tracking of surface evolution at nano/micro layers can be performed using Monte Carlo binary collision approximation models that include most important processes of ions/atoms interactions and time-dependent evolution of target composition. The dynamic version of ITMC (Ion Transport in Materials and Compounds), i.e., ITMC-DYN code includes various implemented interatomic potentials for modeling elastic atomic collisions; several models for inelastic electronic energy loss; dynamic time-dependent update of target composition; multiple number of simultaneously incident ion beams with different parameters; implanted atoms diffusion and mixing; molecular surface recombination and desorption; chemical erosion; and surface segregation8,9. Progress We used the ITMC-DYN package for the analysis and prediction of multiple-ion beams effects on materials erosion and mixing. Integrated analysis of processes that can accompany hydrogen isotopes interaction with plasma facing materials showed that minute impurities, e.g., less than 1% of carbon, significantly increase erosion of metallic walls and Fig.1. Dependence of W erosion / C implantation on accelerate blistering in tungsten9. Self-consistent C content in beam with D ions at 870 K simulations of particles collisional processes, radiation enhanced diffusion, and surface segregation allowed explaining recent experimental results regarding the difference in tungsten erosion and buildup of carbon layer at different target temperatures10,11. Using the same relationship between the diffusion coefficient and the mobility parameter for modeling of surface segregation and conserving all other input parameters, we reproduced the experimental results for different target temperatures. Comparison with experiments was done also for mixed C/D ion beams where relation between sputtered W and implanted C was reproduced for several ion beam compositions (Fig.1). 2 We used also the ITMC-DYN package for initial analyzes of NSTX relevant parameters and conditions which can influence deuterium recycling, surface modification and erosion, impurities distribution, and sputtering. The combination of three main processes was considered in regard to hydrogen isotope motion and recycling from liquid lithium surfaces – reflection, diffusion, and surface recombination and desorption from Li and compounds. Significantly lower diffusion of deuterium in lithium oxide in comparison with pure Li results in deuterium accumulation in compound. Thicker layer of lithium oxide or hydroxide will result in higher deuterium accumulation near the surface due to the reduced diffusion to the bulk, that can increase desorption rate. However, increase of carbon impurities in the edge plasma will lead to carbon accumulation on the surface that will act as barrier to deuterium release from the surface. Specific proposal The above results showed the importance of self-consistent simulations integrating description of all important processes. We propose to continue modeling of mixed materials evolution extending and integrating more models and methods to approach complexity of reactor relevant experimental conditions. Proposed work will be based on the well tested code as well as to leverage our recently built Fig.2. Simulation of ELMs relevant energy deposition facilities for analysis of materials response to various by laser beams12. Surface morphology of the W surface ions/photons fluxes relevant to normal and transient after irradiation with 50 pulses at 0.46 MJ m-2. particles and energy loads12 (Fig. 2). We will consider mixed materials evolution for several candidates for plasma facing materials integrating different interactions during normal and transient plasma operation. Modeling of mixed ions fluxes during normal operation will be combined with modeling of energetic ions and electrons deposition to simulate ELMs regimes. Modeling results for materials mixing and heating will be compared with experimental simulations of PMI at normal operation using ion gun facilities and material response to ELMs energies using laser beams. Anticipated results, impact Comparative analysis of modeling and experimental results will allow prediction of conditions, e.g., for fuzz formation, suppression as well as removal by high energy ions. It will allow also to evaluate the effect of impurities in edge plasma and surface composition on hydrogen isotopes accumulation, desorption or penetration to the bulk. Generally, we propose PMI studies integrating in our research: various interactions and effects on plasma facing surfaces; various materials, ions and impurities; various temporal and energy regimes; comparison of results produced by numerical simulations with experimental simulations in ion gun and laser facilities. References: 1. T. Tanabe, Phys. Scr. T159, 014044 (2014). 2. J. K. Tripathi, T. J. Novakowski, G. Joseph, J. Linke, A. Hassanein, J. Nucl. Mater. (in-press) (2015). 3. P.S. Krstic, J.P. Allain, C.N. Taylor, J. Dadras, S. Maeda, K. Morokuma, J. Jakowski, A. Allouche, C.H. Skinner, Phys. Rev. Lett. 110, 105001 (2013). 4. M.H.J. ’t Hoen, M. Mayer, A.W. Kleyn, H. Schut and P.A. Zeijlmans van Emmichoven, Nucl. Fusion 53, 043003 (2013). 5. A. Suslova, O. El-Atwani, D. Sagapuram, S. S. Harilal and A. Hassanein, Scientific Reports 4, 6845 (2014). 6. G. Miloshevsky and A. Hassanein, Nucl. Fusion 54, 043016 (2014). 7. V. Sizyuk and A. Hassanein, Phys. of Plasmas 22, 013301 (2015). 8. A. Hassanein and D.L. Smith, Nucl. Instr. and Meth. B 13 225 (1986). 9. T. Sizyuk and A. Hassanein, J. Nucl. Mater. 404, 60 (2010). 10. T. Sizyuk and A. Hassanein, J. Nucl. Mater. 458, 312 (2015). 11. H. T. Lee, K. Krieger, J. Nucl. Mater. 390–391, 971 (2009). 12. N. Farid, S.S. Harilal, O. El-Atwani, H. Ding and A. Hassanein, Nucl. Fusion 54, 012002 (2014). 2
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