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