Molecular Dynamics Simulations Of Helium Clustering And Bubble Growth Under Tungsten Surfaces

Plasma-material interaction is one of the maj or challenges in achieving economically viable fusion energy for human's peaceful use.Up to date,tungsten(W)has been well recognized as a leading candidate material for plasma facing material(PFM)owing to its outstanding properties such as good thermo-mechanical performance,strong resistance to erosion,and low tritium retention.However,working as PFM of future steady-operation fusion devices,W will be exposed to huge fluxes of helium(He)and hydrogenic species unremittingly with low incident energy for a long duration.Under such conditions,numerous experiments have shown that the morphology of W surface varies.A nanostructure,"fuzz",has often been observed to form on the W surface.The fuzz can degrade the thermo-mechanical performance,weaken the resistance to erosion,and raise the tritium(as well as deuterium)retention.Therefore,it is important to understand what mechanism underpins the W fuzz formation.Experimental observations imply that the W fuzz is associated with coalescence of He species and growth of He clusters.Previous simulation studies addressed the mechanisms related to the formation of W fuzz mainly at the macroscopic level,by inevitably introducing some of assumptions without solid physical ground,but rarely at the microscopic level.The classical molecular dynamics(MD)simulation is well suitable for studying microscopic or submesocopic problems at the atomic level.Consequently,in the research work towards my PhD,the MD simulation is employed to uncover what mechanism governs the evolutions of He clusters and bubbles(large He clusters)and why the fuzz grows only within a certain parameter range from the atomic level.The simulation studies are performed by using one of the famous and powerful open source MD codes,LAMMPS.The whole dissertation is composed of six chapters,which are introduced in turn as follows:In Chapter 1,the background knowledge related to the PhD project is briefly reviewed first.Then follows the motivation of the upcoming study:it aims to look into the behavior of He atoms under the W surface exposed to high fluxes of low energetic He ions by employing MD simulation at the atomic level to explore the physics behind the relevant experimental findings.In the last section of this chapter,the organization of the thesis is outlined for having a clear overview.Chapter 2 contains three parts.The first part overviews simulation techniques commonly adopted in related themes and explain why the classical MD simulation is chosen as the simulation tool for my study.The second part introduces some specific details of MD simulation for a W-H-He system,in which an interatomic potential including multi-body effects[Bonny et al,J.Phys.Condens.Mat.26(2014)485001]is in particular validated for the project.In the last section of this part,the computational recourse used in the simulation is briefed.In the last part of this chapter,the diffusion properties of He/D clusters are studied by MD simulation with Bonny's EAM potential.Chapter 3 reports the study on the surface response of W(001)to He bombardment by using the MD simulation with a simulation box containing 24400 W atoms.Simulations are performed with high He flux(?1028 m-2 s-1)with its incident energy 80 eV at temperature 2100 K for a long duration of 72 ns.It is found that the saturation of He retention is as high as over 47%.The high saturation can be attributed to the fact that bubbles trap He atoms and prevent them from diffusing to the surface and further back into the plasma.Importantly,it's found that bubbles typically grow in a relatively narrow band of He/V ratios(1?3).In addition,it is observed that the He clusters near the surface lead to the expulsion of He atoms from the W bulk,while some W atoms at the W surface migrate inward concurrently.The coalescence of helium bubbles is also observed.Chapter 4 studies the effects of the incident He flux and the substrate temperature on the size of He clusters underneath W surface under the bombardment of He ions with a fixed incident energy 80 eV but with different He fluxes(?3.0×1027,?1028,and?1029 m-2 s-1)at temperature 300?2100 K.All the simulations are run with a condition variable and other conditions fixed for different periods but with the same fluence?2.5×1020 m-2.The results indicate that the He cluster size depends on the magnitude of He flux:the higher flux leads to the He clusters forming in smaller size in W bulk but with larger number and shallower depth.The coalescence of He atoms with He bubbles depends on the W temperature:at elevated temperatures around 2000 K.the incident He species in W bulk slow down more rapidly than at 1000 K but the number of vacancies per He cluster is smaller.A discussion is also made on how these results are correlated with the fuzz.In Chapter 5,MD simulations are performed to study the effects of the W surface orientation and the incident He energy with a flux?1028 m-2 s-1 on the behavior of He clusters under W surfaces for a period of 24 ns.The simulation results indicate that the surface orientation plays an important role not only in determining the depth distribution but also in determining the He retention and cluster size:at planes(110).the retention rate of He atoms is the lowest,and at faces(111),the clusters grow most easily in the lateral direction.The present simulation results suggest that the(001)surface is the most favorable for fuzz growth.The incident energy has a strong effect on the retention of He atoms:The helium retention rate increases with the incident energy;however,the He retention depends weakly on temperature in the low energy range of interest.In Chapter 6,major conclusions are first summarized and then three maj or innovation points are refined and presented,and finally,the prospect for the future research is discussed.In short,the results obtained in the present thesis provide insight to the reasons why the fuzz grows only within a certain parameter range at the atomic level.However,further research is much needed to relate these atomic mechanisms with the growth of nano-sized W fuzz in order to get a comprehensive understanding of the effects of low energy He on W structure.