Molecular dynamics simulations of interactions between energetic dust and plasma-facing materials

Abstract

The interactions between dust and plasma-facing material (PFM) relate to the lifetime of PFM and impurity production. Series results have been obtained theoretically and experimentally but more detailed studies are needed. In present research, we investigate the evolution of kinetic, potential and total energy of plasma-facing material (PFM) in order to understand the dust/PFM interaction process. Three typical impacting energy are selected, i.e., 1, 10 and 100 keV/dust for low-, high- and hyper-energy impacting cases. For low impacting energy, dust particles stick on PFM surface without damaging it. Two typical time points exist and the temperature of PFM grows all the time but PFM structure experience a modifying process. Under high energy case, three typical points appear. The temperature curve fluctuates in the whole interaction process which indicates there are dust/PFM and kinetic/potential energy exchanges. In the hyper-energy case in present simulation, the violence dust/PFM interactions cause sputtering and crater investigating on energy evolution curves. We further propose the statistics of energy distribution. Results show that about half of impacting energy consumes on heating plasma-facing material meanwhile the other half on PFM structure deformation. Only a small proportion becomes kinetic energy of interstitial or sputtering atoms.

Introduction

Over the past few years progress towards the understanding of dust behaviors in fusion devices has been significant through experiments, modelling and model validation in it [1], [2], [3]. Many plasma processes such as flaking of blisters [4], volume polymerization [5], cracking development [6], arcing [7], brittle destruction [3] and surface melting [9] even ELM and disruption events [10], [11] contribute to the production of dust particles. Dust particles increases the operation difficulties of plasma since the dusts can contaminate the core plasma [12]. The contamination can increase the amount of impurity concentration in plasma or even lead to disruption [13], [14]. Dust particles can also worsen the operation safety of fusion devices, including the risk of exploding and tritium leaking. As a result dust generation and its transport have been attached great importance to.

Because of the dust-PFM (plasma-facing materials) interaction associated with the life time of PFM and impurity generations, it is also a crucial issue in fusion science. Impacting velocities, angle, materials of dust, number of impacting dust, etc. are the factors influencing dust-PFM interaction. Although some studies are being performed for the interaction, it remains a major unsolved problem. One of the reasons is lacking of diagnose methods on dust-PFM interaction process. To the author's knowledge, few effective diagnose techniques can investigate dust-PFM interaction at present. Our knowledge on the interaction is mainly from numeric simulation results, such as finite element method or molecular dynamics [15], [16], [17]. Moreover, previous dust studies basically focus on carbon dust owing to the widely use of carbon plasma-facing materials (such as CFC or graphite) [18], [19]. However, the characteristics of carbon and metal dust are quite different. Carbon materials are active in hydrogen's isotopes which is used as fusion fuel [14], [21], [22], [23], [24], [25]. The chemical erosion is the primary erosion mechanism for carbon materials. But metal materials are more stable in hydrogen environment (especially tungsten) thus the physical sputtering caused by heavy ions or dust particles becomes a more important erosion process than chemical erosion comparing to carbon PFM. As metal materials will be used in ITER, the research on metal dust-PFM interactions has become necessary and urgent.

The rest of this paper is organized as follows. Section 2, we briefly overview the molecular dynamics (MD) simulation methods. In Section 3, we present dust-PFM interaction process and investigate the effect of tungsten (W) dust particles impacting into W surfaces. Finally, we summarize our results and provide concluding remarks in Section 4.