Molecular dynamics simulations of atomic carbon on tungsten surface

Abstract

By means of molecular dynamics simulations using bond-order potential, we have investigated the behavior of atomic carbon on both bcc tungsten (0 0 1) surface, and amorphous tungsten surface that is formed by a simulated annealing process, at finite temperature (T = 300 K) with incident energy ranging from 0.5 eV to 200 eV. The particle and energy reflection coefficients as well as mean range distribution as a function of incident energy at normal incidence have been calculated and the channeling effect in the energy range above 100 eV has also been discussed. The results are compared with those calculated by Eckstein using binary collision (BC) Code TRIM.SP in the energy range from 55 eV to 200 eV.

Introduction

Tungsten (W) and its alloys have been considered as candidate plasma-facing materials (PFMs) for ITER [1], future DEMO reactors [2], [3] and EAST [4]. As long as both elements, tungsten and carbon, are introduced in the main chamber of a fusion device, mixing with carbon on the tungsten material surface will be unavoidable because carbon based material is easy to be eroded and the carbon atoms will generally migrate to other locations due to long-range plasma transport processes and interact with the tungsten surface [1], [5], [6]. Therefore, it is necessary to understand and predict the tungsten surface properties and performance in the presence of C impurities.

The existing experimental data were completely limited in the energy range of keV for carbon projectiles interacting with tungsten surface [7], [8], [9] and simulations were mainly based on the binary collision approximation and adopted amorphous target materials [7], [10]. In the lower incident energy range, which is more important in a fusion device, experimental data are absent. When the mean free path of ions at low incident energies approaches the average atomic spacing in the target, BC model breaks down and many-body nature has a dramatic effect on the interaction between projectiles and target atoms, requiring molecular dynamics (MD) for accurate simulation. In this work, the particle and energy reflection coefficients as well as mean range distributions of incident atomic carbon on tungsten surface have been calculated using classical MD simulations, and an analytical bond-order potential for modeling non-equilibrium processes in the ternary W–C–H system [11]. Due to the scarcity of experimental results, we will compare our simulation results to Eckstein’s data in Ref. [12] where gives a collection of sputtering, reflection and range values calculated by a Monte Carlo (MC) program TRIM.SP [13] in the energy range from 55 eV to 40,000 eV at normal incidence.