Hypervelocity Impacts on Plasma Facing Materials through Molecular Dynamics Simulations

It has been recognized that the production and dynamics of dust in the vacuum chamber of tokamaks are important problems in the framework of the safety and tokamak performance [1]. It is expected that during plasma discharges most of the dust particles concentrate in the scrape-off layer close to the chamber walls [2]. For almost all materials the hypervelocity regime (when the speed of an impact exceeds the speed of the compression waves both in the target and in the projectile) is reached when the impact speed exceeds a few km/s; it is therefore common to consider velocities above 2–3 km/s as hypervelocity impacts [3]. In studies related with plasma facing materials (PFMs) for future nuclear fusion technology, high velocity impacts have been reported, with velocities being around 500 m/s and up to several km/s [4, 5].<br/>In this study we focus on understanding the fundamental characteristics of the mechanisms underlying the crater formation caused by nanoparticles impacts on PFMs. In particular, tungsten (W) is currently the main candidate as plasma facing components for nuclear fusion reactors. For this reason, from molecular dynamics involving very large samples (up to 40 million atoms), we determined the detailed atomistic and thermodynamic aspects of crater formation mechanisms after hypervelocity (v up to 9 km/s) impacts of W projectiles in W.<br/>Further, the atomistic mechanisms of damage initiation during hypervelocity impacts were investigated. Various aspects of the impact at high velocities where the projectile and part of the target materials undergo massive plastic deformation, breakup, melting, or vaporization were analysed. Different stages of the penetration process were identified. Whether the damage occurring in the subsurface of the target is described by collision cascades or as the effect of shock waves will be discussed.<br/>Different stages of the penetration process were identified, and a model developed to understand the damage produced by hypervelocity impact in terms of geometrically necessary dislocations, much like in classic indentation theory, will be presented.<br/><br/><b>References: </b><br/>[1] Federici G. et al 2001 Nucl. Fusion 41 1967.<br/>[2] Tsytovich V.N. and Winter J. 1998 Phys.—Usp. 41 815.<br/>[3] Burchell M.J. et al. 1999 Meas. Sci. Technol. 10 41.<br/>[4] Castaldo C. et al. 2007 Nucl. Fusion 47 L5-L9.<br/>[5] Ratynskaia S. et al. 2008 Nucl. Fusion 48 015006.