Macroscopic erosion of plasma facing and nearby components during plasma instabilities: the droplet shielding phenomenon

Erosion losses due to ablation during a plasma disruption can be extremely high. This can severely limit divertor system lifetime to only a few disruptions. Ablation is mass loss in the form of macroscopic particles (MPs), i.e., droplets of liquid metals or large pieces (grains or crystallites) of non-melting materials such as carbon-based materials. Results of self-consistent magnetohydrodynamic (MHD) calculations are obtained that couple the dynamics of both vapor cloud and MP interaction to incoming ions and electrons from the collisionless scrape-off layer during the disruption. The equation of motion of the MPs is solved inside the inhomogeneous vapor-cloud conditions. For a well-confined vapor cloud, the flight time of MPs in the vapor is short, and complete vaporization of the emitted MPs occurs. This will result in further reduction of net radiation power to the surface, i.e., `droplet shieldingʹ effect. However, if the vapor cloud has a high velocity component along the divertor surface, the MPs are quickly removed from the hot vapor region. A shorter flight time of the MPs in the vapor cloud can result in significant mass losses and extremely short erosion lifetime of the divertor surface and nearby components.