Modelling of Kelvin–Helmholtz instability and splashing of melt layers from plasma-facing components in tokamaks under plasma impact

Abstract

Plasma-facing components (PFCs) in tokamaks are exposed to high-heat loads during abnormal events such as plasma disruptions and edge-localized modes. The most significant erosion and plasma contamination problem is macroscopic melt splashes and losses from metallic divertor plates and wall materials into core plasma. The classical linear stability analysis is used to assess the initial conditions for development and growth of surface waves at the plasma–liquid metal interface. The maximum velocity difference and critical wavelengths are predicted. The effects of plasma density, surface tension and magnetic field on the stability of plasma–liquid tungsten flows are analytically investigated. The numerical modelling predicts that macroscopic motion and melt-layer losses involve the onset of disturbances on the surface of the tungsten melt layer with relatively long wavelengths compared with the melt thickness, the formation of liquid tungsten ligaments at wave crests and their elongation by the plasma stream with splitting of the bulk of the melt, and the development of extremely long, thin threads that eventually break into liquid droplets. Ejection of these droplets in the form of fine spray can lead to significant plasma contamination and enhanced erosion of PFCs. The numerical results advance the current understanding of the physics involved in the mechanism of melt-layer breakdown and droplet generation processes. These findings may also have implications for free surface liquid metal flows considered as the first wall in the design of several types of future fusion reactors.