Melt-layer motion and droplet ejection under divertor-relevant plasma conditions

Accidental melting of metallic plasma-facing materials in future fusion devices poses serious issues regarding the material lifetime and power-handling capabilities as well as core plasma performances. The behaviour of aluminium (as a proxy for beryllium) and tungsten materials was investigated in the Pilot-PSI linear plasma device to study the melt-layer motion and droplet ejection under ITER-relevant plasma conditions. Heat fluxes of up to 50 MW m(-2) raised the surface temperature to values up to 5000 K. The melt-layer rotation was found to depend on the magnetic field (up to 1.6 T) strength and target potential and is attributed to J x B forces caused by radial currents in the plasma. The amount of droplets ejected from the molten surface depends on the material-more droplets ejected from aluminium than from tungsten-and the heat flux to the target. The average droplet velocity was determined to be around 60 m s(-1) for both materials with droplets being ejected mainly in the axial direction. Droplet ejection is only observed during helium discharges, no ejection can be observed with hydrogen plasmas despite similar heat fluxes. Bubble boiling appears to be the main mechanism contributing to the observed droplet ejection.