Influence of electrical currents driven by thermionic emission on tungsten melt motion

Experimental studies of tungsten (W) melt dynamics during repeated ELM transients on both ASDEX Upgrade and JET, together with MEMOS-U melt code analysis, have shown conclusively that melt motion is mainly driven by the Lorentz force due to the magnetic field acting on an electric current passing through the melt layer arising to compensate the thermionic electron emission from the hot plasma-facing surface. To further verify the melt transport model a new experiment has been carried out at ASDEX Upgrade in which transient melting was compared on two samples of identical geometry and at identical exposure positions, but with one sample electrically floating and the other connected to vessel potential. Thermionic emission cannot drive a current on the floating sample, but will instead modify the local sheath potential at the plasma-facing surface. As expected, significant differences were found in the post-exposure morphology of the two samples. Melt motion appears to be weaker at the floating sample surface, supporting the assertion of net replacement current as the main driving mechanism. However, the substantially higher power flux to the floating sample, attributed to an increased local sheath heat transmission factor, complicates the picture, preventing straightforward deductions. Simulations are underway with MEMOS-U to disentangle the two mechanisms.