Free-Surface Liquid Metal Flows through Magnetic Field Gradients in the Divertor Region*

Flowing liquid metals (LM) offer an alternative to solid materials for divertors, advantageous for heat removal and the ability to provide a self-healing surface. Nevertheless, the effect of magnetic field gradients on LM free-surface flows could hinder steady operation, through splashing or flow disruptions. Magnetic field gradients up to 1 T/m were investigated using the LMX-U (Liquid Metal eXperiment Upgrade) channel at PPPL to measure the effects on surface velocity, depth, and streamwise currents. LM flow across a magnetic field generally induces perpendicular currents, whereas flowing through an increasing or decreasing field led to induced currents in the streamwise direction. These streamwise (poloidal) currents with a transverse (toroidal) magnetic field cause Lorentz forces in the surface-normal direction, forcing the LM onto or away from the divertor plate. While the measured effects were relatively small in this experiment, in a larger reactor the interaction of the induced streamwise currents and the toroidal field could generate surface-normal forces that significantly oppose the flow-stabilizing gravitational force. Measurements and simulations using FreeMHD (an OpenFOAM-based custom MHD solver) showed agreement at the LMX-U scale. Simulations at the reactor scale then indicated the toroidal magnetic field gradient in the streamwise direction could be an obstacle for divertor free-surface LM flows.