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.