Surface instability of flowing liquid metal in magnetized fusion plasma

Plasma facing components (PFCs) using liquid metals provide the opportunity to control wall recycling, migrate heat load, and achieve replenishment for surface self-remediation. This results from the manipulation of liquid flow using specifically designed feeding structures. Liquid lithium PFCs have improved fusion operations in various devices in terms of increasing stored energy and accessing radiation dissipation near the PFCs to reduce the wall heat load. Nonetheless, it is possible for the surface to become unstable at which plasma, magnetic and liquid parameters are temporally changed by some causes, suggesting that careful manipulation is required. The present study focuses on establishing a theoretical model using the linearized energy principle with the velocity potential analysis including the viscosity and electrical resistivity of the liquid. We use this to characterize the surface instability of an arbitrary liquid conductor flowing on a flat surface under a plasma sheath electric field and an external magnetic field. Subsequently, several factors (flow speed, surface inclination, thickness, surface tension, liquid density, electrical resistivity, plasma parameters, external magnetic field, and perturbation orientation with respect to magnetic field) were considered on a case-by-case basis to gain understanding on their influence on stability. Finally, the model was compared with camera observations on the flowing liquid Li limiter (FLiLi) of the EAST tokamak to describe the instability events and to estimate the timescale of initial instability development. The model describes well the instability related to the liquid Li of the FLiLi experiments.