Exploratory study of liquid metal response to rapid variation of applied magnetic field

TL;DR

This study investigates how liquid metal in nuclear fusion reactor components responds to rapid magnetic field changes, revealing a two-stage dynamic process and assessing the validity of common modeling assumptions through analysis and simulations.

Contribution

It provides the first detailed analysis of liquid metal response to rapid magnetic variations, including a two-stage behavior and evaluation of model assumptions.

Findings

Liquid metal exhibits a two-stage response with initial rapid currents and forces, followed by oscillations.

The maximum velocity of liquid metal during events is around 0.5 m/s.

Common assumptions like negligible Joule dissipation are validated, but incompressibility may be questionable.

Abstract

Transient plasma events, such as plasma disruptions, are anticipated in the future magnetic-confinement nuclear fusion reactors. The events are accompanied by a rapid change in the magnetic field generated by the plasma current and, accordingly, induction of strong eddy currents and Lorentz forces within the reactor structure. This work targets processes within liquid-metal components of the reactor's breeding blankets. Order-of-magnitude analysis and {exploratory} numerical simulations are performed to understand the response of liquid metal to a rapidly changing magnetic field and to evaluate the accuracy of commonly used simplifying model assumptions. The response is found to consist of two stages: an initial brief stage ($\sim 1$ ms) characterized by a rapid increase in the induced currents, forces, and fluid velocity; and a subsequent stage, which is triggered by the growing velocity of the metal and marked by reversals of Lorentz force, and oscillations and decreases in the amplitude of the induced fields. The transition to the second stage sets the upper limit of the velocity ($\sim 0.5$ m/s in our tests), to which an initially quiescent metal can be accelerated during the event. The simulations indicate that many widely used model assumptions, such as the negligible role of Joule dissipation in the heat balance and the constancy of physical property coefficients, remain valid during the response. However, the assumption of liquid metal incompressibility is found to be questionable due to the potential significant effects of pressure waves.