As the high-intensity electrical current goes through the liquid metal part of the cell, it couples with its own magnetic field to induce a motion of the liquid alumimium. This results in a metal pad that:
- has a curved interface with the bath
- contains vortices of aluminium
- may harbour long-scale motions of the metal-bath interface (waves).
Any industrial process should aim at being as stable and optimised as possible, to maximise the profits it can generate. This is especially true in reduction cells, where any departure from optimality of a given pot design lowers the productivity of a whole plant.
Our experience and MHD modelling allow us to be sure that a design is stable and optimised. In an unstable design, waves triggered by MHD effects significantly decrease the current efficiency of cells and perturb the cell life. A stable, but sub-optimal design means that one is not maximizing the value that can be extracted from given conditions.
Assessing the MHD state and stability of a design is a non-trivial task. Currently, two classes of models exist: the linear stability approach, and the non-linear evolutive approach. The first kind examines the stability of linear modes around an equilibrium configuration (in full 3D), as presented for instance in . The second kind often relies on a shallow-water model to allow for a full time evolution of the metal pad, whose oscillations either grow (unstable configuration) or are damped (stable one). References  and  are examples of that class of models.
Whatever the approach, the goal is to determine if a given cell design is stable or not. This is however only an aspect of the problem. Another, that relies less on computations and more on experience, is to come up with a good design that takes care of the various practical constraints actually existing in a technology, implemented in a particular plant.
 Antille J, Descloux J, Flueck M and Romerio MV, Light Metals 1999, p. 333
 Dupuis M, Bojarevics V and Freibergs J, Light Metals 2004, p. 445
 Zikanov O, Sun H and Ziegler DP, Light Metals 2004, p. 453