The role of MHD in fusion energy

Magnetohydrodynamics (MHD) is a fluid model that describes the macroscopic equilibrium and stability properties of a plasma. Actually, there are several versions of the MHD model. The most basic version is called “ideal MHD” and assumes that the plasma can be represented by a single fluid with infinite electrical conductivity and zero ion gyro radius. Other, more sophisticated versions are often referred to as “extended MHD” or “generalized MHD” and include finite resistivity, two-fluid effects, and kinetic effects (e.g. finite ion gyro radius, trapped particles, energetic particles, etc.). The present volume is focused on the ideal MHD model.

Most researchers agree that MHD equilibrium and stability are necessary requirements for a fusion reactor. If an equilibrium exists but is MHD unstable the result is almost always very undesirable. There can be a violent termination of the plasma known as a major disruption. If no disruption occurs, the result is likely to be a greatly enhanced thermal transport which is highly detrimental to fusion power balance. In order to avoid MHD instabilities it is necessary to limit the regimes of operation so that the plasma pressure and current are below critical values. However, these limiting values must still be high enough to meet the needs of producing fusion power. In fact it is fair to say that the main goal of ideal MHD is the discovery of stable, magnetically confined plasma configurations that have sufficiently high plasma pressure and current to satisfy the requirements of favorable power balance in a fusion reactor.