Plasma stability

A ball at rest in a valley (right) will return to the bottom if moved slightly, or perturbed, and is thus dynamically stable. One on the top of a hill (left) will accelerate away from its rest point if perturbed, and is thus dynamically unstable. Plasmas have many mechanisms that make them fall into the second group under certain conditions.

In plasma physics, plasma stability concerns the stability properties of a plasma in equilibrium and its behavior under small perturbations. The stability of the system determines if the perturbations will grow, oscillate, or be damped out. It is an important consideration in topics such as nuclear fusion and astrophysical plasma.

In many cases, a plasma can be treated as a fluid and analyzed with the theory of magnetohydrodynamics (MHD). MHD stability is necessary for stable operation of magnetic confinement fusion devices and places certain operational limits. The beta limit, for example, sets the maximum achievable plasma beta in tokamaks.

On the other hand, small-scale plasma instabilities (typically described by kinetic theory), such as the drift wave instability, are believed to be the driving mechanism of turbulent transport in tokamaks, which leads to high rate of particle and energy transport across the confining magnetic fields. Plasma instabilities described by kinetic theory can contain aspects such as finite Larmor radius (FLR) effects and resonant wave-particle interactions, which is not captured in fluid models such as MHD.