Plasma equilibria and stability
An important field of plasma physics is the stability of the plasma. It usually only makes sense to analyze the stability of a plasma once it has been established that the plasma is in equilibrium. "Equilibrium" asks whether there are net forces that will accelerate any part of the plasma. If there are not, then "stability" asks whether a small perturbation will grow, oscillate, or be damped out.
In many cases a plasma can be treated as a fluid and its stability analyzed with magnetohydrodynamics (MHD). MHD theory is the simplest representation of a plasma, so MHD stability is a necessity for stable devices to be used for nuclear fusion, specifically magnetic fusion energy. There are, however, other types of instabilities, such as velocity-space instabilities in magnetic mirrors and systems with beams. There are also rare cases of systems, e.g. the Field-Reversed Configuration, predicted by MHD to be unstable, but which are observed to be stable, probably due to kinetic effects.
Plasma instabilities can be divided into two general groups:
Plasma instabilities are also categorised into different modes:
Source: Andre Gsponer, "Physics of high-intensity high-energy particle beam propagation in open air and outer-space plasmas" (2004)
Beta is a ratio of the plasma pressure over the magnetic field strength.
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