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Plasma instability: Difference between revisions
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Instabilities in plasmas are often referred to as 'modes'. Strictly speaking, these terms correspond to different concepts: | Instabilities in plasmas are often referred to as 'modes'. Strictly speaking, these terms correspond to different concepts: | ||
* Instability: the existence of an instability is determined by calculating the growth rate of an infinitesimal perturbation. If this growth rate is positive, the perturbation is unstable. | * Instability: the existence of an instability is determined by calculating the growth rate of an infinitesimal perturbation. If this growth rate is positive, the perturbation is unstable. | ||
* Mode: a (normal) mode is a (global) solution of a (linear) evolution equation. Typically, this solution is a (standing or propagating) wave. The linearity of the equation allows any solution to be | * Mode: a (normal) mode is a (global) solution of a (linear) evolution equation. Typically, this solution is a (standing or propagating) wave. The linearity of the equation allows any solution to be decomposed into a sum of mutually orthogonal normal modes. If the equation is non-linear, the term 'mode' has to be used with care. | ||
Plasma instabilities can be classified into a number of types according to their drive (or free energy source): | Plasma instabilities can be classified into a number of types according to their drive (or free energy source): | ||