# Plasma instability

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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.
- 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 caution.

Plasma instabilities can be classified into a number of types according to their drive (or free energy source):

- Rayleigh-Taylor instabilities due to density gradients or boundaries, associated with non-electromagnetic forces (e.g., gravity)
- Gradient driven instabilities, associated with thermodynamic forces due to gradients in an otherwise Maxwellian plasma
- Kinetic instabilities, associated with deviations from Maxwellianity, i.e., the anisotropy of the velocity distribution function
- Streaming instabilities, associated with energetic particles or electric currents interacting with the plasma, producing waves

Below is a list of some instabilities relevant to fusion plasmas (to be completed).

- Alfvén eigenmodes
- Ballooning instability
- Drift wave instability
- Edge Localized Modes
- Electron Temperature Gradient instability
- Flute instability
- Geodesic Acoustic Mode (GAM)
- Interchange instability
- Ion Temperature Gradient instability
- Kink instability
- Sausage instability
- Tearing mode instability, see also Magnetic island
- Whistler mode