# Plasma physics

**Plasma physics** is the field of physics which studies the dynamic behaviour of plasmas.

Briefly, it is the study of the statistical properties of a field of charged particles, called a plasma. Sometimes called "the fourth state of matter" (besides solid, liquid, and gas), plasma in this context refers to a gas that has been subjected to enough energy to dissociate atoms from their electrons (ionization), producing a cloud of ions and electrons. Because these particles are ionized (charged), the gas behaves in a different fashion than neutral gas in, for instance, the presence of electromagnetic fields.

A common fluid treatment of plasmas comes from a combination of the Navier Stokes Equations of fluid dynamics and Maxwell's equations of electromagnetism. The resulting set of equations, with appropriate approximations, is called Magnetohydrodynamics (or MHD for short).

Plasma physics is important in astrophysics in that many astronomical objects including stars, accretion disks, nebula, and the interstellar medium consist of plasma. It is also important in hypersonic aerodynamics, since at hypersonic speeds the interaction of the shock wave and the boundary layer creates enough heat to ionize the air surrounding a body. This happens, for example, upon re-entry of the Space Shuttle into Earth's atmosphere. Plasma physics is used in work studying nuclear fusion since most known fusion reactions take place in plasma.

Fields of active research include (but of course are not limited to):

- Plasma equilibria and stability
- Nuclear fusion
- Plasma diagnostics
- Plasma sources
- Plasma interactions with waves and beams
- Industrial plasmas
- Plasma theory
- Plasma devices
- Magnetic fusion energy (MFE) -- tokamak,stellarator,reversed field pinch,magnetic mirror
- Inertial fusion energy (IFE) (also Inertial confinement fusion - ICF)

- Space plasmas, e.g. Earth's plasmasphere (an inner portion of the magnetosphere dense with plasma)
- Plasma cosmology