Plasma is a state of matter containing mobile electrons and ions, often together with neutral particles, whose interactions produce collective electromagnetic behavior. Commonly described as the fourth state of matter, it differs from an ordinary gas because its charged constituents carry electric currents and respond strongly to electric and magnetic fields. Plasmas occur in stars and interplanetary space, in lightning, and in laboratory discharges used for lighting, manufacturing, and fusion research. Complete ionization is not necessary: a largely neutral gas can exhibit plasma behavior if its charged component has sufficiently strong collective effects. (energy.gov)
Formation and thermal properties
Plasma formation commonly involves ionization, in which sufficient energy is supplied to remove electrons from an atom or molecule. Heating can produce energetic collisions that cause ionization. Electrical discharges accelerate electrons, which then ionize neutral particles through collisions; intense radiation and laser irradiation can also generate plasmas. The appropriate production method depends on the material, density, and required plasma conditions. (energy.gov)
A plasma does not necessarily have a single temperature. In a thermal plasma near thermodynamic equilibrium, electrons, ions, and neutral particles have approximately comparable temperatures. In a nonthermal plasma, electrons can be much more energetic than the heavier particles. Electric fields preferentially energize the light electrons, while the bulk gas can remain relatively cool. Thus, “cold plasma” does not mean that every particle has little energy; it usually describes a nonequilibrium system with comparatively cool heavy particles. (suli.pppl.gov)
Temperature and degree of ionization describe different properties. Low-temperature plasmas are often weakly ionized, whereas sufficiently hot plasmas can be fully ionized. However, plasma behavior depends on density, interactions, and the length and time scales under consideration, rather than on a universal temperature threshold. (energy.gov)
Collective behavior and characteristic scales
The defining feature of plasma is collective behavior: particles respond to electromagnetic fields generated by many other particles, not only to individual collisions. Many bulk plasmas are approximately quasineutral, meaning that positive and negative charge densities nearly balance over sufficiently large distances. This does not eliminate local charge separation, electric currents, or dynamically important fields. (mrx.pppl.gov)
An important example is Debye shielding. When a charge perturbs a plasma, nearby particles redistribute, reducing its electrostatic influence at greater distances. In a classical, weakly coupled plasma, the electron contribution to the characteristic screening scale, the Debye length, is
[ \lambda_{De}=\sqrt{\frac{\varepsilon_0 k_B T_e}{n_e e^2}}, ]
where (T_e) is electron temperature, (n_e) is electron number density, (e) is the magnitude of electron charge, (k_B) is the Boltzmann constant, and (\varepsilon_0) is vacuum permittivity. Other mobile species can also contribute to screening. (farside.ph.utexas.edu)
Charge separation also produces a restoring electric field, allowing electrons to oscillate relative to ions. The characteristic electron plasma frequency, expressed as an angular frequency, is
[ \omega_{pe}=\sqrt{\frac{n_e e^2}{\varepsilon_0 m_e}}, ]
with (m_e) denoting electron mass. Debye length and plasma frequency help determine when collective behavior becomes important. Conventional descriptions of a bulk plasma generally concern systems much larger than the screening length and processes lasting long enough for a collective response to develop. Boundary layers can nevertheless require analysis at Debye-length scales. (farside.ph.utexas.edu)
Magnetic dynamics and theoretical descriptions
A magnetic field changes charged-particle trajectories through the Lorentz force. In sufficiently magnetized conditions, particles gyrate around magnetic field lines while their guiding centers move along the field or drift across it. These motions help explain both magnetic confinement and the differing transport properties parallel and perpendicular to a field. (farside.ph.utexas.edu)
Plasma theory uses several complementary descriptions. Kinetic models follow particle distributions in position and velocity space, coupled to Maxwell’s equations. Fluid models instead describe quantities such as density, flow velocity, pressure, and temperature. Magnetohydrodynamics treats a plasma as a conducting fluid under suitable large-scale, low-frequency conditions. It does not capture every effect arising from separate electron and ion motions or particle resonances. (farside.ph.utexas.edu)
Plasmas support waves, instabilities, shocks, and turbulence. Another important process is magnetic reconnection, in which magnetic connectivity changes and magnetic energy can be converted into particle motion and heating. It is studied in laboratory experiments and in connection with solar and space phenomena. (mrx.pppl.gov)
Natural occurrence
The Sun and other stars contain extensive regions of plasma. The solar wind carries a tenuous flow of magnetized plasma outward from the Sun, filling interplanetary space and interacting with planetary magnetic environments. These interactions link solar activity with changes in the charged particles surrounding Earth. (pppl.gov)
Earth’s ionosphere is a partially ionized region of the upper atmosphere, continually supplied with charged particles through solar ultraviolet radiation. Its interaction with radio waves affects long-distance communication. Lightning creates transient plasma channels, while auroral light results from energetic particles interacting with atmospheric constituents. (umbra.nascom.nasa.gov)
Research and technological applications
In nuclear fusion research, plasmas provide the conditions in which light atomic nuclei can combine. Magnetic approaches use devices such as tokamaks and stellarators to confine hot plasma. Inertial-confinement approaches rapidly compress and heat small targets. Plasma confinement, instabilities, and energy transport are central research problems in these approaches. (energy.gov)
Industrial plasmas are important in semiconductor fabrication, where they enable controlled material processing. Other applications include surface modification, thin-film production, and lighting. Fluorescent lamps and plasma displays use electrical discharges to produce light, while high-intensity laser–plasma interactions are investigated as a means of accelerating charged particles. (energy.gov)