- Key People:
- Lev Davidovich Landau
- Hannes Alfvén
- On the Web:
- Massachusetts Institute of Technology - Department of Mathematics - What is a Plasma? (PDF) (Mar. 18, 2025)
The most important practical applications of plasmas lie in the future, largely in the field of power production. The major method of generating electric power has been to use heat sources to convert water to steam, which drives turbogenerators. Such heat sources depend on the combustion of fossil fuels, such as coal, oil, and natural gas, and fission processes in nuclear reactors. A potential source of heat might be supplied by a fusion reactor, with a basic element of deuterium-tritium plasma; nuclear fusion collisions between those isotopes of hydrogen would release large amounts of energy to the kinetic energy of the reaction products (the neutrons and the nuclei of hydrogen and helium atoms). By absorbing those products in a surrounding medium, a powerful heat source could be created. To realize a net power output from such a generating station—allowing for plasma radiation and particle losses and for the somewhat inefficient conversion of heat to electricity—plasma temperatures of about 100,000,000 K and a product of particle density times containment time of about 1020 seconds per cubic metre are necessary. For example, at a density of 1020 particles per metre cubed, the containment time must be one second. Such figures are yet to be reached, although there has been much progress.
In general, there are two basic methods of eliminating or minimizing end losses from an artificially created plasma: the production of toroidal plasmas and the use of magnetic mirrors (see nuclear fusion). A toroidal plasma is essentially one in which a plasma of cylindrical cross section is bent in a circle so as to close on itself. For such plasmas to be in equilibrium and stable, however, special magnetic fields are required, the largest component of which is a circular field parallel to the axis of the plasma. In addition, a number of turbulent plasma processes must be controlled to keep the system stable. In 1991 a machine called the JET (Joint European Torus) was able to generate 1.7 million watts of fusion power for almost 2 seconds after researchers injected titrium into the JET’s magnetically confined plasma. It was the first successful controlled production of fusion power in such a confined medium.
Besides generating power, a fusion reactor might desalinate seawater. Approximately two-thirds of the world’s land surface is uninhabited, with one-half of this area being arid. The use of both giant fission and fusion reactors in the large-scale evaporation of seawater could make irrigation of such areas economically feasible. Another possibility in power production is the elimination of the heat–steam–mechanical energy chain. One suggestion depends on the dynamo effect. If a plasma moves perpendicular to a magnetic field, an electromotive force, according to Faraday’s law, is generated in a direction perpendicular to both the direction of flow of the plasma and the magnetic field. This dynamo effect can drive a current in an external circuit connected to electrodes in the plasma, and thus electric power may be produced without the need for steam-driven rotating machinery. This process is referred to as magnetohydrodynamic (MHD) power generation and has been proposed as a method of extracting power from certain types of fission reactors. Such a generator powers the auroras as the Earth’s magnetic field lines tap electrical current from the MHD generator in the solar wind.
The inverse of the dynamo effect, called the motor effect, may be used to accelerate plasma. By pulsing cusp-shaped magnetic fields in a plasma, for example, it is possible to achieve thrusts proportional to the square of the magnetic field. Motors based on such a technique have been proposed for the propulsion of craft in deep space. They have the advantage of being capable of achieving large exhaust velocities, thus minimizing the amount of fuel carried.
A practical application of plasma involves the glow discharge that occurs between two electrodes at pressures of one-thousandth of an atmosphere or thereabouts. Such glow discharges are responsible for the light given off by neon tubes and such other light sources as fluorescent lamps, which operate by virtue of the plasmas they produce in electric discharge. The degree of ionization in such plasmas is usually low, but electron densities of 1016 to 1018 electrons per cubic metre can be achieved with an electron temperature of 100,000 K. The electrons responsible for current flow are produced by ionization in a region near the cathode, with most of the potential difference between the two electrodes occurring there. This region does not contain a plasma, but the region between it and the anode (i.e., the positive electrode) does.
Other applications of the glow discharge include electronic switching devices; it and similar plasmas produced by radio-frequency techniques can be used to provide ions for particle accelerators and act as generators of laser beams. As the current is increased through a glow discharge, a stage is reached when the energy generated at the cathode is sufficient to provide all the conduction electrons directly from the cathode surface, rather than from gas between the electrodes. Under this condition the large cathode potential difference disappears, and the plasma column contracts. This new state of electric discharge is called an arc. Compared with the glow discharge, it is a high-density plasma and will operate over a large range of pressures. Arcs are used as light sources for welding, in electronic switching, for rectification of alternating currents, and in high-temperature chemistry. Running an arc between concentric electrodes and injecting gas into such a region causes a hot, high-density plasma mixture called a plasma jet to be ejected. It has many chemical and metallurgical applications.
Natural plasmas
Extraterrestrial forms
It has been suggested that the universe originated as a violent explosion about 13.8 billion years ago and initially consisted of a fireball of completely ionized hydrogen plasma. Irrespective of the truth of this, there is little matter in the universe now that does not exist in the plasma state. The observed stars are composed of plasmas, as are interstellar and interplanetary media and the outer atmospheres of planets. Scientific knowledge of the universe has come primarily from studies of electromagnetic radiation emitted by plasmas and transmitted through them and, since the 1960s, from space probes within the solar system.
In a star the plasma is bound together by gravitational forces, and the enormous energy it emits originates in thermonuclear fusion reactions within the interior. Heat is transferred from the interior to the exterior by radiation in the outer layers, where convection is of greater importance. In the vicinity of a hot star, the interstellar medium consists almost entirely of completely ionized hydrogen, ionized by the star’s ultraviolet radiation. Such regions are referred to as H II regions. The greater proportion by far of interstellar medium, however, exists in the form of neutral hydrogen clouds referred to as H I regions. Because the heavy atoms in such clouds are ionized by ultraviolet radiation (or photoionized), they also are considered to be plasmas, although the degree of ionization is probably only one part in 10,000. Other components of the interstellar medium are grains of dust and cosmic rays, the latter consisting of very high-energy atomic nuclei completely stripped of electrons. The almost isotropic velocity distribution of the cosmic rays may stem from interactions with waves of the background plasma.
Throughout this universe of plasma there are magnetic fields. In interstellar space magnetic fields are about 5 × 10−6 gauss (a unit of magnetic field strength) and in interplanetary space 5 × 10−5 gauss, whereas in intergalactic space they could be as low as 10−9 gauss. These values are exceedingly small compared with the Earth’s surface field of about 5 × 10−1 gauss. Although small in an absolute sense, these fields are nevertheless gigantic, considering the scales involved. For example, to simulate interstellar phenomena in the laboratory, fields of about 1015 gauss would be necessary. Thus, these fields play a major role in nearly all astrophysical phenomena. On the Sun the average surface field is in the vicinity of 1 to 2 gauss, but magnetic disturbances arise, such as sunspots, in which fields of between 10 and 1,000 gauss occur. Many other stars are also known to have magnetic fields. Field strengths of 10−3 gauss are associated with various extragalactic nebulae from which synchrotron radiation has been observed.
Solar-terrestrial forms
Regions of the Sun
The visible region of the Sun is the photosphere, with its radiation being about the same as the continuum radiation from a 5,800 K blackbody. Lying above the photosphere is the chromosphere, which is observed by the emission of line radiation from various atoms and ions. Outside the chromosphere, the corona expands into the ever-blowing solar wind (see below), which on passing through the planetary system eventually encounters the interstellar medium. The corona can be seen in spectacular fashion when the Moon eclipses the bright photosphere. During the times in which sunspots are greatest in number (called the sunspot maximum), the corona is very extended and the solar wind is fierce. Sunspot activity waxes and wanes with roughly an 11-year cycle. During the mid-1600s and early 1700s, sunspots virtually disappeared for a period known as the Maunder minimum. This time coincided with the Little Ice Age in Europe, and much conjecture has arisen about the possible effect of sunspots on climate. Periodic variations similar to that of sunspots have been observed in tree rings and lake-bed sedimentation. If real, such an effect is important because it implies that the Earth’s climate is fragile.
In 1958 the American astrophysicist Eugene Parker showed that the equations describing the flow of plasma in the Sun’s gravitational field had one solution that allowed the gas to become supersonic and to escape the Sun’s pull. The solution was much like the description of a rocket nozzle in which the constriction in the flow is analogous to the effect of gravity. Parker predicted the Sun’s atmosphere would behave just as this particular solar-wind solution predicts rather than according to the solar-breeze solutions suggested by others. The interplanetary satellite probes of the 1960s proved his solution to be correct.
Interaction of the solar wind and the magnetosphere
The solar wind is a collisionless plasma made up primarily of electrons and protons and carries an outflow of matter moving at supersonic and super-Alfvénic speed. The wind takes with it an extension of the Sun’s magnetic field, which is frozen into the highly conducting fluid. In the region of the Earth, the wind has an average speed of 400 kilometres per second; and, when it encounters the planet’s magnetic field, a shock front develops, the pressures acting to compress the field on the side toward the Sun and elongate it on the nightside (in the Earth’s lee away from the Sun). The Earth’s magnetic field is therefore confined to a cavity called the magnetosphere, into which the direct entry of the solar wind is prohibited. This cavity extends for about 10 Earth radii on the Sun’s side and about 1,000 Earth radii on the nightside.
Inside this vast magnetic field a region of circulating plasma is driven by the transfer of momentum from the solar wind. Plasma flows parallel to the solar wind on the edges of this region and back toward the Earth in its interior. The resulting system acts as a secondary magnetohydrodynamic generator (the primary one being the solar wind itself). Both generators produce potential on the order of 100,000 volts. The solar-wind potential appears across the polar caps of the Earth, while the magnetospheric potential appears across the auroral oval. The latter is the region of the Earth where energetic electrons and ions precipitate into the planet’s atmosphere, creating a spectacular light show. This particle flux is energetic enough to act as a new source of plasmas even when the Sun is no longer shining. The auroral oval becomes a good conductor; and large electric currents flow along it, driven by the potential difference across the system. These currents commonly are on the order of 1,000,000 amperes.
The plasma inside the magnetosphere is extremely hot (1–10 million K) and very tenuous (1–10 particles per cubic centimetre). The particles are heated by a number of interesting plasma effects, the most curious of which is the auroral acceleration process itself. A particle accelerator that may be the prototype for cosmic accelerators throughout the universe is located roughly one Earth radius above the auroral oval and linked to it by all-important magnetic field lines. In this region the auroral electrons are boosted by a potential difference on the order of three to six kilovolts, most likely created by an electric field parallel to the magnetic field lines and directed away from the Earth. Such a field is difficult to explain because magnetic field lines usually act like nearly perfect conductors. The auroras occur on magnetic field lines that—if it were not for the distortion of the Earth’s dipole field—would cross the equatorial plane at a distance of 6–10 Earth radii.
Closer to the Earth, within about 4 Earth radii, the planet wrests control of the system away from the solar wind. Inside this region the plasma rotates with the Earth, just as its atmosphere rotates with it. This system can also be thought of as a magnetohydrodynamic generator in which the rotation of the atmosphere and the ionospheric plasma in it create an electric field that puts the inner magnetosphere in rotation about the Earth’s axis. Since this inner region is in contact with the dayside of the Earth where the Sun creates copious amounts of plasma in the ionosphere, the inner zone fills up with dense, cool plasma to form the plasmasphere. On a planet such as Jupiter, which has both a larger magnetic field and a higher rotation rate than the Earth, planetary control extends much farther from the surface.
