Plasma State of Matter: Definition and Examples of the Fourth State

What plasma is

Plasma is matter in which some of the electrons have broken away from their atoms and move about freely. The atoms that have lost those electrons carry a positive charge and are called ions. The U.S. Department of Energy (DOE) singles this out as the feature that sets plasma apart from the other states: in a solid, a liquid or a gas, electrons stay bound close to the nuclei.

Treccani, the Italian encyclopedia, describes it as a gas with a high share of ionized atoms that carries no net electric charge overall. Both descriptions hold because of quasi-neutrality. In any small region the positive and negative charges balance almost exactly, and a collection of charges has to meet this condition before it can be called a plasma.

Plasma vs solid, liquid and gas: why it is the fourth state

When you look at the states of matter and the changes between solid, liquid and gas, a liquid can be pictured as a solid that has been warmed up. The DOE points out that plasma doesn’t fit that pattern in the same way, because it has properties a gas lacks.

The most obvious one is that it conducts electric current. Free electrons and ions are moving charges, and a cold gas lacks them. NASA explains that plasma conducts so well that it travels along the electromagnetic fields filling space. The same free charges make it respond strongly to magnetic fields, which is what fusion machines rely on.

There is also the way its particles affect each other. Treccani writes that in a gas they usually collide in pairs at short range, while in a plasma long-range electric forces dominate and each particle interacts with a great many others at once. This is the collective behavior of plasma, and according to the DOE the complex interactions between electrons and ions give rise to waves and instabilities.

Feature	Ordinary gas	Plasma
Electrons	Bound to atoms	Partly free
Particles	Neutral atoms or molecules	Free electrons, positive ions and, if ionization is partial, neutral particles
Overall charge	Neutral	Almost neutral (quasi-neutrality)
Electric current	Does not conduct it when cold	Conducts it easily
Magnetic fields	Not strongly affected when cold	Responds strongly
Interactions	In pairs, at short range	Long-range electric forces, collective behavior

The DOE counts plasma as one of four states of matter, next to solids, liquids and gases. NASA calls it the fourth state of matter, and Treccani’s entry gives it the same rank.

How plasma forms: ionization

Plasma comes from ionization, the process by which an atom, or a group of atoms, picks up an electric charge and turns from neutral into an ion. Removing an electron has a cost: the atom must be given a certain amount of energy, known as the ionization energy. According to Treccani, that energy can come from heat (thermal agitation, in the entry’s words), from collisions with charged particles, or from radiation such as light, ultraviolet, X-rays and gamma rays.

Heat works like this. The hotter a gas, the faster its particles move, and at very high temperatures a single collision between two atoms can be enough to free an electron, as happens in the Sun’s atmosphere. Out in space, the DOE adds, plasma can also arise when atoms are struck by very energetic photons, gamma rays among them.

Close to the ground, plasma doesn’t last. In the air we breathe, an electron knocked loose by a cosmic ray, for instance, is soon captured again.

A number called the degree of ionization, written with the Greek letter alpha, says how ionized a plasma is. It equals the electron density divided by the sum of the electron density and the density of neutral particles. If no neutral particles are left, the fraction is 1, and it approaches 0 when free electrons are very scarce. Once the plasma also contains negative ions, this definition is no longer clear-cut.

Hot and cold plasmas

Treccani sorts plasmas by two criteria: density (dense and rarefied plasmas) and temperature (hot and cold ones).

Most of the universe’s plasma, the DOE says, runs hot: it can pass 10,000 degrees Fahrenheit, and all of its atoms may be ionized. Low-temperature plasmas have only some of their atoms ionized and can stay as cool as an ordinary room.

Plasma examples in nature: the Sun, stars and lightning

Estimates of how widespread it is differ from source to source. NASA, which calls it rare on Earth, puts it at 99.9% of the visible universe; for the DOE it is 99% of visible matter, and for Treccani about 99% of the matter in the universe.

Much of this plasma is found in stars, the Sun among them, in nebulae and in interstellar matter. The DOE also describes how a star begins. When gravity squeezes and heats a gas, collisions between atoms strip off electrons and plasma forms. Closer to home, Treccani counts the discharge of a lightning bolt among natural plasmas, like the bolts produced in thunderstorms. The DOE also lists one particular kind of lightning, ball lightning, as plasma.

The ionosphere starts about 70-80 km up, and it is plasma because short-wavelength sunlight up there, from ultraviolet to X-rays, knocks electrons out of atoms. Higher than 200 km, where particles rarely run into one another, it stays ionized by night as well as by day. Between the planets of the solar system flows the solar wind, a plasma moving at about 400 km per second that gives shape to the far reaches of Earth’s magnetic field.

Practical example: the DOE names plasma as the cause of the aurora borealis. The solar wind supplies the energy, and according to NASA a process called magnetic reconnection, on the night side of Earth, can push particles toward the poles and set the aurora glowing.

Plasma in technology: lamps, chips and fusion

The plasma you are most likely to meet at home is inside lamps. Treccani mentions the glowing gas of arc lamps and neon tubes. Plasma screens work with a grid of tiny controlled plasmas, each of which gives off light.

Practical example: a fluorescent lamp holds a thin inert gas mixed with a small amount of mercury vapor. Flip the switch and the current warms the gas and sets its particles moving until it becomes plasma and glows. Neon signs and many street lamps light up by much the same mechanism.

Industry uses plasma to treat the surfaces of materials, for example through plasma etching, and DOE-funded plasma research has improved the manufacture of semiconductors, including those that end up in phones and cars. Plasma torches fire a jet at around 10,000 °C, hot enough to melt and weld metals that are hard to melt, such as molybdenum and tantalum.

In the lab, the DOE explains, plasma is made by hitting atoms with electromagnetic fields, laser beams or high-voltage electricity. Then there is nuclear fusion. According to ITER, plasma is the setting in which light elements fuse and release energy. That holds for stars as much as for fusion machines. In a tokamak a strong electric current ionizes the fuel gas and turns it into plasma, and auxiliary heating systems then bring it to between 150 and 300 million °C. Because its particles are charged, large magnetic coils shape the plasma and hold it away from the walls. The DOE calls confining plasma a key step for tokamaks and stellarators, the machines that could one day supply fusion energy.

Where the word «plasma» comes from

In the late nineteenth century William Crookes was studying electrical discharges in rarefied gases and wrote that he was facing matter in a fourth state, with a gap between it and a gas as wide as the one between a gas and a liquid. He called it radiant matter, a term that Mario J. Pinheiro, in his paper on the origin of the word, credits to Faraday. Later experiments by J. J. Thomson showed that this matter was made of electrified corpuscles, which came to be known as electrons.

The name came from Irving Langmuir (1881-1957), an American chemist and physicist who studied gas discharges at General Electric. Pinheiro traces the first written use of «plasma» for an ionized gas to a 1928 paper by Langmuir. The dates vary, though. His colleague Harold Mott-Smith, in a 1971 letter to Nature, recalled the proposal being made in 1927 during work on discharges in mercury vapor, and Treccani dates the term to 1929. Langmuir went on to win the 1932 Nobel Prize in Chemistry for his work on surface chemistry.

The word, which goes back to the ancient Greek for molding, had been used for decades in the life sciences. The Czech physiologist Jan Evangelista Purkyně had applied it to the fluid left over when the corpuscles are taken out of blood. According to ITER, the choice grew out of an analogy: charged particles and impurities rode along in the discharge, somewhat as blood cells do in blood plasma. It took the scientific community some time to accept a medical term with a new meaning.