Plasma (physics)

Plasma forms when one of the other three classical states—most often a gas—becomes significantly ionized, meaning a large fraction of its particles are charged ions or electrons. Though uncommon on Earth, plasma makes up an estimated 99.9% of all ordinary matter in the universe. Stars are nearly pure plasma, and it also dominates the thin intracluster and intergalactic mediums. Artificially, plasma can be created by heating a neutral gas or exposing it to a strong electromagnetic field. Because it contains charged particles, plasma conducts electricity well. Its behavior—both of individual particles and of the whole—is shaped by collective electromagnetic fields and is highly sensitive to external fields. This response to electromagnetism is used in technologies like plasma televisions and plasma etching. Depending on conditions, some neutral particles may remain, making the plasma partially ionized; neon signs and lightning are examples. Unlike the clear phase transitions between solids, liquids, and gases, the shift to plasma is not sharply defined—it depends on context and interpretation. Whether a given ionization level qualifies as plasma varies with the phenomenon under study. **Etymology** Irving Langmuir introduced the term *plasma* for ionized gas in 1928. He wrote: “Except near the electrodes, where there are sheaths containing very few electrons, the ionized gas contains ions and electrons in about equal numbers so that the resultant space charge is very small. We shall use the name plasma to describe this region containing balanced charges of ions and electrons.” Colleagues Lewi Tonks and Harold Mott-Smith recalled that Langmuir was inspired by blood plasma: the way electrons moved from thermionic filaments reminded him of how blood plasma carries red and white corpuscles and germs. The word *plasma* in blood plasma ultimately comes from the Greek for the result of forming or molding. **History** Plasma science grew from Langmuir’s 1920s work, but its roots lie in the history of electricity. The electric arc was discovered independently by Vasily Petrov and Humphry Davy in 1803. In 1831, Michael Faraday systematically studied electric glow discharge in rarefied gases. Early researchers thought electrical conduction through gases resembled that in liquids. Sir William Crookes, investigating low-pressure discharges, proposed a different model and introduced the idea of a fourth state of matter in a notable 1879 paper. **Definitions**

**The fourth state of matter** Plasma is called the fourth state of matter, after solid, liquid, and gas. In this state, an ionized substance becomes so electrically conductive that long-range electric and magnetic fields dominate its behavior. Plasma is typically quasineutral—its overall charge is roughly zero—and consists of unbound positive and negative particles. Though unbound, these particles are not free from forces: moving charged particles generate electric currents, and each particle’s motion both affects and is affected by fields from other charges. This leads to collective behavior with many variations. Plasma is distinct from other states; describing a low-density plasma as merely an “ionized gas” is misleading, even though it shares with gas the lack of a definite shape or volume. The table below summarizes key differences. **Ideal plasma** Three factors define an ideal plasma: - **Plasma approximation**: The plasma parameter Λ, which counts charge carriers inside the Debye sphere, is much greater than one. This is equivalent to the ratio of electrostatic to thermal energy densities being small; such plasmas are weakly coupled. - **Bulk interactions**: The Debye length is much smaller than the plasma’s physical size, meaning interactions in the bulk dominate over boundary effects. When this holds, the plasma is quasineutral. - **Collisionlessness**: The electron plasma frequency (measuring electron oscillations) is much larger than the electron–neutral collision frequency. Electrostatic interactions then dominate over ordinary gas kinetics; such plasmas are collisionless. **Non-neutral plasma** In most plasmas, the strong electric force and high conductivity keep positive and negative charge densities equal over any sizable region (quasineutrality). A plasma with a significant charge excess—or, in the extreme, composed of a single species—is called non-neutral. Electric fields dominate here. Examples include charged particle beams, an electron cloud in a Penning trap, and positron plasmas. **Dusty plasma** A dusty plasma contains tiny charged dust particles, often found in space. These particles acquire high charges and interact with each other. When the particles are larger, it is called grain plasma. In labs, dusty plasmas are also known as complex plasmas.