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Plasma (physics): the story on HearLore | HearLore
Plasma (physics)
In 1816, a 24-year-old Michael Faraday stood before an audience and proposed a radical idea that matter existed in a state far beyond vaporization, a concept he called radiant matter. This hypothesis, delivered during a series of lectures, suggested that the gaseous form was merely a stepping stone to something more exotic, distinguished from vapor as vapor is from fluidity. Faraday expanded on this notion in 1819, listing it among the four states of matter, though he admitted it was purely hypothetical at the time. He noted experimental evidence and reasons for belief, likely referring to the strange light he observed in electrical discharges through air and other gases at varying pressures. It would take another 63 years for this concept to be formally identified in a laboratory setting. On the 22nd of August 1879, Sir William Crookes presented a lecture to the British Association for the Advancement of Science in Sheffield, where he used the term radiant matter to describe what he had observed, paying tribute to Faraday's far-reaching speculations. The transition from hypothesis to recognized state of matter was slow, but the groundwork laid by these two men established the foundation for understanding the universe's most abundant form of matter.
The Naming of Blood
Systematic studies of plasma began in the 1920s under the direction of Irving Langmuir and his colleagues, yet the name itself carries a biological origin that might surprise modern physicists. Langmuir introduced the term plasma in 1928 to describe ionized gas, but the choice of word was not arbitrary. Lewi Tonks and Harold Mott-Smith, who worked alongside Langmuir during that decade, recalled that the naming came from a specific analogy. Mott-Smith remembered that the transport of electrons from thermionic filaments reminded Langmuir of the way blood plasma carries red and white corpuscles and germs. Just as blood plasma serves as a medium that transports cells and pathogens, the ionized gas served as a medium that transported electrons and ions. This biological metaphor stuck, transforming a description of electrical discharge into a fundamental state of matter. The term plasma now defines a state where charged particles interact collectively, governed by electromagnetic fields rather than simple collisions, a concept that revolutionized how scientists viewed the behavior of matter in the cosmos.
Stars and The Void
While plasma is rarely encountered on Earth in its pure form, it is estimated that 99.9% of all ordinary matter in the universe exists as plasma. Stars are almost pure balls of plasma, and the rarefied intracluster medium and intergalactic medium are dominated by this state. Within the Solar System, interplanetary space is filled with the plasma expelled via the solar wind, extending from the Sun's surface out to the heliopause. Astrophysical plasmas are also observed in accretion disks around stars or compact objects like white dwarfs, neutron stars, or black holes in close binary star systems. Plasma is associated with the ejection of material in astrophysical jets, which have been observed with accreting black holes or in active galaxies like M87's jet that possibly extends out to 5,000 light-years. Even the distant stars and much of interstellar space or intergalactic space are filled with plasma, albeit at very low densities. This ubiquity means that the physics of plasma is not just a laboratory curiosity but the primary language of the universe, governing the dynamics of stars, galaxies, and the space between them.
Common questions
When did Michael Faraday first propose the concept of radiant matter?
Michael Faraday first proposed the concept of radiant matter in 1816 during a series of lectures. He expanded on this notion in 1819, listing it among the four states of matter as a purely hypothetical concept. Faraday noted experimental evidence and reasons for belief, likely referring to the strange light he observed in electrical discharges through air and other gases at varying pressures.
Who coined the term plasma for the state of matter and when was it introduced?
Irving Langmuir introduced the term plasma in 1928 to describe ionized gas. The choice of word came from a specific analogy where Lewi Tonks and Harold Mott-Smith recalled that the transport of electrons from thermionic filaments reminded Langmuir of the way blood plasma carries red and white corpuscles and germs. This biological metaphor transformed a description of electrical discharge into a fundamental state of matter.
What percentage of ordinary matter in the universe exists as plasma?
It is estimated that 99.9% of all ordinary matter in the universe exists as plasma. Stars are almost pure balls of plasma, and the rarefied intracluster medium and intergalactic medium are dominated by this state. Within the Solar System, interplanetary space is filled with the plasma expelled via the solar wind, extending from the Sun's surface out to the heliopause.
How much energy does a typical lightning discharge release in terms of current and voltage?
Typical lightning discharges 30 kiloamperes at up to 100 megavolts. Plasma temperatures in these events can approach 30,000 kelvin, and electron densities may exceed 10 to the power of 24 per cubic meter. Lightning emits radio waves, light, X-rays, and even gamma rays during these events.
When did researchers demonstrate that low-temperature atmospheric pressure plasma could inactivate bacterial cells?
In the mid-1990s, researchers demonstrated that low-temperature atmospheric pressure plasma could effectively inactivate bacterial cells. This work led to the establishment of a new field of research known as plasma medicine. Dielectric barrier discharge became a key tool in this field, allowing for the manipulation of matter without the extreme heat that would destroy biological tissues or delicate materials.
What is the quantitative criterion for a particle to be considered magnetized in a plasma?
A common quantitative criterion is that a particle on average completes at least one gyration around the magnetic-field line before making a collision. It is often the case that the electrons are magnetized while the ions are not, making magnetized plasmas anisotropic. When used in combination with a high Hall parameter, a critical value triggers the problematic electrothermal instability which limited these technological developments.
On Earth, plasma manifests in dramatic and often dangerous forms, with lightning serving as the most visible example. Typical lightning discharges 30 kiloamperes at up to 100 megavolts, emitting radio waves, light, X-rays, and even gamma rays. Plasma temperatures in these events can approach 30,000 kelvin, and electron densities may exceed 10 to the power of 24 per cubic meter. Above the Earth's surface, the ionosphere is a plasma, and the magnetosphere contains plasma that interacts with the solar wind. The polar aurorae are another manifestation, where plasma energy pours back into the atmosphere, creating the green and red lights of the aurora borealis. Upper-atmospheric lightning, including sprites, blue jets, blue starters, gigantic jets, and ELVES, represents a complex family of transient luminous events. Even St. Elmo's fire, a static discharge phenomenon, and fire itself, if sufficiently hot, can be considered plasma. These natural occurrences demonstrate that plasma is not confined to the stars but is an active participant in Earth's atmospheric and magnetic environment.
The Electric Arc
The generation of artificial plasma relies on a common principle: energy input must be applied to produce and sustain it. When an electric current is applied across a dielectric gas, the potential difference and subsequent electric field pull bound electrons toward the anode while the cathode pulls the nucleus. As the voltage increases, the current stresses the material beyond its dielectric limit, leading to electrical breakdown marked by an electric spark. The underlying process is the Townsend avalanche, where collisions between electrons and neutral gas atoms create more ions and electrons. The first impact of an electron on an atom results in one ion and two electrons, causing the number of charged particles to increase rapidly in millions after about 20 successive sets of collisions. This cascade process forms an electric arc, a continuous electric discharge between two electrodes similar to lightning. With ample current density, the discharge forms a luminous arc where the inter-electrode material undergoes various stages of saturation, breakdown, glow, transition, and thermal arc. Electrical resistance along the arc creates heat, which dissociates more gas molecules and ionizes the resulting atoms, allowing the plasma to conduct electricity and generate light.
The Cold Plasma Revolution
In the mid-1990s, a significant shift occurred in the application of plasma technology when researchers demonstrated that low-temperature atmospheric pressure plasma could effectively inactivate bacterial cells. This work, along with later experiments using mammalian cells, led to the establishment of a new field of research known as plasma medicine. Dielectric barrier discharge, a non-thermal discharge generated by applying high voltages across small gaps, became a key tool in this field. The application of this discharge to synthetic fabrics and plastics functionalizes the surface, allowing paints, glues, and similar materials to adhere. Plasma jets produced by fast propagating guided ionization waves, known as plasma bullets, are now used in medical treatments and industrial processes. These non-thermal plasmas, often called cold plasmas, allow for the manipulation of matter without the extreme heat that would destroy biological tissues or delicate materials. The ability to generate stable impermeable plasma with no magnetic confinement using only an ultrahigh-pressure blanket of cold gas has opened new avenues for synthesis of different nanostructures, suggesting that plasma can act as a physical barrier and a catalyst for complex reactions.
The Magnetized Dance
The existence of charged particles causes the plasma to generate and be affected by magnetic fields, creating a dynamic interplay known as magnetization. A common quantitative criterion is that a particle on average completes at least one gyration around the magnetic-field line before making a collision. It is often the case that the electrons are magnetized while the ions are not, making magnetized plasmas anisotropic, meaning their properties in the direction parallel to the magnetic field are different from those perpendicular to it. In magnetized plasmas, a gyrokinetic approach can substantially reduce the computational expense of a fully kinetic simulation. The study of such magnetized nonthermal weakly ionized gases involves resistive magnetohydrodynamics with low magnetic Reynolds number, a challenging field where calculations require dyadic tensors in a 7-dimensional phase space. When used in combination with a high Hall parameter, a critical value triggers the problematic electrothermal instability which limited these technological developments. Despite these challenges, magnetized plasmas are essential for understanding phenomena ranging from the Earth's magnetosphere to the confinement of fusion energy in experimental reactors.
Complexity and Filaments
Although the underlying equations governing plasmas are relatively simple, plasma behavior is extraordinarily varied and subtle, often exhibiting unexpected behavior from a simple model. The spontaneous formation of interesting spatial features on a wide range of length scales is one manifestation of plasma complexity. These features are interesting because they are very sharp, spatially intermittent, or have a fractal form. Filamentation, for instance, refers to the self-focusing of a high power laser pulse, where the nonlinear part of the index of refraction becomes important and causes a higher index of refraction in the center of the laser beam. The tighter focused laser has a higher peak brightness that forms a plasma, which in turn has an index of refraction lower than one, causing a defocusing of the laser beam. The interplay of the focusing index of refraction and the defocusing plasma makes the formation of a long filament of plasma that can be micrometers to kilometers in length. Striations or string-like structures are seen in many plasmas, like the plasma ball, the aurora, lightning, electric arcs, solar flares, and supernova remnants. These complex structures demonstrate that plasma lies on the boundary between ordered and disordered behavior, defying simple mathematical descriptions and requiring sophisticated models to capture its full complexity.