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The fourth state of matter

How is plasma state defined?

A plasma is an ionized gas consisting of a quasi-neutral mixture of free electrons, ions (atomic or molecular) and neutral species interacting among themselves. As the temperature rises, the matter transforms itself, changing its aggregation state. Plasma can be considered the fourth state of matter, in addition to its solid state, liquid and gas form; It is obtained providing the molecules of a gas at a given pressure an amount of thermal energy sufficient to dissociate the molecules and ionize the atoms and the molecules of the gas itself. The transition between gaseous state and plasma can not, however, be considered a phase transition in the thermodynamic sense because it occurs gradually as the temperature rises.

99.9% of visible matter in the Universe is in the plasma state: the interior of the stars, interstellar space (an example is shown in the figure), ionosphere, boreal aurora (illustrated in the image taken by the International Space Station), lightning, flames. To natural plasmas present in nature, plasma generated in the laboratory are also added: neon tubes, plasma balls, electric arches, radiofrequency discharges for industrial applications, up to high temperature plasmas for controlled thermonuclear fusion research.

Nebulosa (NGC 6559). La radiazione di colore rosso è dovuta alla ricombinazione dell’idrogeno interstellare, ionizzato dalla radiazione emessa dalle stelle vicine, con gli elettroni. Piccole particelle di polvere (dust) riflettono la luce blu proveniente dalle stelle vicine, altre particelle di polvere assorbono la luce dando origine a filamenti scuri [Fonte: Nasa].
Aurora Boreale
Fotografia di un’aurora boreale scattata dalla Stazione Spaziale Internazionale (ISS-NASA). L'aurora boreale è un fenomeno dovuto all’interazione tra i costituenti della ionosfera e le particelle cariche (vento solare) intrappolate dal campo magnetico terrestre in corrispondenza dei poli. Ioni ed elettroni eccitano gli atomi e le molecole neutre che costituiscono la ionosfera, le quali diseccitandosi emettono luce di diverse lunghezze d’onda [Fonte: Nasa].

There is also a category of plasmas where the constituents include small aggregates of solid matter (varying in dimension from nanometers to millimeters) that are charged negatively due to the greater mobility of electrons than ion. These are the so-called "dusty plasma" with dynamics characterized by the fact that the electric charge of "powder" granules varies rapidly in time: we can include among these plasmas those that make up comets, planet rings, nebulae, flames, but also those produced during volcanic eruptions, atmospheric aerosols, desert sand transported by the wind. The presence of dust is also documented in thermonuclear fusion reactors and reactors for industrial processes.

Where does the term "plasma" come from?

The term "plasma" was used for the first time in 1927 by American Nobel Physicist Award Irving Langmuir to indicate an ionized gas whose behavior is similar to that of a fluid that carries electrons, ions, and impurities. The term was suggested by analogy with blood plasma, a term introduced in the 19th century by Czech doctor Purkinje to indicate the fluid that carries white blood cells, red blood cells, and nutrients.

Typical plasma parameters

How much a plasma is ionized when compared to ordinary gas?

The degree of ionization of a plasma dominated by collision processes depends on the nature of the gas, on the concentration of the atoms that compose it and on its temperature. In the case of local thermodynamic equilibrium, it is possible to evaluate the degree of ionization using the formula known as Saha equation. In the simplest case of a monoatomic hydrogen gas, this equation takes the analytical form

ni/nt = y/2 [√(1+4⁄y)-1]

where y=(2πme KB T)^(3⁄2)/(h^3 nt ) e^(-χ/(KB T))nt=nn+ni  Is the total concentration of H atom (nn) and ion (ni), χ is the ionization potential of H (χ = 13.6 eV), me the mass of the electron, T the gas temperature, h and KB the constants of Planck and Boltzmann respectively. In the figure the ratio ni / nt as a function of the electron temperature is shown for different values of the total density nt of hydrogen atoms. It is noted that for 1 eV temperatures it is possible to have complete gas ionization despite the ionization potential of 13.6 eV. This is made possible in a plasma in an equilibrium state by the presence of electrons distributed over a wide range of energies, centered on thermal energy but extending up to much higher values and thus able to ionize neutral atoms. in nature or in laboratory products, the ratio can take values that vary from many orders of magnitude depending on the density and temperatures. This value ranges from 10-6 to 10-1 for radiofrequency discharges, is about 1.5 in the Sun nucleus, in the order of 1013 for magnetically confined plasmas in tokamak and about 1018 in plasma in the solar corona; Fusion plasmas and solar corona can therefore be considered completely ionized (ni/nt ≈1).


In the figure, the degree of ionization ni / nt  of a plasma of hydrogen at the variation of the gas temperature for different values of the total density nt, in the case of the local thermodynamic equilibrium.

Plasma state characteristics: collective effects

One of the key features of plasmas is the presence of collective effects. Unlike neutral gases where electrically neutral atoms or molecules collide amongst themselves as point masses, in a plasma the charged particles, in addition to interactions with neutral atoms by electric polarization fields (in binary collisions with much smaller radius of interaction of the average interatomic distance) also interact simultaneously with numerous other charged particles through Coulombian electrostatic fields, which have a far greater range of interaction than the size of the particles themselves. Indeed, as the electrostatic potential generated by an electric charge decreases slowly away from the source (proportionally to 1/r) the intensity of the interaction remains at a not negligible level even away from the source charge, up to a characteristic distance called Debye length λD and then rapidly decreases. For this reason, a charge q interacts simultaneously with a "collection" of many near loads, all contained in the radius sphere λD. To define the Debye length λD, we can consider the electrical potential φ (r) generated by the test charge q at a given point of the plasma assumed as the source of the reference system. In the vacuum the electrostatic potential produced by q, solution of the Laplace equation takes the form φ (r) = 1 / (4πε0) q / r. If the particle is in a plasma, the opposite sign charges thicken around it, while those of the same sign move away (on the scale λD the plasma does not maintain its electrical neutrality) thus generating (for r> λD), a spatial distribution of electrostatic potential whose overall effect is a significant reduction in the value of the coulombian potential, with respect to the corresponding value in the vacuum. The charges therefore lie around the test charge to exponentially shield the electrostatic potential generated by it at a distance of the order λD = √ ((ε0 kB Te) / (e2ne))Te and ne represent the plasma temperature and density respectively.



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