In addition to the three states listed above, a substance can be in a fourth state of aggregation - plasma , which was discovered relatively recently. The plasma state occurs when a substance in a gaseous state is exposed to such strong ionizing factors as ultra-high temperatures (several million degrees), powerful electrical discharges or electromagnetic radiation. In this case, the molecules and atoms of the substance are destroyed and transformed into a mixture consisting of positively charged nuclei and electrons moving at colossal speeds. For this reason, plasma is sometimes called an electron-nuclear gas.

There are two types of plasma: isothermal and gas-discharge.

Isothermal plasma It is obtained at high temperatures, under the influence of which thermal dissociation of the atoms of the substance takes place, and can exist indefinitely. This type of plasma is the substance of stars, as well as ball lightning. The Earth's ionosphere is also a special type of plasma; however, in this case, ionization occurs under the influence of ultraviolet radiation from the Sun.

Isothermal plasma plays an extremely important role in space processes. Three other states of matter in outer space are exceptions.

Gas discharge plasma is formed during an electrical discharge and is therefore stable only in the presence of an electric field. As soon as the action of the external field ceases, the gas-discharge plasma, due to the formation of neutral atoms from ions and electrons, disappears within 10 –5 -10 –4 s.

One of the remarkable properties of plasma is its high electrical conductivity. The higher the temperature of the plasma, the higher its conductivity. Because of this, currents of hundreds of thousands and millions of amperes can be passed through plasma.

By passing such currents through a plasma, it is possible to raise its temperature to tens and even hundreds of millions of degrees, and its pressure to tens of gigapascals. Such conditions are known to be close to holding thermonuclear fusion reactions , which can produce colossal amounts of energy.

As is known, energy is released not only during the fission of nuclei, but also during their fusion, i.e., during the fusion of lighter nuclei into heavier ones. The task in this case is to overcome electrical repulsion and bring light nuclei closer to sufficiently small distances where nuclear attractive forces begin to act between them. So, for example, if it were possible to force two protons and two neutrons to combine into the nucleus of a helium atom, then enormous energy would be released. By heating to high temperatures as a result of ordinary collisions, nuclei can approach such small distances that nuclear forces come into play and fusion occurs. Once started, the fusion process, as calculations show, can provide the amount of heat needed to maintain the high temperature necessary for further nuclear fusions, i.e. the process will continue continuously. This produces such a powerful source of thermal energy that its amount can be controlled only by the amount of material required. This is the essence of conducting a controlled thermonuclear fusion reaction.

When an electric current passes through a plasma, it creates a strong magnetic field that compresses the flow of electrons and ions into plasma cord This achieves thermal insulation of the plasma from the walls of the vessel. As the current increases, the electromagnetic compression of the plasma becomes more pronounced. This is the essence of the so-called pinch effect .As research has shown, the pinch effect and the forces created by external magnetic fields varying according to a certain law can be successfully used to hold plasma in a “magnetic bottle” where the fusion reaction occurs.

CHEMICAL BOND THEORY

General provisions of the doctrine of chemical bonds. Covalent bond

The concept of a chemical bond is one of the fundamental ones in modern science. Without knowledge of the nature of the interaction of atoms, it is impossible to understand the mechanism of formation of chemical compounds, their composition and reactivity, and even more so, to predict the properties of new materials.

The very first and not entirely clear ideas about chemical bonds were introduced by Kekule in 1857. He pointed out that the number of atoms bonded to an atom of another element depends on the basicity of the constituent parts .

For the first time, the term “chemical bond” was introduced by A.M. Butlerov in 1863. In the creation of the doctrine of chemical bonds, his theory of chemical structure, proposed in 1861, played a large role. However, having formulated the main provisions of the theory, Butlerov did not yet use the term “chemical bond”. The tenets of his teaching are as follows:

1. Atoms in molecules are connected to each other in a certain sequence. Changing this sequence leads to the formation of a new substance with new properties.

2. The connection of atoms occurs in accordance with their valency.

3. The properties of substances depend not only on the composition, but also on their “chemical structure”, i.e. on the order of connection of atoms in molecules and the nature of their mutual influence.

Thus, the properties of substances are determined not only by their qualitative and quantitative composition, but also by the internal structure of the molecules.

In 1863, in his work “On various explanations of some cases of isomerism,” Butlerov already spoke about “the method of chemical bonding between atoms,” about “the chemical bonding of individual atoms.”

What does the term “chemical bond” mean?

A number of definitions of this concept can be given, but the most obvious of them is that chemical bond – this is the interaction that occurs between atoms during the formation of substances.

A scientific explanation of the nature of the chemical bond could appear only after the emergence of the doctrine of the structure of the atom. In 1916, the American physical chemist Lewis suggested that a chemical bond arises by pairing electrons belonging to different atoms. This idea was the starting point for modern covalent chemical bond theory .

In the same year, the German scientist Kossel suggested that when two atoms interact, one of them gives away and the other accepts electrons. The electrostatic interaction of the resulting ions leads to the formation of a stable compound. The development of Kossel's ideas led to the creation ionic bond theory .

In any case, the chemical bond is of electrical origin, because is ultimately due to the interaction of electrons.

One of the reasons for the emergence of a chemical bond is the desire of atoms to assume a more stable state. A necessary condition for the formation of a chemical bond is a decrease in the potential energy of a system of interacting atoms.

During chemical reactions, the nuclei of atoms and the inner electron shells do not undergo changes. Chemical bonding occurs through the interaction of the electrons most distant from the nucleus, called valence .

Valence elements are: for s-elements - s-electrons of the outer energy level, for p-elements - s- and p-electrons of the outer energy level, for d-elements - s-electrons of the outer and d-electrons of the pre-external energy levels, for f- elements - s-electrons of the outer and f-electrons of the third outside energy levels.

There are usually five main types of chemical bonding: ionic, covalent, metallic, hydrogen, and intermolecular interactions , caused by van der Waals forces, and the first three types of connection are significantly stronger than the last two.

The modern doctrine of chemical bonding is based on quantum mechanical concepts. Two methods are currently widely used to describe chemical bonds: valence bond method(MVS) and molecular orbital method(MMO).

The BC method is simpler and more visual, so we will begin our consideration of the theory of chemical bonding with it.

Let's consider the most common covalent chemical bond.

Valence bond method

The BC method is based on the following provisions.

1. A covalent chemical bond is formed by two electrons with oppositely directed spins, and this electron pair belongs simultaneously to two atoms. The atoms themselves retain their individuality.

2. A covalent chemical bond is stronger the more the interacting electron clouds overlap.

In the broad sense of the word covalent bond is a chemical bond between atoms carried out by sharing electrons. A covalent bond can be considered as a universal, most common type of chemical bond.

To accurately describe the state of an electron in a molecule, it is necessary to solve the Schrödinger equation for the corresponding system of electrons and nuclei, specifying the condition of minimum energy. However, at present, solving the Schrödinger equation is possible only for the simplest systems. The first approximate calculation of the electron wave function was made in 1927 by Heitler and London for the hydrogen molecule.

Rice. 4.1. Dependence of the energy of a system of two hydrogen atoms on

internuclear distance for electrons with parallel (1) and

antiparallel (2) spins.

As a result of their work, they obtained an equation relating the potential energy of the system to the distance between the nuclei of two hydrogen atoms. It turned out that the calculation results depend on whether the spins of both electrons are the same or opposite in sign.

With parallel spins, the approach of atoms leads to a continuous increase in the energy of the system. With oppositely directed spins, atoms approach each other to a certain distance r 0 is accompanied by a decrease in the energy of the system, after which it begins to increase again (Fig. 4.1).

Thus, if the electron spins are parallel, the formation of a chemical bond does not occur for energy reasons, but in the case of oppositely directed electron spins, an H2 molecule is formed - a stable system of two hydrogen atoms, the distance between the nuclei of which is r 0 .

This is the distance r 0 significantly less than twice the atomic radius (for a hydrogen molecule - 0.074 and 0.106 nm, respectively), therefore, when a chemical bond is formed, mutual overlap of electron clouds and reacting atoms occurs (Fig. 3.2).

Rice. 4.2. Scheme of electron cloud overlap during formation

hydrogen molecules

Due to the overlapping of the clouds, the electron density between the nuclei increases, and the attractive forces between this region of negative charge and the positively charged nuclei of interacting atoms increase. An increase in attractive forces is accompanied by the release of energy, which leads to the formation of a chemical bond.

When depicting structural formulas, a bond is indicated by a dash or two dots (a dot denotes an electron):

N – N N: N

In the case considered, electrons located in the s-orbitals of hydrogen atoms are shared. The hydrogen atom has no other electrons. In the case of, for example, halogens, each interacting atom also has three pairs of electrons at the external energy level that are not involved in the formation of a chemical bond (two s-electrons and four p-electrons):

The chemical bond in the F2 molecule is formed due to the interaction of unpaired electrons located in atomic p-orbitals; the remaining electrons do not take part in the formation of the chemical bond (they are often called lone electron pairs).

Only one electron from each atom takes part in the formation of H 2 and F 2 molecules. A covalent bond formed by one pair of electrons is called single communication

A bond formed by two or three pairs of electrons is called multiple communication Thus, oxygen and nitrogen atoms contain two and three unpaired electrons, respectively:

Consequently, two or three electrons from each atom, respectively, take part in the formation of O 2 and N 2 molecules. Thus, the bond in the oxygen molecule is double, and in the nitrogen molecule it is triple:

How can a multiple bond be formed? Are all connections equal in these cases? To answer this and other related questions, we should consider the basic characteristics of a covalent bond.

And others. A change in the state of aggregation can be accompanied by an abrupt change in free energy, entropy, density and other basic physical properties.

It is known that any substance can exist only in one of three states: solid, liquid or gaseous, a classic example of which is water, which can be in the form of ice, liquid and vapor. However, if we take the entire Universe as a whole, there are very few substances that are in these considered indisputable and widespread states. They are unlikely to exceed what is considered negligible traces in chemistry. All other matter in the Universe is in the so-called plasma state.

1. What is plasma?

The word “plasma” (from the Greek “plasma” - “formed”) in the middle of the 19th century

V. began to be called the colorless part of the blood (without red and white bodies) and

liquid that fills living cells. In 1929, American physicists Irving Langmuir (1881–1957) and Levi Tonko (1897–1971) called ionized gas in a gas discharge tube plasma.

English physicist William Crookes (1832-1919), who studied electrical

discharge in tubes with rarefied air, wrote: “Phenomena in evacuated

tubes open up a new world for physical science, in which matter can exist in a fourth state.”

Depending on the temperature, any substance changes its

state. Thus, water at negative (Celsius) temperatures is in a solid state, in the range from 0 to 100 °C - in a liquid state, above 100 °C - in a gaseous state. If the temperature continues to rise, atoms and molecules begin to lose their electrons - they become ionized and gas turns into plasma. At temperatures above 1,000,000 ° C, plasma is absolutely ionized - it consists only of electrons and positive ions. Plasma is the most common state of matter in nature, it accounts for about 99% of the mass of the Universe, the majority of stars, nebulae. completely ionized plasma. The outer part of the earth's atmosphere (ionosphere) is also plasma.

Even higher are the radiation belts containing plasma.

Auroras, lightning, including globular lightning, are all different types of plasma that can be observed under natural conditions on Earth. And only an insignificant part of the Universe is made up of solid matter - planets, asteroids and dust nebulae.

In physics, plasma is understood as a gas consisting of electrically

charged and neutral particles, in which the total electric charge is zero, i.e. the condition of quasineutrality is satisfied (therefore, for example, a beam of electrons flying in a vacuum is not plasma: it carries a negative charge).

1.1. Most typical forms of plasma

Properties and parameters of plasma

Plasma has the following properties:

Low-temperature plasma is characterized by a low degree of ionization (up to 1%). Since such plasmas are quite often used in technological processes, they are sometimes called technological plasmas. Most often, they are created using electric fields that accelerate electrons, which in turn ionize atoms. Electric fields are introduced into the gas through inductive or capacitive coupling (see inductively coupled plasma). Typical applications of low temperature plasma include plasma modification of surface properties (diamond films, metal nitridation, wettability modification), plasma etching of surfaces (semiconductor industry), purification of gases and liquids (ozonation of water and combustion of soot particles in diesel engines).

Hot plasma is almost always completely ionized (ionization degree ~100%). Usually it is precisely this that is understood as the “fourth state of matter”. An example is the Sun.

2.4. Density

Besides temperature, which is fundamental to the very existence of a plasma, the second most important property of a plasma is its density. The phrase plasma density usually means electron density, that is, the number of free electrons per unit volume (strictly speaking, here, density is called concentration - not the mass of a unit volume, but the number of particles per unit volume). In a quasineutral plasma, the ion density is related to it through the average charge number of the ions: . The next important quantity is the density of neutral atoms n0. In a hot plasma, n0 is small, but can nevertheless be important for the physics of processes in plasma. When considering processes in a dense, nonideal plasma, the characteristic density parameter becomes rs, which is defined as the ratio of the average interparticle distance to the Bohr radius.

2.5. Quasi-neutrality

Since plasma is a very good conductor, electrical properties are important. The plasma potential or space potential is the average value of the electric potential at a given point in space. If any body is introduced into the plasma, its potential will generally be less than the plasma potential due to the appearance of the Debye layer. This potential is called floating potential. Due to its good electrical conductivity, plasma tends to shield all electric fields. This leads to the phenomenon of quasineutrality - the density of negative charges is equal to the density of positive charges with good accuracy (). Due to the good electrical conductivity of plasma, the separation of positive and negative charges is impossible at distances greater than the Debye length and at times greater than the period of plasma oscillations.

An example of a non-quasineutral plasma is an electron beam. However, the density of non-neutral plasmas must be very small, otherwise they will quickly decay due to Coulomb repulsion.

Mathematical description

Plasma can be described at various levels of detail. Usually plasma is described separately from electromagnetic fields.

3.1. Fluid (liquid) model

In the fluid model, electrons are described in terms of density, temperature, and average velocity. The model is based on: the balance equation for density, the momentum conservation equation, and the electron energy balance equation. In the two-fluid model, ions are treated in the same way.

3.2. Kinetic description

Sometimes the liquid model is not sufficient to describe plasma. A more detailed description is given by the kinetic model, in which the plasma is described in terms of the distribution function of electrons over coordinates and momenta. The model is based on the Boltzmann equation. The Boltzmann equation is not applicable to describe a plasma of charged particles with Coulomb interaction due to the long-range nature of Coulomb forces. Therefore, to describe plasma with Coulomb interaction, the Vlasov equation with a self-consistent electromagnetic field created by charged plasma particles is used. The kinetic description must be used in the absence of thermodynamic equilibrium or in the presence of strong plasma inhomogeneities.

3.3. Particle-In-Cell (particle in a cell)

Particle-In-Cell models are more detailed than kinetic models. They incorporate kinetic information by tracking the trajectories of large numbers of individual particles. Electrical density charge and current are determined by summing particles in cells that are small compared to the problem under consideration, but nevertheless contain a large number of particles. Email and mag. The fields are found from the charge and current densities at the cell boundaries.

4. Use of plasma

Plasma is most widely used in lighting technology - in gas-discharge lamps that illuminate streets and fluorescent lamps used indoors. And in addition, in a variety of gas-discharge devices: electric current rectifiers, voltage stabilizers, plasma amplifiers and ultra-high frequency (microwave) generators, cosmic particle counters. All so-called gas lasers (helium-neon, krypton, carbon dioxide, etc.) are actually plasma: the gas mixtures in them are ionized by an electric discharge. Properties characteristic of plasma are possessed by conduction electrons in the metal (ions rigidly fixed in the crystal lattice neutralize their charges), a set of free electrons and mobile “holes” (vacancies) in semiconductors. Therefore, such systems are called solid-state plasma. Gas plasma is usually divided into low temperature - up to 100 thousand degrees and high temperature - up to 100 million degrees. There are generators of low-temperature plasma - plasmatrons, which use an electric arc. Using a plasma torch, you can heat almost any gas to 7000-10000 degrees in hundredths and thousandths of a second. With the creation of the plasma torch, a new field of science arose - plasma chemistry: many chemical reactions are accelerated or occur only in a plasma jet. Plasmatrons are used in the mining industry and for cutting metals. Plasma engines and magnetohydrodynamic power plants have also been created. Various schemes for plasma acceleration of charged particles are being developed. The central problem of plasma physics is the problem of controlled thermonuclear fusion. Thermonuclear reactions are the synthesis of heavier nuclei from the nuclei of light elements (primarily hydrogen isotopes - deuterium D and tritium T), occurring at very high temperatures (> 108 K and above). Under natural conditions, thermonuclear reactions occur in the Sun: hydrogen nuclei combine with each other to form helium nuclei, releasing a significant amount of energy. An artificial thermonuclear fusion reaction was carried out in a hydrogen bomb.

Conclusion

Plasma is still a little-studied object not only in physics, but also in chemistry (plasma chemistry), astronomy and many other sciences. Therefore, the most important technical principles of plasma physics have not yet left the stage of laboratory development. Currently, plasma is being actively studied because is of great importance for science and technology. This topic is also interesting because plasma is the fourth state of matter, the existence of which people did not suspect until the 20th century.

Bibliography

Wurzel F.B., Polak L.S. Plasmochemistry, M, Znanie, 1985.
Oraevsky N.V. Plasma on Earth and in space, K, Naukova Dumka, 1980.

I think everyone knows the 3 main states of matter: liquid, solid and gaseous. We encounter these states of matter every day and everywhere. Most often they are considered using the example of water. The liquid state of water is most familiar to us. We constantly drink liquid water, it flows from our tap, and we ourselves are 70% liquid water. The second physical state of water is ordinary ice, which we see on the street in winter. Water is also easy to find in gaseous form in everyday life. In the gaseous state, water is, as we all know, steam. It can be seen when, for example, we boil a kettle. Yes, it is at 100 degrees that water changes from liquid to gaseous.

These are the three states of matter that are familiar to us. But did you know that there are actually 4 of them? I think everyone has heard the word “plasma” at least once. And today I want you to also learn more about plasma - the fourth state of matter.

Plasma is a partially or fully ionized gas with equal densities of both positive and negative charges. Plasma can be obtained from gas - from the 3rd state of aggregation of a substance by strong heating. The state of aggregation in general, in fact, completely depends on temperature. The first state of aggregation is the lowest temperature at which the body remains solid, the second state of aggregation is the temperature at which the body begins to melt and become liquid, the third state of aggregation is the highest temperature, at which the substance becomes a gas. For each body, substance, the temperature of transition from one state of aggregation to another is completely different, for some it is lower, for some it is higher, but for everyone it is strictly in this sequence. At what temperature does a substance become plasma? Since this is the fourth state, it means that the temperature of transition to it is higher than that of each previous one. And indeed it is. In order to ionize a gas, a very high temperature is required. The lowest temperature and low ionized (about 1%) plasma is characterized by a temperature of up to 100 thousand degrees. Under terrestrial conditions, such plasma can be observed in the form of lightning. The temperature of the lightning channel can exceed 30 thousand degrees, which is 6 times higher than the temperature of the surface of the Sun. By the way, the Sun and all other stars are also plasma, most often high-temperature. Science proves that about 99% of all matter in the Universe is plasma.

Unlike low-temperature plasma, high-temperature plasma has almost 100% ionization and a temperature of up to 100 million degrees. This is truly a stellar temperature. On Earth, such plasma is found only in one case - for thermonuclear fusion experiments. A controlled reaction is quite complex and energy-consuming, but an uncontrolled reaction has proven itself to be a weapon of colossal power - a thermonuclear bomb tested by the USSR on August 12, 1953.

Plasma is classified not only by temperature and degree of ionization, but also by density and quasi-neutrality. Collocation plasma density usually means electron density, that is, the number of free electrons per unit volume. Well, with this, I think everything is clear. But not everyone knows what quasi-neutrality is. Plasma quasineutrality is one of its most important properties, which consists in the almost exact equality of the densities of the positive ions and electrons included in its composition. Due to the good electrical conductivity of plasma, the separation of positive and negative charges is impossible at distances greater than the Debye length and at times greater than the period of plasma oscillations. Almost all plasma is quasi-neutral. An example of a non-quasineutral plasma is an electron beam. However, the density of non-neutral plasmas must be very small, otherwise they will quickly decay due to Coulomb repulsion.

We have looked at very few terrestrial examples of plasma. But there are quite a lot of them. Man has learned to use plasma for his own benefit. Thanks to the fourth state of matter, we can use gas-discharge lamps, plasma TVs, electric arc welding, and lasers. Conventional fluorescent discharge lamps are also plasma. There is also a plasma lamp in our world. It is mainly used in science to study and, most importantly, see some of the most complex plasma phenomena, including filamentation. A photograph of such a lamp can be seen in the picture below:

In addition to household plasma devices, natural plasma can also often be seen on Earth. We have already talked about one of her examples. This is lightning. But in addition to lightning, plasma phenomena can be called the northern lights, “St. Elmo’s fire,” the Earth’s ionosphere and, of course, fire.

Notice that fire, lightning, and other manifestations of plasma, as we call it, burn. What causes such a bright light emission from plasma? Plasma glow is caused by the transition of electrons from a high-energy state to a low-energy state after recombination with ions. This process results in radiation with a spectrum corresponding to the excited gas. This is why plasma glows.

I would also like to talk a little about the history of plasma. After all, once upon a time only such substances as the liquid component of milk and the colorless component of blood were called plasma. Everything changed in 1879. It was in that year that the famous English scientist William Crookes, while studying electrical conductivity in gases, discovered the phenomenon of plasma. True, this state of matter was called plasma only in 1928. And this was done by Irving Langmuir.

In conclusion, I want to say that such an interesting and mysterious phenomenon as ball lightning, which I have written about more than once on this site, is, of course, also a plasmoid, like ordinary lightning. This is perhaps the most unusual plasmoid of all terrestrial plasma phenomena. After all, there are about 400 different theories about ball lightning, but not one of them has been recognized as truly correct. In laboratory conditions, similar but short-term phenomena were obtained in several different ways, so the question about the nature of ball lightning remains open.

Ordinary plasma, of course, was also created in laboratories. This was once difficult, but now such an experiment is not particularly difficult. Since plasma has firmly entered our everyday arsenal, they are experimenting a lot on it in laboratories.

The most interesting discovery in the field of plasma was experiments with plasma in zero gravity. It turns out that plasma crystallizes in a vacuum. It happens like this: charged plasma particles begin to repel each other, and when they have a limited volume, they occupy the space that is allotted to them, scattering in different directions. This is quite similar to a crystal lattice. Doesn't this mean that plasma is the closing link between the first state of matter and the third? After all, it becomes a plasma due to the ionization of the gas, and in a vacuum the plasma again becomes as if solid. But this is just my guess.

Plasma crystals in space also have a rather strange structure. This structure can only be observed and studied in space, in the real vacuum of space. Even if you create a vacuum on Earth and place plasma there, gravity will simply compress the entire “picture” that forms inside. In space, plasma crystals simply take off, forming a three-dimensional three-dimensional structure of a strange shape. After sending the results of observing plasma in orbit to scientists on Earth, it turned out that the vortices in the plasma strangely repeat the structure of our galaxy. This means that in the future it will be possible to understand how our galaxy was born by studying plasma. The photographs below show the same crystallized plasma.

Typical plasma examples

Plasma is the most common state of matter. More than 99% of what is observed consists of plasma. The following forms of plasma are well known:

Laboratory and industrial
- Flames
- Welding arc
- Rocket exhaust
- Plasma for controlled thermonuclear fusion
Natural
- and others (formed by thermonuclear fusion)
- Interstellar gas

Properties

The term plasma is used for systems of charged particles large enough to produce collective effects. Microscopic small amounts of charged particles (eg ion beams in ion traps) are not plasma. Plasma has the following properties:

The Debye screening length is small compared to the characteristic size of the plasma.
- r_D/L<<1\,
Inside the sphere c there is a large number of charged particles.
- r_D^3N>>1\,, Where N\,- concentration of charged particles
The average time between particle collisions is long compared to the period of plasma oscillations.
- \tau\omega_(pl)>>1\,

Classification

Plasma is usually divided into low temperature And high temperature, equilibrium And nonequilibrium, and quite often cold plasma is nonequilibrium, and hot plasma is equilibrium.

Temperature

In nonequilibrium plasmas, the electron temperature significantly exceeds the ion temperature. This occurs due to the difference in the masses of the ion and electron, which makes the process of energy exchange difficult. This situation occurs in gas discharges, when the ions have a temperature of about hundreds, and the electrons have a temperature of about tens of thousands of degrees.

In equilibrium plasmas both temperatures are equal. Since the ionization process requires temperatures comparable to the ionization potential, equilibrium plasmas are usually hot (with temperatures greater than several thousand degrees).

Concept high temperature plasma usually used for thermonuclear fusion plasma, which requires temperatures of millions of degrees.

Degree of ionization

The degree of ionization is defined as the ratio of the number of ionized particles to the total number of particles. Low-temperature plasmas are characterized by low degrees of ionization (<1 %). Так как такие плазмы довольно часто употребляются в plasma technologies they are sometimes called technological plasmas. Most often, they are created using electric fields that accelerate electrons, which in turn ionize atoms. Electric fields are introduced into the gas through inductive or capacitive coupling. Typical applications of low-temperature plasmas include plasma modification of surface properties (diamond films, metal nitridation, wettability modification), plasma etching of surfaces (semiconductor industry), purification of gases and liquids (ozonation of water and combustion of soot particles in diesel engines).

Hot plasmas almost always completely ionized (ionization degree ~100%). Usually they are understood as the “fourth state of matter.” An example is the Sun.

Density

Besides temperature, which is fundamental to the very existence of a plasma, the second most important property of a plasma is its density. Word plasma density usually means electron density, that is, the number of free electrons per unit volume (strictly speaking, here, density is called concentration - not the mass of a unit volume, but the number of particles per unit volume). Ion density connected to it through the average charge number of ions \langle Z\rangle: n_e=\langle Z\rangle n_i. The next important quantity is the density of neutral atoms n 0 . In hot plasma n 0 is small, but can nevertheless be important for the physics of processes in plasma.

Quasi-neutrality

Since plasma is a very good conductor, electrical properties are important. Plasma potential or potential of space is called the average value of the electric potential at a given point in space. If any body is introduced into the plasma, its potential will generally be less than the plasma potential due to the appearance of the Debye layer. This potential is called floating potential. Due to its good electrical conductivity, plasma tends to shield all electric fields. This leads to the phenomenon of quasineutrality - the density of negative charges is equal to the density of positive charges with good accuracy ( n_e=\langle Z\rangle n_i). Due to the good electrical conductivity of plasma, the separation of positive and negative charges is impossible at distances greater than the Debye length and at times greater than the period of plasma oscillations.

An example of a non-quasineutral plasma is an electron beam. However, the density of non-neutral plasmas must be very small, otherwise they will quickly decay due to Coulomb repulsion.

Differences from the gaseous state

Plasma is often called fourth state of matter. It differs from the three less energetic states of matter, although it is similar to the gas phase in that it does not have a specific shape or volume. There is still debate about whether plasma is a separate state of aggregation, or simply a hot gas. Most physicists believe that plasma is more than a gas due to the following differences:

Property	Gas	Plasma
Electrical conductivity	Very small	Very high Despite the fact that when current flows, although a small but nevertheless finite drop in potential occurs, in many cases the electric field in the plasma can be considered equal to zero. Density gradients associated with the presence of an electric field can be expressed through the Boltzmann distribution. The ability to conduct currents makes the plasma highly susceptible to the influence of a magnetic field, which leads to phenomena such as filamentation, the appearance of layers and jets. The presence of collective effects is typical, since electric and magnetic forces are much stronger than gravitational ones.
Number of particle types	One	Two or three Electrons, ions and neutral particles are distinguished by their electron sign. charge and can behave independently of each other - have different speeds and even temperatures, which causes the appearance of new phenomena, such as waves and instabilities.
Speed distribution	Maxwell's	May be non-Maxwellian Electric fields have a different effect on particle velocities than collisions, which always lead to a Maxwellization of the velocity distribution. The velocity dependence of the Coulomb collision cross section can enhance this difference, leading to effects such as two-temperature distributions and runaway electrons.
Type of interactions	Binary As a rule, two-particle collisions, three-particle collisions are extremely rare.	Collective Each particle interacts with many at once. These collective interactions have a much greater impact than two-particle interactions.

Mathematical description

Plasma can be described at various levels of detail. Usually plasma is described separately from electromagnetic fields. A joint description of a conducting fluid and electromagnetic fields is given in the theory of magnetohydrodynamic phenomena or MHD theory.

Fluid (liquid) model

Kinetic description

Sometimes the liquid model is not sufficient to describe plasma. A more detailed description is given by the kinetic model. Plasma is described in terms of the Electron Velocity Distribution Function. The model is based on. When describing plasma and electricity together. fields, the Vlasov system of equations is used. The kinetic description must be used in the absence of thermodynamic equilibrium or in the presence of strong plasma inhomogeneities.

Particle-In-Cell (particle in a cell)

Particle-In-Cell models are more detailed than kinetic models. They incorporate kinetic information by tracking the trajectories of large numbers of individual particles. Electrical density charge and current are determined by summing particles in cells that are small compared to the problem under consideration but nevertheless contain a large number of particles. Email and mag. The fields are found from the charge and current densities at the cell boundaries.

Basic plasma characteristics

All quantities are given in Gaussian units except temperature, which is given in eV and ion mass, which is given in proton mass units. μ = m i / m p ; Z– charge number; k– Boltzmann constant; TO– wavelength; γ - adiabatic index; ln Λ - Coulomb logarithm.

Frequencies

Larmor frequency of electron, angular frequency of the electron’s circular motion in a plane perpendicular to the magnetic field:

\omega_(ce) = eB/m_ec = 1.76 \times 10^7 B \mbox(rad/s)

Larmor frequency of the ion, angular frequency of the circular motion of the ion in a plane perpendicular to the magnetic field:

\omega_(ci) = eB/m_ic = 9.58 \times 10^3 Z \mu^(-1) B \mbox(rad/s)

plasma frequency(plasma oscillation frequency), the frequency with which electrons oscillate around the equilibrium position when displaced relative to the ions:

\omega_(pe) = (4\pi n_ee^2/m_e)^(1/2) = 5.64 \times 10^4 n_e^(1/2) \mbox(rad/s)

ion plasma frequency:

\omega_(pe) = (4\pi n_iZ^2e^2/m_i)^(1/2) = 1.32 \times 10^3 Z \mu^(-1/2) n_i^(1/2) \mbox (rad/s)

electron collision frequency

\nu_e = 2.91 \times 10^(-6) n_e\,\ln\Lambda\,T_e^(-3/2) \mbox(s)^(-1)

ion collision frequency

\nu_i = 4.80 \times 10^(-8) Z^4 \mu^(-1/2) n_i\,\ln\Lambda\,T_i^(-3/2) \mbox(s)^(-1 )

Lengths

De Broglie electron wavelength, electron wavelength in quantum mechanics:

\lambda\!\!\!\!- = \hbar/(m_ekT_e)^(1/2) = 2.76\times10^(-8)\,T_e^(-1/2)\,\mbox(cm)

minimum approach distance in the classical case, the minimum distance at which two charged particles can approach each other in a head-on collision and the initial speed corresponding to the temperature of the particles, neglecting quantum mechanical effects:

e^2/kT=1.44\times10^(-7)\,T^(-1)\,\mbox(cm)

electron gyromagnetic radius, radius of circular motion of an electron in a plane perpendicular to the magnetic field:

r_e = v_(Te)/\omega_(ce) = 2.38\,T_e^(1/2)B^(-1)\,\mbox(cm)

ion gyromagnetic radius, radius of circular motion of the ion in a plane perpendicular to the magnetic field:

r_i = v_(Ti)/\omega_(ci) = 1.02\times10^2\,\mu^(1/2)Z^(-1)T_i^(1/2)B^(-1)\,\ mbox(cm)

plasma skin layer size, the distance at which electromagnetic waves can penetrate the plasma:

c/\omega_(pe) = 5.31\times10^5\,n_e^(-1/2)\,\mbox(cm)

(Debye length), the distance at which electric fields are screened due to the redistribution of electrons:

\lambda_D = (kT/4\pi ne^2)^(1/2) = 7.43\times10^2\,T^(1/2)n^(-1/2)\,\mbox(cm)

Speeds

thermal electron velocity, formula for estimating the speed of electrons at . Average speed, most probable speed and root mean square speed differ from this expression only by factors of the order of unity:

v_(Te) = (kT_e/m_e)^(1/2) = 4.19\times10^7\,T_e^(1/2)\,\mbox(cm/s)

thermal ion velocity, formula for estimating the ion velocity at

These are the three states of matter that are familiar to us. But did you know that there are actually 4 of them? I think everyone has heard the word " plasma" And today I want you to also learn more about plasma - the fourth state of matter.

Unlike low-temperature plasma, high-temperature plasma has almost 100% ionization and a temperature of up to 100 million degrees. This is truly a stellar temperature. On Earth, such plasma is found only in one case - for thermo-nuclear fusion experiments. Controlling the reaction is quite complex and energy-intensive, but uncontrolled reaction is quite early - behaved like a weapon of colossal power - a thermo-nuclear bomb, tested by the USSR on August 12, 1953.

We have looked at very few terrestrial examples of plasma. But there are quite a lot of them. Man has learned to use plasma for his own benefit. Thanks to the fourth aggregate state of matter, we can use gas-discharge lamps, plasma televisions, zoo-rami, arc-electric welding, laser-rami. Conventional gas-discharge fluorescent lamps are also plasma. There is also a plasma lamp in our world. It is mainly used in science to study and, most importantly, see some of the most complex plasma phenomena, including filamentation. A photograph of such a lamp can be seen in the picture below:

That's all I would like to say on the topic of plasma. I hope it interested and surprised you. After all, this is truly an amazing phenomenon, or rather a state - the 4th state of matter.

Message on the topic of gas and plasma. Plasma is the fourth state of matter. From Faraday to Langmuir

1. What is plasma?

1.1. Most typical forms of plasma

Properties and parameters of plasma

2.4. Density

2.5. Quasi-neutrality

Mathematical description

3.1. Fluid (liquid) model

3.2. Kinetic description

3.3. Particle-In-Cell (particle in a cell)

4. Use of plasma

Conclusion

Bibliography

Typical plasma examples