Plasma physics. Basics of Plasma Physics

The times when plasma was associated with something unreal, incomprehensible, fantastic, have long since passed. Nowadays this concept is actively used. Plasma is used in industry. Most large-scale use in lighting equipment. An example is gas-discharge lamps that illuminate the streets. But in the fluorescent lamps it is present. It is also in electric welding. After all, the arc of welding is a plasma generated by a plasmatron. There are many other examples.

Plasma physics is an important branch of science. Therefore, it is necessary to understand the basic concepts related to it. Our article is devoted to this.

Definition and types of plasma

What is plasma? The definition in physics is quite clear. Plasma refers to a state of matter when in the latter there is a significant (commensurate with the total number of particles) number of charged particles (carriers) capable of moving more or less freely inside the substance. One can single out the following main types of plasma in physics. If the carriers belong to particles of one kind (and the particles of the opposite charge sign that neutralize the system do not have the freedom of movement), it is called one-component. In the opposite case, it is - two-or multi-component.

Features of plasma

So, we briefly described the notion of plasma. Physics is an exact science, so you can not do without definitions. Let us now describe the main features of this state of matter.

The properties of plasma in physics are as follows. First of all, in this state, under the action of already small electromagnetic forces, there is a motion of carriers-a current that flows in this way and until these forces disappear through the screening of their sources. Therefore, the plasma eventually turns into a state when it is quasineutral. In other words, its volumes, large of some microscopic magnitude, have zero charge. The second feature of the plasma is related to the long-range nature of the Coulomb and Ampere forces. It consists in the fact that the motions in this state, as a rule, are of a collective nature, involving a large number of charged particles. These are the basic properties of plasma in physics. It would be useful to remember them.

Both of these features lead to the fact that plasma physics is unusually rich and diverse. Its brightest manifestation is the ease with which various kinds of instabilities arise. They are a serious obstacle hampering the practical application of plasma. Physics is a science that is constantly evolving. Therefore, we can hope that over time these obstacles will be eliminated.

Plasma in liquids

Turning to concrete examples of structures, we begin by considering the plasma subsystems in condensed matter. Among liquids, liquid metals should be named first of all - an example to which the plasma subsystem corresponds - a one-component carrier electron plasma. Strictly speaking, to the category of interest we should include liquid electrolytes, in which there are carriers - ions of both signs. However, for various reasons, electrolytes are not considered to be in this category. One of them is that there are no light, mobile carriers, such as electrons, in the electrolyte. Therefore, the above properties of the plasma are much less pronounced.

Plasma in crystals

Plasma in crystals has a special name - solid-state plasma. In ionic crystals, although there are charges, but they are immobile. Therefore, there is no plasma there. In metals, however, these are conduction electrons that make up a single-component plasma. Its charge is compensated by the charge of immobile ions (more accurately speaking, unable to shift over long distances).

Plasma in semiconductors

Considering the basics of plasma physics, it must be noted that in semiconductors the situation is more diverse. Briefly describe it. One-component plasma in these substances can arise if appropriate impurities are introduced into them. If impurities easily give off electrons (donors), then carriers of n-type - electrons appear. If impurities, on the other hand, easily select electrons (acceptors), then p-type carriers - holes (empty places in the electron distribution) that behave like particles with a positive charge appear. A two-component plasma, formed by electrons and holes, appears in semiconductors in an even simpler way. For example, it appears under the action of light pumping, which sends electrons from the valence band to the conduction band. We note that under certain conditions, electrons and holes attracted to each other can form a bound state, similar to the hydrogen atom, an exciton, and if the pump is intense and the density of excitons is large, they merge together and form a drop of electron-hole liquid. Sometimes this state is considered a new state of matter.

Ionization of gas

These examples relate to special cases of the plasma state, and the plasma in the pure form is called ionized gas. Many factors can lead to its ionization: an electric field (a gas discharge, a thunderstorm), a light flux (photoionization), fast particles (radiation of radioactive sources, cosmic rays, which were discovered in increasing degrees of ionization with altitude). However, the main factor is the heating of the gas (thermal ionization). In this case, to the detachment of the electron from the atom, the other particle of gas, which has sufficient kinetic energy due to the high temperature, collides with the latter.

High-temperature and low-temperature plasma

The physics of low-temperature plasma is what we come into contact with almost every day. Examples of such a state can serve as a flame, a substance in a gas discharge and lightning, various kinds of cold cosmic plasma (ionosphere and magnetosphere of planets and stars), a working substance in various technical devices (MHD generators, plasma engines, burners, etc.). . Examples of high-temperature plasma are the matter of stars at all stages of their evolution, except for early childhood and old age, the working substance in installations for controlled thermonuclear fusion (tokamaks, laser devices, beam devices, etc.).

The fourth state of matter

A century and a half ago, many physicists and chemists believed that matter consists only of molecules and atoms. They are combined in a combination either completely unordered or more or less ordered. It was believed that there are three phases - gaseous, liquid and solid. Substances take them under the influence of external conditions.

However, at the present time it can be said that there are 4 states of matter. It is the plasma that can be considered a new, fourth. Its difference from the condensed (solid and liquid) states is that, like gas, it has not only shear elasticity, but also a fixed intrinsic volume. On the other hand, the plasma is related to the condensed state by the presence of short-range order, that is, the correlation of positions and composition of particles adjacent to a given plasma charge. In this case, such a correlation is generated not by intermolecular forces but by Coulomb forces: this charge repels the charges of the same name with it and attracts the opposite charges.

The physics of plasma was briefly considered. This topic is quite extensive, so we can only say that we have uncovered its foundations. Plasma physics certainly deserves further consideration.