The times when we associated plasma with something unreal, incomprehensible, fantastic, are long gone. Today, this concept is actively used. Plasma is used in industry. It is most widely used in lighting engineering. An example is gas discharge lamps illuminating the streets. But it is also present in fluorescent lamps. It is also in electric welding. After all, the welding arc is a plasma generated by a plasma torch. Many other examples could be given.
Plasma physics is an important branch of science. Therefore, it is worth understanding the basic concepts related to it. This is what our article is dedicated to.
Definition and types of plasma
What is plasma? The definition in physics is quite clear. A plasma state is such a state of matter when the latter has a significant (commensurate with the total number of particles) number of charged particles (carriers) that can more or less freely move inside the substance. The following main types of plasma in physics can be distinguished. If the carriers belong to particles of the same type (andparticles of opposite charge, neutralizing the system, do not have freedom of movement), it is called one-component. Otherwise, it is - two- or multi-component.
Plasma features
So, we have briefly described the concept of plasma. Physics is an exact science, so definitions are indispensable here. Let us now tell about the main features of this state of matter.
Plasma properties in physics are as follows. First of all, in this state, under the action of already small electromagnetic forces, the movement of carriers arises - a current that flows in this way until these forces disappear due to the screening of their sources. Therefore, the plasma eventually passes into a state where it is quasi-neutral. In other words, its volumes, larger than some microscopic value, have zero charge. The second feature of plasma is related to the long-range nature of the Coulomb and Ampère forces. It consists in the fact that motions in this state, as a rule, have a collective character, 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 most striking manifestation is the ease of occurrence of various kinds of instabilities. They are a serious obstacle hindering the practical application of plasma. Physics is a science that is constantly evolving. Therefore, it can be hoped that over time these obstacleswill be eliminated.
Plasma in liquids
Turning to concrete examples of structures, let's start with the consideration of plasma subsystems in condensed matter. Among liquids, one should first of all name liquid metals - an example to which the plasma subsystem corresponds - a single-component plasma of electron carriers. Strictly speaking, the category of interest to us should also include electrolyte liquids in which there are carriers - ions of both signs. However, for various reasons, electrolytes are not included in this category. One of them is that there are no light, mobile carriers, such as electrons, in the electrolyte. Therefore, the above plasma properties are expressed much weaker.
Plasma in crystals
Plasma in crystals has a special name - solid state plasma. In ionic crystals, although there are charges, they are motionless. Therefore, there is no plasma. In metals, these are conduction electrons that make up a one-component plasma. Its charge is compensated by the charge of immobile (more precisely, unable to move long distances) ions.
Plasma in semiconductors
Considering the basics of plasma physics, it should be noted that the situation in semiconductors is more diverse. Let's briefly characterize it. A one-component plasma in these substances can arise if appropriate impurities are introduced into them. If impurities easily donate electrons (donors), then n-type carriers appear - electrons. If impurities, on the contrary, easily take away electrons (acceptors), then p-type carriers arise- holes (empty places in the distribution of electrons), which behave like particles with a positive charge. A two-component plasma formed by electrons and holes arises in semiconductors in an even simpler way. For example, it appears under the action of light pumping, which throws electrons from the valence band into the conduction band. We note that under certain conditions, electrons and holes attracted to each other can form a bound state similar to a hydrogen atom - an exciton, and if the pumping is intense and the density of excitons is high, then they merge together and form a drop of electron-hole liquid. Sometimes such a state is considered a new state of matter.
Gas ionization
The above examples referred to special cases of the plasma state, and plasma in its pure form is called ionized gas. Many factors can lead to its ionization: electric field (gas discharge, thunderstorm), light flux (photoionization), fast particles (radiation from radioactive sources, cosmic rays, which were discovered by increasing the degree of ionization with height). However, the main factor is the heating of the gas (thermal ionization). In this case, the separation of an electron from an atom leads to a collision with the latter of another gas particle, which has sufficient kinetic energy due to high temperature.
High and low temperature plasma
Low-temperature plasma physics is what we come into contact with almost every day. Examples of such a state are flames,substance in a gas discharge and lightning, various types of cold space plasma (iono- and magnetospheres of planets and stars), 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 controlled thermonuclear fusion facilities (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 combinations either completely disordered or more or less ordered. It was believed that there are three phases - gaseous, liquid and solid. Substances accept them under the influence of external conditions.
However, currently we can say that there are 4 states of matter. It is plasma that can be considered new, the fourth. Its difference from the condensed (solid and liquid) states lies in the fact that, like a gas, it does not have not only shear elasticity, but also a fixed volume. On the other hand, a plasma has in common with a condensed state the presence of short-range order, i.e., the correlation of the positions and composition of particles adjacent to a given plasma charge. In this case, such a correlation is generated not by intermolecular, but by Coulomb forces: a given charge repels charges of the same name with itself and attracts opposite ones.
Plasma physics was briefly reviewed by us. This topic is quite voluminous, so we can only say that we have revealed its basics. Plasma physics certainly deserves further consideration.
