The times when plasma was associated with something unreal, incomprehensible, fantastic, have long passed. Nowadays, this concept is actively used. Plasma is used in industry. It is most widely used in lighting technology. 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, a welding arc is a plasma generated by a plasmatron. Many other examples can be given.
Plasma physics is an important branch of science. Therefore, it is worthwhile to understand the basic concepts related to it. This is what our article is devoted to.
Definition and types of plasma
What is plasma? The definition in physics is quite clear. A state of matter is called a plasma state when in the latter there is a significant (commensurate with the total number of particles) number of charged particles (carriers) capable of more or less freely moving inside the substance. The following main types of plasma in physics can be distinguished. If the carriers belong to particles of the same sort (and particles of the opposite sign of the charge, which neutralize the system, do not have freedom of movement), it is called one-component. In the opposite case, it is two- or multicomponent.
Plasma features
So, we briefly described the concept of plasma. Physics is an exact science, therefore one cannot do without definitions. We will now talk about 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 influence of already small electromagnetic forces, carrier movement arises - a current that flows in this way until these forces disappear due to the screening of their sources. Therefore, the plasma eventually goes into a state where it is quasineutral. In other words, its volumes, large of a certain microscopic size, have zero charge. The second feature of the plasma is associated with the long-range nature of the Coulomb and Ampere forces. It consists in the fact that the movements in this state, as a rule, are collective in 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 most striking manifestation is the ease of occurrence of various kinds of instabilities. They are a serious obstacle that impedes the practical use of plasma. Physics is a science that is constantly evolving. Therefore, it is hoped that over time these obstacles will be removed.
Plasma in liquids
Turning to specific examples of structures, we begin by examining the plasma subsystems in condensed matter. Among liquids, liquid metals should first be mentioned - an example to which the plasma subsystem corresponds - a one-component plasma of electron carriers. Strictly speaking, the category of interest to us should also include liquid electrolytes in which there are carriers — ions of both signs. However, for various reasons, electrolytes do not belong to this category. One of them is that in the electrolyte there are no light, mobile carriers, such as electrons. Therefore, the above plasma properties are much less pronounced.
Plasma in crystals
Plasma in crystals has a special name - solid state plasma. Although there are charges in ionic crystals, they are motionless. Therefore, there is no plasma there. In metals, these are conduction electrons that make up a one-component plasma. Its charge is compensated by the charge of motionless (more precisely, incapable of moving long distances) ions.
Semiconductor Plasma
Considering the basics of plasma physics, it should be noted that the situation in semiconductors is more diverse. We briefly describe it. Unicomponent plasma in these substances can arise if appropriate impurities are introduced into them. If impurities easily give up electrons (donors), then n-type carriers arise - electrons. If, on the contrary, impurities easily pick up electrons (acceptors), then p-type carriers arise - holes (empty spaces in the distribution of electrons) that behave like particles with a positive charge. The two-component plasma, formed by electrons and holes, arises in semiconductors in an even simpler way. For example, it appears under the influence of light pumping, casting electrons from the valence band into the conduction band. 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, they merge together and form a drop of electron-hole liquid. Sometimes this condition is considered a new state of matter.
Gas ionization
The above examples relate to special cases of a plasma state, and pure ionized gas is called plasma . 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 degree of ionization). However, the main factor is gas heating (thermal ionization). In this case, the collision of the electron with the atom leads to the collision with the latter of another gas particle having sufficient kinetic energy due to high temperature.
High temperature and low temperature plasma
Low-temperature plasma physics is something we come into contact with almost every day. Examples of such a state are flame, matter in a gas discharge and lightning, various types of cold cosmic plasma (ionic 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 installations for controlled thermonuclear fusion (tokamaks, laser devices, beam devices, etc.).
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 take them under the influence of external conditions.
However, at present, we can say that there are 4 states of matter. It is plasma that can be considered new, fourth. Its difference from condensed (solid and liquid) states is that it, like gas, does not have 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, 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: this charge repels the charges of the same name from itself and attracts opposite charges.
Plasma physics was briefly reviewed by us. This topic is quite voluminous, so we can only talk about the fact that we have revealed its basics. Plasma physics certainly deserves further consideration.