We are accustomed to classifying matter on Earth into three states of matter: solid, liquid, and gaseous. Differences between aggregate states are reflected through the strength of the bonds that bind the building blocks of matter together. These bonds are strongest between atoms or molecules in a solid. These bonds are much weaker in a liquid state, and in the gaseous state, they are almost non-existent. The exact ratio between the thermal energy of atoms or molecules and the binding energy determines the state of matter. Roughly speaking, the physical state of matter depends on its temperature. Take, for example, (solid) ice. If the ice is heated to the melting point, it begins to melt, and we get water in a liquid state (liquid). Such a phase transition takes place at a constant temperature if the pressure is also constant. If the liquid is heated again, its temperature rises to the boiling point when the water evaporates; we got steam. This, too, is a phase transition that takes place at a constant temperature. However, as the gas composed of neutral atoms or molecules heats, the proportion of ionized atoms or molecules increases gradually, and plasma can be formed. Here the phase transition does not take place at a constant temperature, but the gas’s temperature increases all the time during heating. In nature, plasma is found in forms such as, e.g., flame or lightning during storms, where enough energy is released to ionize the surrounding air. Plasma predominates in the upper atmosphere and the universe in stars, such as the solar wind, solar corona, sunspots, and comet tails are also in the plasma state. It is also found in interplanetary and interstellar space. Therefore, in space, most matter is in a state of plasma, while on Earth, it must be artificially created.
One way to generate plasma is to heat the gas to a temperature where atoms or molecules’ ionization reaches a sufficient level. Such a plasma is called thermal equilibrium plasma because the gas is in thermodynamic equilibrium (concentrations of electrons, ions, and neutral atoms and molecules are uniformly dependent on temperature, and the kinetic energy of particles’ thermal motion is uniformly dependent on temperature). We need a temperature higher than 104 K to generate such plasma, where a significant degree of ionization is achieved. Thus, thermal equilibrium plasma is not suitable for use in laboratories. Hence, we use various other ionization forms, which allow significantly higher ion density at much lower temperatures. More important ways to ionize atoms or molecules are photoionization, multiphoton excitation, and gas discharge.
Plasma can be generated by different types of discharges (smoldering, direct current with a hot cathode, high frequency – radio frequency (RF), and microwave (MV) or combined). When we apply a discharge by a strong external electric field in gas, we cause an accelerated movement of free electrons, which are in any case present in the gas at low densities, to energy suitable for the ionization of atoms or molecules. The external electric field can be direct or alternating. It is only essential that the electrons are accelerated to sufficiently high energy to allow ionization. The energy transfer of a high-frequency electric field is much more efficient for light electrons than for heavy ions. In plasmas generated by high-frequency discharge, the electron temperature (thermal energy of electrons) is much higher than that of ions. However, when the plasma source (electric field) is switched off, the plasma disappears quickly (within a few milliseconds) due to the recombination of charged particles into neutral gas molecules.
