Plasma is an ionized gas, that is, a gas where a significant part of its molecules are not in electrical equilibrium. This means that there are ions (positive or negative) and also free electrons inside the gas. In this condition, the gas has the ability to conduct electric current, and the higher the degree of ionization, the greater the ability to conduct electric current. The degree of ionization is defined as the volume of ions and electrons over the total volume.

Ionization is not a spontaneous phenomenon. It needs an external agent to happen, which can be the incidence of electromagnetic wave, heating, or the collision of other elements. When the external agent is removed, the ions and electrons tend to recombine, and the gas ceases to be a plasma.

Plasma is of great interest to industry because chemical reactions of interest take place inside it. A common way to produce plasma artificially is to subject the gas to an electric field (which is generated by a potential difference between electrodes) where the few pre-existing ions and electrons are accelerated and collide with other atoms or molecules. When the speed at the instant of collision is great, electrons are removed from the atoms/molecules hit, which generates other ions and free electrons, which in turn are also accelerated and collide, generating a chain reaction that causes a large part of the volume of the gas to be formed of ions. If the potential difference is maintained and the gas is highly ionized, an electric current flows through the gas. The lower intensity electric current is called “Dark Discharge”. As the current increases, the name changes to “Glow discharge” and “Voltaic Arc”. Plasma reactors are devices that work in conjunction with a power supply to ionize a gas and maintain the conditions for the reactions of interest to take place.