In the physics of plasmas, it is often described as a state of matter, like a liquid or a gas. Unlike biology where a plasma has a different meaning. A plasma in physics may however form within a gas. For most telestial plasmas the gas and plasma state coexist. Plasma can also coexist with the solid state as demonstrated by the recent experiments in plasma crystals. In a partially ionised plasma some of the atoms or molecules are ionized, meaning that the electrons have enough energy to escape their nuclei.
Above we see what a gas plasma on earth might look like. Not all the gas is ionized; the black dots are neutral gas atoms or molecules. The blue dots represent atoms that have lost an electron and become positive charged ions. The lost electron is seen as the free electrons, the red dots. In general, it is thought that plasma is the fourth state of matter in order of solid, liquid, gas and finally plasma. The order is set by the energy of the particles. The more energy the particles in the system have then the higher the velocity of particles and the temperature of the system. This picture is accurate for equilibrium systems. It would take a temperature of many 10′s of thousands of degrees Celsius to start to ionise a gas to get to the plasma state. This concept is somewhat misleading as plasmas are not defined by the temperature of the particles. It is possible to ionise the gas by using a non-equilibrium system such as a beam of electrons not in thermal equilibrium with the gas. This is referred to as a cold, low temperature plasma and is in fact the most common plasma seen on earth. It is the cold plasma that has the most practical uses. I will explain that later.
The best definition of a plasma is then not as a hot gas but in terms of its collective behaviour. Plasma are a collection of charged particles, both positive and negative that behave in a collective way because of attractive and repelling electric forces. So this is the important feature, plasmas behave collectively. The interaction that makes them behave together is the electric fields created by the free charges.
In the centre of a plasma it is considered to be electrically neutral, that is the same number of negative electrons as positive ions in any given volume. The net charge is zero and the average electrical field is zero. If you put a metal probe into a plasma and put a voltage of +100 Volts on the metal probe the plasma would react to shield the probe so that the electrical field created by the probe would go to zero very close to the probe. This shielding is called Debye shielding.
There needs to be enough charged particles in a given volume so that the plasma can act to shield an external potential. This means there needs to be many thousands of charged particles of both charges (positive and negative) inside a radius which is equal to the Debye length. The volume of the plasma needs to be big enough so that is covers many Debye lengths in each direction. If it takes a Debye length to shield a potential and the plasma is just one Debye length long then the shielding will not work. The number of collisions that the shielding particle has with neutral gas particles has to be small enough as not to prevent the shielding being effective. It is normally the electrons that do the shielding so that the time between electron-neutral collisions has to be longer that the response time of an electron to an electromagnetic field.
The electromagnetic force is generally observed to create structure: e.g., stable atoms and molecules, crystalline solids. In fact, the most widely studied consequences of the electromagnetic force form the subject matter of Chemistry and Solid-State Physics, which are both disciplines developed to understand essentially static structures.
Structured systems have binding energies larger than the ambient thermal energy. Placed in a sufficiently hot environment, they decompose: e.g., crystals melt, molecules disassociate. At temperatures near or exceeding atomic ionization energies, atoms similarly decompose into negatively charged electrons and positively charged ions. These charged particles are by no means free: in fact, they are strongly affected by each others’ electromagnetic fields. Nevertheless, because the charges are no longer bound, their assemblage becomes capable of collective motions of great vigor and complexity. Such an assemblage is termed a plasma in the physics world.
A plasma will form when the electron temperature reaches a few electron volts in a gas argon. Nomally, in plasma physics temperatures are converted to electron volts (eV) for convenience. One electron volt equals about 40 times room temperature or over 11,000K. With an electron temperature of 2eV the energy of electrons follows a logarithm scaling with energy, there will exist enough electrons with energies above the ionisation energy of the atoms in argon. It is not necessary that all atoms are ionised for a plasma to form. At low pressures of one millionth of atmosphere a plasma will exist when one part in a million of the atoms are ionised. It is important to understand that in plasma physics it is not necessary for the gas atoms and electrons to be in thermal equilibrium. In fact, in most industrial applications the electrons are in thermal equilibrium with themselves but not with the gas. Therefore we talk about a gas temperature and an electron temperature. By heating the electrons using a radio frequency (RF) field the electrons lose energy in collisions with neutrals but are weakly coupled because of the difference in mass of the electron and gas atom. It is not uncommon for a Argon plasma at very low gas pressure to have an electron temperture of 3eV and a gas temperature of 0.02eV or room temperatrue. Such a plasma is self sustaining so only the electrons are hot.
Of course, bound systems can display extreme complexity of structure: e.g., a protein molecule. Complexity in a plasma is somewhat different, being expressed temporally as much as spatially. It is predominately characterized by the excitation of an enormous variety of collective dynamical modes.
Since thermal decomposition breaks interatomic bonds before ionizing, most terrestrial plasmas begin as gases. In fact, a plasma is sometimes defined as a gas that is sufficiently ionized to exhibit plasma-like behaviour. Note that plasma-like behaviour ensues after a remarkably small fraction of the gas has undergone ionization. Thus, fractionally ionized gases exhibit most of the exotic phenomena characteristic of fully ionized gases.
Plasmas resulting from ionization of neutral gases generally contain equal numbers of positive and negative charge carriers. In this situation, the oppositely charged fluids are strongly coupled, and tend to electrically neutralize one another on macroscopic length-scales. Such plasmas are termed quasi-neutral (“quasi” because the small deviations from exact neutrality have important dynamical consequences for certain types of plasma mode). Strongly non-neutral plasmas, which may even contain charges of only one sign, occur primarily in laboratory experiments: their equilibrium depends on the existence of intense magnetic fields, about which the charged fluid rotates.
It is sometimes remarked that 95% (or 99%, depending on whom you are trying to impress) of the baryonic content of the Universe consists of plasma. This statement has the double merit of being extremely flattering to Plasma Physics, and quite impossible to disprove (or verify). Nevertheless, it is worth pointing out the prevalence of the plasma state. In earlier epochs of the Universe, everything was plasma. In the present epoch, stars, nebulae, and even interstellar space, are filled with plasma. The Solar System is also permeated with plasma, in the form of the solar wind, and the Earth is completely surrounded by plasma trapped within its magnetic field.
Terrestrial plasmas are also not hard to find. They occur in lightning, fluorescent lamps, a variety of laboratory experiments, and a growing array of industrial processes. In fact, the glow discharge has recently become the mainstay of the micro-circuit fabrication industry. Liquid and even solid-state systems can occasionally display the collective electromagnetic effects that characterize plasma: e.g., liquid mercury exhibits many dynamical modes, such as Alfvén waves, which occur in conventional plasmas physics.
Video Presentation related to this material available on Youtube What is a Plasma in Physics