The plasma physics and the plasma state in nature

The plasmas are usually partially ionized gases, a mixture of positive and negatively charged particles where Ni, Ne and Na respectively denote the number of positive ions, electrons and neutral atoms Na by unit volume. In the equilibrium state, this medium is quasineutral and such statement means that the negative and positive charge densities are equals (Ni = Ne) and hence the plasma bulk electrically neutral. These physical systems are made of free charges that are also characterized by average kinetic energy of each particle group or quivalently, by their temperatures; as the electron kBTe, ion kBTi and neutral gas kBTa temperatures. The involved average energies of particles are usually high and are currently expressed in units of electron volts (eV) being 1 eV equivalent to 11.600 Kelvin, and this fact explains why plasmas are usually associated with high energies. All particle temperatures are equal in the thermodynamic equilibrium, but this is not the most frequent case in nature and plasmas are commonly found in stationary multithermal states.

The electric charges in plasmas are free to move under forces exerted by electric and magnetic fields. In addition to collisions, the long range Coulomb interaction between charged particles makes the motions of the particles to rely not only on local conditions, but also on the state of remote regions as well. In consequence, the charged species in plasmas exhibit collective motions.

Therefore, we may define the plasmas as quasineutral gases of charged and neutral particles that exhibit collective behaviors.

In addition to electrons and positive ions, more complex plasmas also might have negative ions or complex molecules (i.e. NO2-, ...etc) or small charged grains of dust. In this latter case they are denominated dusty plasmas.

The plasmas are everywhere present in nature and could be roughly classified according to the plasma density (N= Ni = Ne) and temperature (kBT = kBTe = kBTe). Both magnitudes exhibit large variations, of seven orders of magnitude in temperature and more than twenty in density as shows the side graph.

This huge range is because Plasma Physics covers a very different phenomena of interest in Science and Engineering as electric gas discharges, the interplanetary plasma, the solar corona, the Earth ionosphere, ion thrusters for space propulsion, controlled thermonuclear fusion...etc.

The huge range of the diagram may be realized when we notice that water (point labeled, H2O) and air (labeled, Air STP) under standard conditios only differ in density by 103. The water and white dwarf star are separated by 105 while the massive neutron stars and water by a factor of 1015. In the diagram, the plasma densities range from 1 to 1028 cm-3 and the temperatures from 0.01 up to 106 eV. Despite a common beckaground remains, the kind of physics involved in fusion reactor plasmas is quite different from phenomena involved in ionosperic and space plasmas, placed at the bottom of the diagram.

In laboratory experiments, the plasmas are often in a non equilibrium state (the temperature of electrons, ions and neutral atoms are different) and are originated by ionizing a low pressure neutral gas by different means. For cold, weakly ionized plasmas the relative content of charged particles is small, leading to a low ionization degree, even below 10-6. Then, only a small fraction of atoms in the gas are ionized and the typical values for plasmas of our interest are Ne between 106 and 108 cm-3 and kBTe about 2-3 eV. The temperatures of ions and neutral atoms are approximately equal to ambient neutral gas temperature, about 0.05 eV.

For further reading the interested reader is referred to any textbook of Plasma Physics as Introduction to Plasma Physics and Controlled Fusion. Vol.   1, Plasma Physics. by F.F. Chen, Plenum Press, New York  (1984).