Ionized gases and plasmas

Plasmas are the fourth state of matter and account for almost 99% of the Universe. They exist either in their natural state (solar corona, Sun, interior of stars, ionosphere, interior of white dwarfs, etc.), or in laboratories, where they are generally produced by electrical discharges. They are gases containing atoms, molecules and ions in their ground state or in an excited state, and electrons and photons resulting from the de-excitation of excited states. Electrons, which are very light compared to ions and neutrals, are strongly accelerated by electric and/or magnetic fields and play a very special role. There are many different types of plasma, depending on electron density and temperature (or energy). They can be distinguished by a number of criteria:

• their ionization state, which can range from very weak (a few electrons in an "ocean" of neutrals) to totally ionized (there are only electrons and ions left);

• collisions between the particles that make them up, with plasmas based on classical kinetics and the Boltzmann equation. Here again, we can separate them into thermal plasmas, where collisions are very numerous (pressure close to atmospheric pressure) and ionization is due to a thermal effect, and discharge plasmas, where the distance covered between two collisions is sufficiently large for ionization to take place by direct collision;

• interaction-free (or virtually collision-free) plasmas, where charged particles move under the effect of electric and magnetic fields (atmospheres with very low pressures < 0.1 Pa);

• relativistic plasmas where particle velocities approach that of light;

• very dense, fully ionized plasmas that behave like solids or liquids (charged submicron particles);

• plasmas based on quantum statistics such as Fermi-Dirac, e.g. an electron gas.

Wave propagation in plasmas is also highly complex, as they contain charged particles of very different masses (electrons and ions) which will participate in the wave. In addition, magnetic fields give them an anisotropic character. Once again, we can classify them into different families in terms of wave propagation.

This article is just a very brief introduction to plasmas.

Acknowledgements: this text is based on that of Professor J.L. Delcroix, published in Techniques de l'Ingénieur in 1980. I would like to pay tribute to Professor Delcroix, from whom I drew a great deal of inspiration, and from whom I have taken some passages that are still relevant today.

The table of notations and symbols can be found at the end of...