Thermal plasmas: fundamental aspects

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, 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 making them up, with a separation between thermal plasmas and discharge plasmas;

• interaction-free (or virtually collision-free) plasmas 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. a gas of electrons.

Among all these plasmas, we will limit our discussion to thermal plasmas i.e. mainly produced by electric arcs and Radio Frequency discharges. The main characteristic of thermal plasmas is that they are generated at or near atmospheric pressure, i.e. for simplicity between 10 ⁴ and 10 ⁶ Pa. They fall within the scope of classical kinetics and the Boltzmann equation. Because of the pressure, collisions between particles are very numerous and ionization is essentially due to a thermal effect. They correspond to electron densities between 10 ²⁰ and 10 ²⁴ m ^-3 and temperatures between 6,000 and 25,000 K. In these plasmas, electric fields (except in the vicinity of electrodes in arcs) and mean free paths are too low to produce an ionization state by direct inelastic collision. Ionization is therefore essentially a thermal phenomenon due to elastic collisions.