Electrothermics

2.2. Plasma and its characteristics

2.2.1. Plasmas

Plasmas are the fourth state of matter after solids, liquids, and gases, and over 99% of the universe is in plasma state [IGN 92]. Plasma is an ionized gas, considered to be electrically neutral both at the macroscopic scale and in its entirety: this property is known as quasi-neutrality (and does not only applies to the neutrality of the plasma as a whole, but also to certain parts thereof) [BAC 95]. It consists of a mix of different cohabiting species: light particles (electrons), heavy particles in either their ground or excited states (atoms, molecules, and ions), and massless particles (photons).

Since plasmas can exist in a large pressure range, it is more convenient to class them as a function of electronic temperature and electronic density (1 eV = 7,740 K, as per the Stefan–Boltzmann distribution) [FAU 87]. Figure 2.1 shows the different types of plasma, which imply the existence of some natural plasmas in the universe.

Figure 2.1. Temperature diagram as a function of electronic density

In thermal plasma, also known as hot plasma, the kinetic energies of the electrons and the heavy particles are in the same order of magnitude. Non-thermal plasma (also known as cold plasma), however, is a plasma in thermal disequilibrium, characterized by a large gap between the kinetic energies of the particles. Figure 2.2 shows the temperatures ...