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Phases, the Three Fourth Phase In the solid state, the atoms are firmly imprisoned inside a rigid network (like ice for example). When we raise the temperature, they go into a liquid state (the ice melts), where the atoms may slide around in relation to the others, thus enabling a liquid to adapt to the shape of a container. If we heat it up even more, we arrive at the gas state: atoms then move around freely and independently of each other, i.e., water turns into steam (Figure 13-11). Finally, when we get to very high temperatures (typically several million degrees), the ingredients of the atom separate, nuclei and electrons move around independently and form a globally neutral mixture: this is the plasma state (See Figure 13-12).

Figure 13-11 The Three Phases
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Figure 13-12 The Fourth Phase

This fourth state of matter, found in the stars and the interstellar environment, makes up most of our universe (around 99 %). On Earth, it does not exist in a natural form, apart in lightning and the Aurora Borealis. In our everyday life, plasmas have many applications (micro-electronics, television flat screens and so on), of which the commonest is the neon tube. (See Figure 13-13.)
Plasma Depending on the temperature, the atoms may be partially or wholly ionized. A plasma may thus be considered as a mixture of positively charged ions and negatively charged electrons, possibly co-existing with neutral atoms and molecules. For example, in our luminescent tube, the ions and electrons is a small proportion in relation to atoms and molecules. On the other hand, in plasmas produced for fusion experiments, the gas is strongly ionised, and the atoms and molecules are in low proportion, even completely absent in the heart of the pulse. In both cases, the description of plasmas comes from the physics of fluid mechanics and controlled by the force of electromagnetic interaction. The system is described by the usual macroscopic features such as density, temperature, pressure and rate of flow.

Figure 13-13 Plasma Occurrence

See "Application to Thermo-nuclear Fusion" and "Plasma Confinement and Pinch Effect" for further detail on the subject.

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