1928: introduction by I. Langmuir of the term “plasma”, which comes from the Greek, to evoke gelatinous or moldable matter. While observing the behavior of ionized gas in discharge tubes, I. Langmuir noticed that the oscillations of the ions resembled the oscillations in a gelatinous medium, hence the name "plasma" to designate the 4th state of matter. .

This is the beginning of the development of plasma physics which will also be supported by research in radiocommunications.

What do we know about plasma then?

Plasma looks like a gas, except that the particles that make it up are not neutral but charged, which means that the plasma conducts electricity and is sensitive to electromagnetic fields. It therefore ultimately behaves quite differently from a gas!

1901: G. Marconi observes the reflection of the waves (according to him on the atmosphere, in reality we will later discover that it was the ionosphere).

1925: E. Appleton introduces the hypothesis that the Earth's atmosphere is ionized above a certain altitude. It thus opens the way to the study of natural (astrophysical) plasmas, while so-called "laboratory" plasmas continue to be explored, particularly with regard to electron beams as sources of coherent radiation.

Since 1955:

•  Rise in research on controlled nuclear fusion.
•  Development of space exploration which first made it possible to explore the ionosphere, then to go further and further into the solar system.
•  Development of research on interactions between plasmas and surfaces, leading to surface treatments in mechanics or microelectronics using plasmas.
•  It has also been shown that laser-created plasmas can behave as sources of fast particles or radiation, that is, miniature accelerators, which present an alternative to traditional accelerators.

Since the 2000s :

Research in plasma physics is booming, far from being limited to astrophysics or fusion, industrial applications are numerous and promising. Plasmas are particularly at the heart of major societal issues such as the environment (CO2 conversion), medicine and agronomy. Space meteorology has also recently entered the field of plasma research.
* Source: Introduction to plasma physics, Gérard Belmont (CNRS), Laurence Rezeau, Caterina Riconda and Arnaud Zaslavsky (Sorbonne University), July 2018, ISTE éditions

To answer it, we offer you an extract from a book to understand plasmas in all their diversity: Introduction to plasma physics, Gérard Belmont (CNRS), Laurence Rezeau, Caterina Riconda and Arnaud Zaslavsky (Sorbonne University), 2019, ISTE editions.
“It can be said that the “plasma” state constitutes the “fourth state of matter”. It is less well known than the other three states of matter (solid, liquid and gas) because it is less present in our immediate environment. However, it must be known that neutral matter, which constitutes the largest part of this environment, is an exception in the universe. In most of the latter, matter consists either totally or partially of charged particles (particularly, electrons and protons) which are free and not bound within neutral atoms and molecules; these charged particle gases are called “plasmas”. Their main property, which distinguishes them from neutral gases, is that they closely interact with the electromagnetic field, on the one hand, because the movement of particles is governed by fields, and on the other hand, because the ensemble of particles is itself a source of fields by the charge density and the currents that these movements cause.
Plasma physics is thus at the intersection of statistical physics and electromagnetism.
– For the physics of natural plasmas, its most developed fields of application are external geophysics (ionosphere/magnetosphere of the Earth and other planets, aurora borealis, etc.), solar and stellar physics (solar corona, solar wind, etc.) and astrophysics (galactic jets, etc.).
– For laboratory plasmas, they play a very important role in studies concerning nuclear fusion, either by magnetic confinement (tokamaks) or by inertial confinement (laser fusion) and in the production of energy particles by plasma accelerator. "