The fourth state of matter : the plasma state
Due to its high ionization rate which induces specific behaviors, the plasma state is a singular state of matter, compared to solid, liquid and gas states which make up our earth environment. That is why it is coined the fourth of the matter. This plasma state is multifaceted and moreover ubiquitous in the universe, whatever they are natural or artificially created on earth; in the latter case, they appear very often within extreme conditions which make plasmas and large research facilities closely linked. Plasma physics is both a specific and interdisciplinary research domain. It shows unique opportunities to make in laboratories states of matter, either very diluted or very dense, very cold or very hot which can be found in universe on a large scale. Furthermore it is structured by large cognitive research programs, on one hand, in sciences of the matter, astrophysics, planetology, ultrahigh laser intensities and by applied research, on the other hand, focused on key societal issues gathering thermonuclear fusion, innovating processes for structuration and resistance of the materials, environment, health, propulsion.
In order to better identify the various plasmas, it is usual to consider the density of particles (or equivalently the average inter-particle distance) and to consider the averaged kinetic energy of the particles. The plasmas can then be labeled, diluted or dense if not showing quantum degeneracy, on the one hand, and cold or hot, on the other hand. Due to the wide variety of both the instruments which are used and the undertaken projects, the research community is usually structured in four categories: natural plasmas, cold plasmas for processes, magnetic fusion plasmas and plasmas created by laser-matter interaction. Plasmas are central for a large category of applications: materials and environment, satellites, atmospheric reentry, electron and ion acceleration, sources of X ray and energy, more specifically within the framework of the magnetic or inertial confinement thermonuclear fusion. The energetic challenge associated to the latter application takes form on the national territory via the building of large facilities, namely the International Thermonuclear Experimental Reactor (ITER) at Cadarache (South-East of France) and the Laser Megajoule (LMJ) in Bordeaux (South-West of France).
The researches in France
The knowledge of these plasmas, whatever they are natural in space or artificially-made on earth, the associated physics and the technologies allowing the scientists to create, to observe and to control them originates from advanced researches from plasmas physics, astrophysics or technology labs held in universities and engineer schools, public labs such as the Centre National de la Recherche Scientifique (CNRS), the Commissariat à l’Energie atomique et aux Energies Alternatives (CEA), the Office National des Etudes et Recherches Aeronautiques (ONERA) and private companies.
France gathers also an important research community on natural plasmas such as plasmas met in our solar system and earth magnetosphere. The development and the scientific analysis of major satellite missions, present (SOHO, CLUSTER, ACE, …) or future (Solar Orbiter, MMS – Magnetosphere Multiscale, TARANIS, …) include all the characteristic scales and their evolution, from close ionosphere up to sun environment, allowing to better understand the space meteorology which is the « applied science» component of this topic.
The scientific community involved in cold plasmas in France is concerned with basic study of discharges and plasmas, their interaction with gases, liquids and solids, for a large range of applications including energy (plasma assisted combustion, energy conversion), environment (depollution), surface treatment and material functionalization, processes, aeronautics and aerospatial industry, and, more recently, biological and medical applications.
France is very active on major research programs devoted to thermonuclear research for magnetic confinement fusion. Researches built within CEA, CNRS, universities and engineer schools during the sixties with specifically the tokamak located within the CEA site at Fontenay aux Roses (Paris area) and then since the late eighties the tokamak Tore-Supra featuring very long plasma discharges confined by using superconducting coils, located within the CEA site in Cadarache (South-East of France). This tokamak lately renamed WEST is close to the future tokamak ITER, under construction, a large international project conducted in partnership by European Union, Japan, China, South Korea, Russia, USA and India.
France is very active as well on the other thermonuclear research program, using inertial confinement fusion, with as its centerpiece the MegaJoule (LMJ) laser now under construction close to Bordeaux. The researches closely couple CEA, CNRS, universities and engineer schools around hot and dense plasmas whose characteristics moke those of the cores of stars. Closely associated because of the specific skills which are involved, the researchers are also very much involved in the domain of ultrahigh laser intensities. France is home to the PETAL laser coupled with the LMJ laser for radiographic diagnosis of plasmas created by the LMJ laser. France will be soon home for a large facility, namely the Apollon laser, under construction on the Saclay plateau. Its power never achieved before will give rise to generation of radiation or charged particles with strong current making then possible new basic research thematic but applied thematic as well for electron and ion acceleration, for diagnosing the dense matter, etc.
In many regards, the physics of charged particle beams is very close of the neutral plasmas and the technologies of the particle accelerators shared with plasma generators: vacuum, cryogenics, superconductivity, magnetism, radiofrequency cavities, diagnostics. CNRS or CEA laboratories are particularly active in the large national, European and international and engineers cooperate to serve various public or industrial researches: particle physics, nuclear, materials, biology, chemistry, medicine, environment, archeology, art….
These programs are widely open to the European and global scientific community. Start-up and operation of these large equipments for fusion which is intended to continue for over two decades, requires the development of a broad enough scientific highly skilled community, educated at master level and doctorate level.
Due to the major investment they need, the specific research devoted to fusion is addressed within large-scale networks: national- (EURATOM-CEA association), European- (European Fusion Development Agreement organization for JET tokamak) and world-scale (ITER, International Fusion Energy Organization) for magnetic confinement fusion and national- (Institut Laser Plasmas) for inertial confinement fusion.