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Accueil > Grand Public > LPP studies all aspects of plasma physics and technology from space to the laboratory, from theory to experiments

LPP studies all aspects of plasma physics and technology from space to the laboratory, from theory to experiments

 The plasma state - a distinct state of matter

A plasma is a hot ionized gas with a significant number of free electrically charged particles. These free charges of ions and electrons makes the plasma strongly influenced by electric and magnetic fields in contrast to neutral gases. Both internal and external fields induce therefore strong collective behaviors of the plasma which can span over large distances.
For example, electrons are significantly more mobile than ions (due to the large mass ratio) and causes localized charge separation within the plasma. This again creates internal electric and magnetic fields where the corresponding forces on the charged particles impose characteristic fluctuations and oscillations in the plasma.
The type of atoms and molecules, the ratio of ionised to neutral particles, the particle energies, and the external "electromagnetic" environment all results in a broad spectrum of plasmas of different nature.
More than 99% of the visible universe are plasmas of different kinds, but the plasma state is rarely a natural phenomenon here on the surface of the earth. However, we are able to create plasmas on earth, with unique qualities that have become vital for a number of applications important to our lives and to the world around us.
The figure below shows a classification of plasmas ordered by their energy (energy/temperature of their electrons) and the density of charged particles (or ionization rate ?)...

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Classification of natural and laboratory plasmas in a logarithmic diagram charges density/temperature

For simplicity plasma physics can be classified into three main sections :

- Space plasmas

- Industrial plasmas

- Fusion plasmas

However, the general questions are similar (the collective behavior, particle and energy transport, interaction between two plasmas of different nature....). Our research is therefore many times addressed to both the natural world and the commercial world at the same time.

 The plasma universe - space plasma

In our solar system, the Sun, the interplanetary medium, the magnetospheres and the ionospheres of the Earth and other planets, as well as the ionospheres of comets and certain planetary moons all consist of plasmas. Very far away, the stars, the stellar and extragalactic jets, and the interstellar medium are examples of astrophysical plasmas.

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M16 eagle nebula. Hubble telescope photography (NASA, 2-11-1995)

So basically almost all visible matter above about 100 km altitude from the earth (within and above the ionosphere) are plasmas of different nature, and constitute more than 99% of the visible universe. (The term visible is important, as roughly 90% of the mass of the universe is thought to be contained in "dark matter," the composition and state of which are unknown).

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Storm and archways of the solar corona-Astronomy picture of the day (NASA,15-11-2000)

The plasmas in space are extremely tenuous and hot, with densities dramatically lower and temperatures significantly higher than those achieved in the laboratory. But even if the density is "low" compared to the laboratory plasmas, the ionization degree can vary from fully ionized plasmas to just a few percent. One of the notable characteristics of space plasma is its tendency to form sharp boundaries between plasmas with different properties. These sharp boundaries can have profound astrophysical implications such as generating electric fields in space and providing sources of energy for driving electric currents over very large distances. In addition, all plasmas in space are penetrated by magnetic fields that influence their physical properties in various and often dramatic ways. In some cases the magnetic force is locally much greater than the gravitational force, in other cases the conductivity of the plasma is so high that the magnetic field is "frozen" in the plasma. One example of the complexity of space plasmas is the solar wind and its influence for us here on earth.

 The Sun Earth connection

The sun emits a high conducting plasma at supersonic speeds of about 500 km/s into the interplanetary space. (Considering the gravitation of the sun this acceleration is quite astonishing and still not fully understood). This plasma is called the solar wind and consist mainly of protons and electrons with a few percent of helium ions. Because of the high conductivity the solar magnetic field is frozen to the plasma and drawn outwards with the expanding solar wind.
When the solar wind (with its interplanetary magnetic field) hits the earth’s dipolar magnetic field it can not simply penetrate it so it deflects around it. Since the solar wind hits the "obstacle" with supersonic speed, a bow shock wave is generated. The solar wind is slowed down in this process and a substantial fraction of its particle kinetic energy is transferred into thermal energy. The kinetic pressure of the solar wind plasma distorts the outer part of the terrestrial magnetic field. On the day side it compresses the field while on the night side the field is stretched out into a long magnetotail which reaches far beyond the lunar orbit. The plasma in the magnetosphere (see figure) is not evenly distributed, but grouped into different regions with plasmas of different densities and temperature.

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Scheme of the interaction between the solar wind and the terrestrial magnetic field

 Cold plasmas

Cold plasmas are partially ionized gas in which neutrals and ions have a low temperature (10^2-10^3 K) whereas electrons can reach very high temperatures (10^4-10^5 K). These plasmas are therefore out of thermodynamic equilibrium. The very high-energy electrons are responsible for the generation of chemically reactive ions and radicals, which allow the use of cold plasmas at atmospheric pressure for the air and water cleaning (see picture), or for the sterilization with a low energetic cost. Concerning the low-pressure applications, the plasma uniformity over large volumes allows using them for lighting, surface treatments or for etching and deposit in micro-electronic, and more recently for space propulsion. Thus, cold plasmas concern engineering and contain rich and complex physics, which make it an interesting and important subject for both industries and researchers.

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Filamentary discharge in liquid medium from Paul Ceccato (full extent of the image is 3cm)

 Fusion plasmas

One of the greatest challenges in the 21st century involving plasma physics aim at recreating the fusion reaction which occurs in the centre of the sun, in a confined chamber her on earth, to produce electrical energy : this project or experiment is called ITER. The most likely fusion reaction to occur, written just below, consists in the fusion of deuterium and tritium to form a helium nucleus (an alpha particle), and a high-energy neutron. The products being lighter than the reagents, this reaction occurs with an important burst of energy of 17.6 MeV mainly contained by the neutron. The access energy leaves the plasma, and serves to heat up water that goes into a turbine to produce electrical energy.
1D2 + 1T3→2He4+0n1 + 17.6 MeV

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Artist view of ITER reactor

To realize this fusion reaction, one needs to fulfil one criterion that depends on 3 parameters :
-  Temperature T. The colliding particles must have a sufficient energy to overcome the “Coulombian” potential close to the nucleis. These atoms reach typically a temperature around 200 millions of Kelvins and at this temperature, the gas is fully ionized.
-  The density n. The colliding particles must have a sufficient density so that the reaction taking place produces enough energy to be self-sustained.
-  Confinement time te. The plasma must be sufficiently confined to minimize energy losses at the wall. Indeed, as no materials can withstand this temperature, plasma is magnetically confined : Coils (in red) are creating a powerful toroïdal magnetic field, which appears on the figure with the field lines in blue. Electrons and ions trajectories wind around these lines, confining the plasma in the centre of the reactor. However, turbulence and other mechanisms can destroy the confinement and allow the plasma to reache the reactor walls. Confinement time measures the plasma diffusion time to the walls.

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Magnetic field streamlines

Lawson have shown that the product of these 3 parameters must reach a certain level so that the fusion reaction can produce electrical energy.
ITER will the first machine with a tokamak configuration which will be able to reach this criterion.
Nevertheless, several aspects are still in development to realize this project :

-  Control of fusion reaction, particularly self-sustained reaction

-  Massive production of tritium

-  Find a material which can resist to high neutron flux for confinement walls

Since the russian works at the end of the 50’s, the magnetic compression (so-called Z-pinch) is a rich and exciting topic. It has stimulated fundamental studies on plasma heating by a pulsed B field and on instabilities, or the development of MHD codes as well as pulsed power science and technologies. Namely, the Z-pinches are renowned sources of copious hard radiation having multiple applications.

 Pulsed hot plasmas

Having been considered originally as a way to inertial confinement fusion, then outclassed by magnetic confinement and laser, the late 90’s saw a renaissance of the Z-pinches with successful shots using multiwire array loads on the Z-machine at the Sandia National Laboratories ( USA). Today, this physics is under study on large scale terawatt machines in USA (ZR, Zebra), Russia (GIT, S300), UK (MAGPIE) and France (SPHINX).

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Schematic implosion of a multiwire array load. Left to right : (i) initial configuration, the current induces an azimuthal B field then a centripetal Lorentz force, (ii) implosion, the ionized matter is heated and accelerated onto axis (iii) stagnation, prolonged plasma heating and emission of X-ray bursts (from SNL documents)

Many domains can be addressed by these plasmas :

- Astrophysics and space physics (opacity, equation of state, spectroscopy, instability of supernovae, thermonuclear reactions).

- Hydrodynamics (shocks) and condensed matter (equation of state, opacity, hardening).

- Plasma physics (Z-pinches stability, instability and turbulence, heat transfer).

- Radiation sources (continuous and line spectra, uncoherent and coherent X-rays).

CNRS Ecole Polytechnique Sorbonne Université Université Paris-Saclay Observatoire de Paris
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Exploitant du site : Laboratoire de Physique des Plasmas, Ecole Polytechnique route de Saclay F-91128 PALAISEAU CEDEX
Hébergeur : Laboratoire de Physique des Plasmas, Ecole Polytechnique route de Saclay F-91128 PALAISEAU CEDEX
Directeur de la publication : Dominique Fontaine (Directrice)