The production of an abundant and clean energy is one of the grail of physics and modern technology. Among the candidates identified, nuclear fusion is among the favorites. After several decades of efforts, scientists are able to overcome one by one the obstacles they face in achieving control this form of energy. Two developments in this area have been announced in recent days.
The reactions of nuclear fusion
The nuclear fusion reactions are those that take place inside stars. In this process, nuclei of light atoms fuse to form heavier atoms. The reaction product, the same mass of fuel, 4 to 5 times more energy than fission reactions that are used in existing nuclear power plants. However, this nuclear energy is much more difficult to master.
Fusion reactions occur at temperatures of several tens of millions of degrees. In these circumstances, the matter is in the form of a plasma. The first challenge is therefore confined to this set of highly energetic charged particles. The second is to obtain a density very high regard for pushing the nuclei of atoms, which naturally repel the effect of electrical forces to meet and merge. In the case of a star, obtaining high temperatures and high densities is carried out simultaneously by the gravitational collapse of the star under its own weight. Fusion reactions occur when the energy required to offset the collapse and ensure stability of the star. At least until the total use of fuels which causes the death of the star.
Fission and fusion
To reproduce the nuclear fusion on Earth, we must succeed in maintaining a state controlled to a certain quantity of matter at temperatures and densities very high to cause fusion reactions and especially sustain the process to provide energy continuously. This condition is essential and is a difference between the reactions of fusion and fission.
The fission reactions occur in string. An neutron posted a solid nucleus destabilizes its energy causing fission into two nuclei of mass less and producing neutrons which in turn will cause the fission of nuclei. The reaction self-sustaining, continues as the fuel is available. This presents a major advantage to ensure the continued production of energy or when trying to make a very powerful bomb. Advantage can quickly become a disadvantage if the reaction gets carried away and causes the complete fusion reactor and radioactive pollution that ensues as happened at Chernobyl in 1986. The risk of runaway reactions is not the case in the process of nuclear fusion as soon as the temperature and / or density of matter are not met, the reaction stops.
The issue of energy production by fission is therefore to prevent the risk of runaway reaction while the production of fusion energy is to keep the system under conditions permitting the realization of reactions. The fission process is simple to implement, but risky when the merger is a complex process but does not present similar risks. Fusion has also other advantages over fission. The proposed reagents, deuterium and tritium (heavy isotope of hydrogen) are relatively abundant. Chemically equivalent to hydrogen, deuterium replaces in molecules of up to 0.015%. This may seem low but the abundance of hydrogen-water molecule contains two atoms to one oxygen atom, provides abundance of deuterium (it should however be extracted molecules). If tritium is different. This is a radioactive element with a period of very short life of just over 12 years. Thus there is only very small quantities naturally but its synthesis is under control for several decades. Combustible materials for nuclear fusion are far more abundant than those available for fission, that is to say uranium.
The other major advantage of fusion over fission of waste products. The reactions of nuclear fission produces radioactive waste whose lifetimes are very long, the order of several hundreds of thousands of years. The problem with these waste is not so much of their dangerousness as the difficulty to store securely over periods as long. What are the geological terrain that will remain stable for the next 200,000 years? How to ensure transmission of the memory of those storages? Simply put: what should we write about the waste to be sure of being understood in 100,000 years? In the case of fusion, radioactive waste, in much smaller amounts, have lifetimes shorter of the order of hundred years.
Control of energy production by nuclear fusion presents so many advantages: the existence of relatively abundant fuel, a very high ratio between the amount of fuel required and the energy produced a quantity of hazardous waste relatively low. However, the key processes occurring inside stars and to start the fusion reaction-the-gravitational collapse can not be reproduced on Earth. To create the conditions for nuclear fusion, two processes have been investigated: magnetic confinement and inertial confinement.
The method of magnetic confinement of plasma is one that comes closest to what happens inside stars. Fusion reactions will take place continuously. They are thus hundreds of millions of tons of hydrogen are consumed every second by the sun. The method of magnetic confinement is intended to control a “burning plasma” . This is to maintain the plasma in the temperature and density that can maintain the fusion reactions permanently. In this device, energy is produced continuously, provided that it incorporates into the plasma fuel as and as it is consumed. The plasma is controlled in a chamber of magnetic confinement of toroidal called “tokamak”. Magnets located around the room and create within it a toroidal magnetic field that keeps the plasma at a safe distance from the surface of the tokamak. We must indeed avoid contact between the chamber and the plasma since the temperature of the latter.
Even if it is several decades of containment experiments take place, many challenges remain to be surpassed. These problems are theoretical in nature as regards the behavior of hot plasmas, in particular instabilities that must be understood and modeled. But they are also technical difficulties in the field of material sciences for the achievement of tokamaks. The proposed International Thermonuclear Experimental Reactor (ITER) , mentioned in the note on the 2011 budget, which is currently under construction in Cadarache, is based on the principle of the tokamak.
A new method of magnetic confinement
Researchers at the Massachusetts Institute of Technology (MIT) have recently proposed a new method of containment. The Levitation Dipole Experiment (LDX)  is to maintain levitation, the center of a room, a magnet shaped ring. This magnet produces a toroidal field which can confine a plasma within the chamber. Thus, while the traditional magnetic confinement to control the plasma inside the tokamak, and therefore the magnets, the magnetic levitation method to control the plasma outside the magnet. They talk about reverse containment. The creation of this new device is based on the results of astronomical research on the magnetosphere. The magnetic field created by the planets is a toroidal field. This field can therefore confine charged particles as happens in Earth’s magnetosphere in regions called the Van Allen belts.
The discovery made recently on the random turbulence observed in plasma. They normally have the effect of dispersing the particles in the plasma. However, it was observed that the plasma confined by this new method could also be densified by the turbulence. The local contraction by turbulence was observed in the area but had never been reproduced in the laboratory before. Knowing the importance of increasing the density of plasma to induce the nuclear fusion reactions, this discovery could lead to benefits for the realization of magnetic confinement fusion.
However, the LDX is designed first to reproduce the magnetic fields created by planets. It is primarily a device for studying magnetospheres. The experiment carried opens new perspectives in the production of energy by fusion but requires many developments to guide it in this way. The LDX is however a vital tool to develop knowledge about plasmas and their containment and to provide training for researchers, resources needed for further research on fusion.
Inertial confinement: Laser fusion
It is important to note first that the development of laser equipment used in the experiments of inertial confinement fusion is primarily linked to the imperatives of defense and nuclear weapons. The cessation of nuclear testing is a fundamental problem for the defense: how to ensure that the nuclear arsenal remains operational without conducting tests? This point is crucial because it was on this assurance on the effectiveness of the weapon is based on the principle of nuclear deterrence.
To abandon nuclear testing, it must develop alternative means of experimentation. This has been done in France by the creation of the simulation program . This program includes a first component of numerical simulation to understand and analyze the phenomena involved in the explosion of a nuclear weapon. However, the models used need to be confronted with a form of experimentation. The second component is thus an experimental part in which the laser facilities play an essential role.
These facilities, the National Ignition Facility (NIF)  in the United States and MegaJoule Laser (LMJ)  in France, can reproduce the thermodynamic conditions under which reactions occur during the explosion of a nuclear weapon. These conditions are also those needed for nuclear fusion to produce energy, these plants naturally contribute to research in this field using the method of inertial confinement laser. In this device, tens of laser beams are sending a very short pulse and high energy on a solid target of two millimeters in diameter. Absorption of laser energy by the target causes the expansion of the outer layers and, in response, compression of the internal layers. The density and temperature within the target, composed of deuterium and tritium, a sharp increase would produce such a hot spot then start fusion reactions.
The device required is huge compared with the size of the target. The NIF building covers the area of three football fields. The major part is dedicated lines that can produce the 192 laser beams. The beams converge into a sphere 10 meters in diameter in which the target is placed in a gold cylinder, called Hohlraum, a few millimeters in length only.
NIF researchers recently announced that the initial conditions for ignition could be met by the end of the year is only a few months after the commissioning of the instrument. This breakthrough was possible by developing a technique for measuring the Plasma Science and Fusion Center (PSFC) at MIT . This technique, the “Merger backlighting method”, proposed in 2008, has been used successfully to measure the electromagnetic properties reigning around the target at the University of Rochester. This experience allows us to understand the conditions occurring within the Hohlraum and improve the mechanism to achieve ignition.
It should be noted here that the term ignition has different meanings depending on whether one speaks of magnetic or inertial confinement. In the first case, ignition is achieved when the energy emitted by the plasma in fusion reached the level of energy required to start and sustain the reaction. Beyond the ignition, the reactor produces so much energy it consumes. In the second case, the ignition is when nuclear reactions begin within the target.
The problems to solve in order to produce energy by inertial confinement are mainly related to the difficulty of obtaining the initial conditions of temperature and density. The compression and simultaneous warming of the target by the laser pulse has initial difficulties. It does not seem possible to achieve levels of density and temperature just high enough compression. An alternative, preferred to NIF and LMJ, is to follow the laser pulse compression by another impetus to raise the temperature. This is called Rapid Start. We must also get at the hot spot temperature and density sufficient for the reaction self-sustaining and consumes all of the target to get the best performance.
The main obstacle to overcome in the case of nuclear fusion by inertial confinement for the continuous production of energy. The burning of a target takes a few tens of picoseconds. Energy is produced by discretely. To be able to produce energy continuously, it must be able to repeat the merge many hundreds or even thousands of times per second. We must therefore ensure achieving this rate despite the extreme accuracy required, including the positioning of the target.
Energy production by nuclear fusion is a major issue internationally. Even if the economic returns are not expected until the second half of the century, major scientific powers spend budgets for the implementation of instruments that either nationally with laser facilities or internationally including through the ITER project . If multiple pathways may lead to the realization of fusion reactor, it is still too early to know which lead is to say, offer a return to industrial production of energy. Different methods are therefore in a process of emulation rather than competition. They all have obstacles still to overcome and require theoretical advances as well as technical.
In addition to this competition, the production of this form of nuclear power is competitive with all forms of renewable energy. The recovery of solar energy for example, could steal the limelight from nuclear fusion, if its performance could be significantly improved in the coming decades. What looks like the Holy Grail? Nobody knows yet.
The author thanks Dr. James Van Dam, director of the Institute for Fusion Studies (IFS), University of Texas at Austin , for his insights into the subject.
 The site of the national organization of researchers working on the properties of plasmas in fusion: http://burningplasma.org/home.html
-  The ITER site: http://www.iter.org/default.aspx
-  The site LDX: http://www.psfc.mit.edu/ldx/
-  The simulation program site: http://www-lmj.cea.fr/fr/programme_simulation/index.htm
-  The site of the National Ignition Facility: https: / / lasers.llnl.gov/
-  The website of Laser MegaJoule http://www-lmj.cea.fr/index.htm
-  The site of the PSFC: http://www.psfc.mit.edu/
-  The site of the IFS: http://hagar.ph.utexas.edu/ifs/
- Levitating Magnet brings space physics to fusion – MIT News – David L. Chandler – 25/01/2010 — http://web.mit.edu/newsoffice/2010/fusion-ldx-0125.html
- Peering inside an artificial sun – MIT News – David L. Chandler – 27/01/2010 — http://web.mit.edu/newsoffice/2010/plasma-science.html
|Category: Nanotechnology, physics||Tags: electrical forces, Nuclear Fusion, nuclear power plants, plasma, radioactive pollution|