Nuclear Fusion

Fitting one of a total of 4,320 laser glass plates in a support. The high purity SCHOTT material facilitates the release of the enormous amounts of energy required for nuclear fusion.
Dr. Manfred Weber, Science journalist
Frankfurt/Main, Germany

A Real Mega Project

A research facility is currently being set up in the south of France near Bordeaux that is heralding in a new era in laser technology. SCHOTT is supplying the laser glass and other important optical components.

The amount of energy supplied by a million power stations all at once. That is what the French Laser Mégajoule (LMJ) will require – for a few billionths of a second – when it goes into service for the first time in 2008. Together with a comparable facility in the USA, the LMJ will be the most powerful laser in the world.

The project carries its name because of the energy of almost two megajoules that the laser will deliver to a so-called pellet. This tiny ball of about 2 millimeters contains a mixture of deuterium and tritium, both heavier relatives of hydrogen. Under the impact of the 0,35 micron laser radiation, a fusion reaction will take place producing excess energy, about ten times as much as the laser energy needed to ignite the fuel. It creates conditions similar to those in the interior of the sun for a few fractions of a second – long enough to give researchers a rich field of operation.

Gigantic Dimensions

Diagram of the Megajoule facility which will be installed in a 300 meter long building.
But not only the laser energy is in the “mega” range. The LMJ building will be 300 meters long and 150 meters wide. Each of the 240 beams is produced using 18 slabs of laser glass, requiring a total of 4,320 pieces, each measuring 45 by 80 centimeters. Thus, an enormous amount of material has to be manufactured and processed. That makes 150 tonnes of laser glass altogether. Equally imposing are 35 tonnes of fused silica and 66 tonnes of glass for mirrors plus the 4,000 square meters of flat glass surfaces that have to be polished.

Even though 2008 seems to be a distant future, the project is proceeding full force. Already by the end of this year, a first test facility will begin operating. The LIL (Ligne d’integration laser – integrated laser line) contains a package of eight laser beams and puts all materials to the test. “A crucial step towards the LMJ”, say Evelyne Le Page and Michel Le Guennec, “which is very important for us”. The small team of SCHOTT France has been with the LMJ project since its conception in 1996. They work on the outskirts of Paris and are in constant contact with the customer, the CEA (Commissariat à l’énergie atomique – the French atomic energy authority). “This involves supplying more than the laser glass,” comments Le Page, naming the first transport mirror as an example. “Also we offer interesting alternatives all along the beam route with our filter lenses and absorption and reflector glass. If there should be any tests where other SCHOTT products could provide a solution we will be ready and willing to give our advice and support.”

Interesting Field of Research

The heart of the Megalaser: the target chamber where all the beams meet on the fuel capsule known as a “pellet”.
If SCHOTT LITHOTEC beats the competition, another SCHOTT component could be used at the end of the beam path, namely a deflection and focusing system comprised of fused silica grids. “The CEA has chosen a new approach here for directing the beam to the target”, explains Le Guennec. “While the American design favors lenses, the LMJ prefers to keep grids.” These do two things at the same time. They focus the beam from its original 40 by 40 centimeter size down to fractions of a millimeter. At the same time they aim it at the tiny pellet that is positioned with an accuracy of a twentieth of a millimeter – in a target chamber some ten meters in diameter. The same precision is called for here as in lithography. Grids are in fact normally used there to deflect short-wave radiation. This produces tiny structures on silicon chips to make computers faster and faster.

For a 1.3 billion euro price tag, people justifiably want to know what the purpose of the facility is. The purposes are at least as manifold as the number of scientific experiments scheduled to be carried out there. More than a hundred a year are planned. They are expected to provide insight into thermonuclear plasmas so that the behavior of stars can be better understood. The dream of fusion as a virtually inexhaustible source of energy is getting a good deal closer to reality. And, furthermore thanks to the LMJ, France will be able to do without atomic weapon tests in the future without adverse effects on the safety and maintenance of existing stocks.

SCHOTT is devoting a large part of its activities in France to the LMJ. Quite apart from laser technology, initial spin-offs in such areas as coating technology and satellite optics have been formed. Still, several hurdles have to be taken before the first laser pulses are emitted. But that is not surprising with this mega project?