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Laser fusion achieves prototype milestone

28 Aug 2002

Huge sums are being invested in laser-fusion research at laboratories in France and the US. Jacqueline Hewett looks into the progress made so far by the French team.

From Opto & Laser Europe September 2002

More than € 5 bn of funding is to be ploughed into the world's two high-power laser facilities for nuclear fusion. In the US, the National Ignition Facility (NIF) project is set to receive almost € 3 bn of public money, while the Laser Mégajoule (LMJ) project in France will come in at a cost of some € 2.1 bn. Both ventures are scheduled for completion at the end of the decade. So how are these cash injections being spent in the meantime?

The two projects are striving towards the same goal: achieving inertial confinement fusion. Scientists at NIF and LMJ are building lasers to generate 1.8 MJ energy pulses. When fired at a gold target, these pulses produce X-rays that compress and then fuse a deuterium and tritium pellet, generating a surplus of energy. The process, however, is complex: it involves the use of more than 30,000 optical components.

Work in progress At the Lawrence Livermore National Laboratory, the building that will host NIF is starting to take shape. Meanwhile, on the outskirts of Bordeaux, researchers at the French Atomic Energy Authority (CEA) are building a small-scale prototype of LMJ, called the Ligne d'Intégration Laser (LIL).

Once completed in 2009, the LMJ will form the cornerstone of France's nuclear weapons simulation scheme (the decision to launch the programme was taken in 1995 and France stopped actively testing nuclear weapons in 1996). Didier Besnard is the programme director. He told OLE: "LIL is a prototype laser system. It has only eight beams compared with the 240 for the Mégajoule laser." He added: "Even if we cannot obtain gain at LIL, we can do lots of physics already and prepare for the experiments on the LMJ."

The construction of LIL is a major step towards the successful completion of LMJ. "What's very difficult with the LMJ laser is not so much the underlying technology, but the very stringent specifications you have to meet," said Besnard. "You push the technology to the extreme for each component of the laser system. That is why we decided to build LIL." After six years in development, the prototype system is now operational with four beams.

All of the fundamental components of the LIL laser chain were in place by the end of 2001, producing four beams at a wavelength of 1.06 µm. Besnard and his colleagues achieved "first light" at the facility in March this year. "LIL is now helping us to validate the technological choices we have made for LMJ. By the end of 2002, we expect this validation to be complete," he said.

The next step is converting the near-infrared beam to 350 nm, as the ultraviolet wavelength generates fusion-inducing X-rays much more efficiently.

"We are now looking at the laser beam itself, how it behaves, its time and spatial shapes, and how it is aligned," Besnard told OLE. "We estimate that the 350 nm regime will be available by the end of 2003 at an energy level of 30 kJ." The first experiments with the LIL will take place in its 3 m diameter target chamber once this regime has been achieved. The remaining four beams will be installed by the end of 2005, which will increase the pulse energy to 60 kJ.

The completion of the validation process coincides with the start of the construction phase of LMJ. Work will begin next year to erect the building that will house the entire LMJ system. This will include laser lines scaled up from 130 m in LIL to 290 m; a 40 m high experimental hall; and a target chamber with a 10 m diameter.

Although both the NIF and the LMJ teams are aiming to produce 1.8 MJ pulses, the two groups have slightly different approaches. NIF will be using 192 beams, whereas LMJ is to use 240 beams to do the same job. But although the number of beams used may be different, the amplification processes are similar and the teams will collaborate on the production of their optics.

All beams at NIF and LMJ will start with a small source known as a pilot. At NIF, the pilot is a nanojoule energy pulse from a diode-pumped fibre laser. It will supply pulses of 1-20 ns duration. Besnard says that the LMJ pilot is yet to be decided: "Our current pilot is also a fibre laser, but what we will decide for the LMJ depends on the results we obtain on the LIL." The pilot beam will carry a pulse with the correct temporal and spatial shape into the next phase - amplification.

First, a preamplifier boosts the pulse energy to a few joules before it is passed into the main amplifier. The amplifier stage must increase the input pulse energy to the necessary level, while maintaining the beam's spatial, spectral and temporal characteristics. Both teams plan to use neodymium-doped glass for this, and are collaborating with two firms - Germany-based Schott and Japan-based Hoya - on production. At NIF alone, 3072 such slabs will be required.

To minimize the size of the laser chain and the building housing it, the beam will be reflected to take four passes through the amplifiers. At this stage, the beam size will also be expanded using 40 x 40 cm optics.

Pockels cells based on potassium dihydrogen (KDP) will sit in the amplifiers. These will "clean" the beams and prevent any stray wavelengths from entering the chain before reaching the final stage - frequency conversion. Huge KDP crystals will then convert the original 1.06 µm wavelength to 350 nm, before each beam is directed into the target chamber. Having passed through a building some 290 m long, all the laser energy will be directed towards a spherical pellet just 1 mm in diameter.

Once inside the 10 m target chamber, LMJ's 240 beams will be focused onto a 1 cm long gold cylinder holding the pellet. This cylinder converts the laser pulses into X-rays, acting like an X-ray oven.

Contract and implode "The beams are absorbed by the internal walls of the holder [called a holraum] and are converted into X-rays. This is a smooth way of depositing energy onto the outer part of the pellet," explained Besnard. Initially, the outer shell of the pellet is vaporized. The rest of the pellet, containing the deuterium and tritium, contracts and reaches the right conditions for fusion ignition. "Imagine the laser and pellet as a coupled system," said Besnard. "You have to balance the laser system and the pellet specifications to smoothly deposit over the whole pellet surface, or it will not implode."

The deuterium and tritium isotopes (DTIs) are made at 20 K. The pellet, which contains just 300 µm of DTI material, is compressed to some 1000 gcm-3 and held at a temperature close to absolute zero until the laser pulse arrives. "Then the DTIs go up tens of millions of degrees centigrade," said Besnard. "This is an exciting and beautiful project, not just in terms of the laser technology but also the work behind the pellets."

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