IN DEPTH

Positioning a target in the reaction chamber of Mégajoule, France’s new €3 billion laser fusion laboratory.

PHYSICS Laser fusion, with a difference on January 9, 2015 Europe’s Laser Mégajoule project blazes its own trail toward nuclear ignition

By Daniel Clery, in Le Barp, Aquitaine, France if needed without having to test them. The LMJ’s project leader. But generating power French facility, like its U.S. counterpart, will from laser fusion will be left to outside aca- o anyone familiar with laser fusion also pursue a sideline in inertial fusion en- demic researchers, and they won’t get their

research, a visit to Laser Mégajoule ergy (IFE) research: crushing capsules of hands on the machine for another 2 years. www.sciencemag.org (LMJ), a €3 billion research facility hydrogen isotopes with laser pulses so that When they do, some key design differences completed late last year near France’s the isotopes fuse into helium, releasing vast may give LMJ a better chance of triggering Atlantic coast, triggers instant déjà stores of energy that might one day be har- ignition than NIF has. vu. The site is a dead ringer for the nessed in a power plant. At their core, the two facilities are near- Tworld’s leading lab, the National Ignition But in a major departure from NIF’s ini- twins. Just as at NIF, LMJ researchers use Facility (NIF) in California. LMJ has the tial approach, LMJ is putting top-secret a to produce a pulse of same stadium-sized building, the same weapons research first. Once NIF was com- light that lasts a few billionths of a sec-

shiny white metal framework, ond, with just billionths of a joule Downloaded from the same square beam tubes and of energy. This weak pulse then 10-meter-wide reaction chamber. passes into preamplifiers, slabs of The coffee is richer than NIF’s, the “The target of ignition drives the design -doped glass that are security less obtrusive, the visitors’ of the machine.” pumped full of energy by xenon area larger and more informative. flash lamps just before the pulse Overall, however, walking through Pierre Vivini, LMJ project leader comes through. They dump that LMJ’s doors is like stepping into a energy into the beam, boosting it parallel universe in which Lawrence Liver- plete in 2009, Livermore researchers im- to about a joule, before the light is split into more National Laboratory, the U.S. nuclear mediately embarked on a crash program many parallel beams and sent to the main weapons laboratory that runs NIF, has to achieve ignition—generating a self- amplifiers (the same neodymium glass and somehow come under French control. sustaining fusion reaction that produces flash lamps, only bigger). The similarities are no coincidence. Both as much energy as went into triggering it. LMJ has 22 main amplifier chains, ar- sites were designed for the same purpose— They failed to reach that goal and have since ranged in four vast halls around the build- to train scores of powerful laser beams on changed their approach (Science, 21 Septem- ing, and each amplifier accommodates a single target, subjecting it, for an instant, ber 2012, p. 1444). eight parallel beams at a time. During a to outlandish extremes of temperature and France’s Alternative Energy and Atomic laser shot, the eight beams are bounced pressure. The two labs have collaborated Energy Commission (CEA), which built back and forth through the amplifier four extensively, and the primary mission of LMJ, also wants to achieve ignition, be- times to multiply their energy by a factor of each is military: replicating nuclear explo- cause it’s key to both weapons research and 20,000. An elaborate array of mirrors will sions in miniature so that weapons scien- energy. “The target of ignition drives the direct the 176 beams around all sides of the

PHOTO: PATRICK LANDMANN/SCIENCE SOURCE LANDMANN/SCIENCE PATRICK PHOTO: tists can ensure their bombs will detonate design of the machine,” says Pierre Vivini, spherical reaction chamber; then a final set

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Published by AAAS NEWS | IN DEPTH of optics will convert the beams from infra- The hope is that using PETAL in this way tion and pioneered at the Osaka University red to (UV) light and focus them will allow IFE researchers to avoid some in Japan, is to use a short, high-powered to a needle-sharp point at the center of the of the pitfalls that have hobbled NIF. The laser pulse as the spark—the sort of pulse chamber. Recombining all the beams deliv- key element of any IFE scheme is the fuel that PETAL can produce. Last decade, Eu- ers 1.5 megajoules of energy into the target capsule, a plastic sphere about the size of ropean IFE researchers proposed building at the center of the chamber—roughly the a peppercorn containing frozen a demonstration IFE reactor based on fast same as the kinetic energy of a 2-tonne and —isotopes of hydrogen that are ignition, called HiPER. That plan lost some truck traveling at 140 kilometers per hour. the fuel of fusion. Placed at the center of the momentum when NIF fell far short of igni- NIF’s laser delivers 1.8 megajoules. reaction chamber, the plastic of the capsule tion, but its proponents hope LMJ-PETAL Because of funding constraints, only one is vaporized by the intense heating from will give it new impetus. main amplifier chain is now online. But that the laser pulse, causing an implosion that Recent experiments on lower powered is enough to get started on nuclear weap- crushes the fuel to 100 times the density of machines have suggested that PETAL might ons research, says François Geleznikoff, di- lead and heats it to 100 million K—which not pack enough punch to trigger fast ig- rector of nuclear weapons at CEA: “With should be sufficient for fusion to ignite. nition. But another alternative approach, eight beams we can do good physics. We At NIF and in the weapons research ex- pioneered at the University of Roches- don’t need all the beams to study weapons.” periments at LMJ, researchers trigger the ter in New York, might save the day. The The facility will add at least another two implosion indirectly by enclosing the cap- technique, known as shock ignition, com- chains (16 beams) each year until it reaches sule in a metal can that is heated by the presses the fuel capsule with a laser pulse full capacity sometime in the next 10 years. laser and in turn bombards the capsule from the main laser as in other techniques. Well before then, academic IFE research- with x-rays. That approach offers some But at the end of the pulse, the laser adds ers from across Europe a sudden spike of power to pro- have been promised at least duce a shock wave converging 20% of the machine’s time. on the center of the fuel. When “With 50 shots per year we the shock hits the center, the can really develop a seri- sudden hike in pressure sparks ous program,” says the reaction. “Experiments at physicist Dimitri Batani of Omega [Rochester’s laser] and the University of Bordeaux elsewhere are encouraging, in France. and the laser requirements Their game plan includes [for shock ignition] look rather some significant deviations more benign than fast ignition from NIF’s strategy. For ex- at this stage,” says Chris Ed- ample, they’ve won funding wards, a fusion researcher at for a separate laser to be the at the installed at LMJ, provid- United Kingdom’s Rutherford ing pulses much shorter Appleton Laboratory and one and more powerful than of the leaders of HiPER. anything NIF can match. In their quest for fusion The PETawatt Aquitaine energy with LMJ-PETAL, re- Laser (PETAL) will gener- searchers face sociological and ate pulses with a relatively political challenges, too. The modest 3.5 kilojoules of en- A peppercorn-sized fusion fuel capsule suspended inside its metal can. European IFE community is ergy, but that energy will small and is not used to work- be crammed into a trillionth of a second, advantages—it smoothes out imperfections ing with such a huge machine or with producing a power of more than a thou- in the laser beam, and x-rays are better weapons lab levels of security. “LMJ alone sand trillion watts—a hundred times that of than UV light at driving the implosion—but is like a cathedral in the desert,” Batani LMJ’s pulses. PETAL’s pulses won’t be split it makes the target complex and expensive, says. “Researchers are interested, but suspi- and delivered from all directions; they’ll not what you want for energy generation. cious. Many are not convinced it is a good come from a single direction timed to co- NIF researchers have struggled to make tool for research.” CEA also needs to over- incide with a pulse from the main laser. In this approach work: Energy is lost in the come its reluctance to share simulation experiments, they could provide a sudden process of converting light into x-rays, and codes with academic researchers for fear of intense kick of power or could be used like the implosions do not progress smoothly. helping rogue nations develop thermonu- a strobe light to take snapshots of what’s IFE researchers outside the weapons clear weapons, Batani says: “We need reli- going on. labs want to do things differently. By get- able simulations, but there is no open code.” Experiments combining PETAL and LMJ ting rid of the can and targeting beams And Europe has traditionally focused on a will mimic the conditions in the interiors directly on the capsule, they can avoid the different approach, magnetic confinement of stars and other astrophysical objects. complications and energy loss of convert- fusion, which has its own cathedral not far Researchers will also use the powerful la- ing UV light to x-rays. To get a smooth, away: the multibillion-euro ITER, under ser jolts to accelerate protons—an approach symmetrical implosion, many advocate construction in Cadarache, France. that could yield compact accelerators for driving it more slowly. But then the com- “If shock ignition on LMJ works, politi- cancer therapy. But what excites laser fu- pressed fuel won’t get hot enough to start cians could become more positive,” Batani sion researchers is the prospect that the reacting on its own; it will need an extra says. And Europe—always an also-ran in short, sharp blasts from PETAL could act as spark to start it off. this branch of fusion energy—might just

a spark plug for fusion reactions. One possible solution, known as fast igni- some boasting rights. ■ SOURCE LANDMANN/SCIENCE PATRICK PHOTO:

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