CONTROVERSES NUCLEAIRES !
La FUSION?!
2009
mai
In Hot Pursuit of Fusion (or Folly)
ADIT, http://www.nytimes.com/2009/05/26/science/26fusi.html?_r=1&ref=science

RAYS OF HOPE The National Ignition Facility in California, to be dedicated this week.

From Laser Light to a Miniature Star


A rendering of a pellet hit by lasers to create nuclear fusion.

LIVERMORE, Calif. — Here in a dry California valley, outside a small town, a cathedral of light is to be dedicated on Friday. Like the cathedrals of antiquity, it is built on an unrivaled scale with unmatched technology, and it embodies a scientific doctrine that, if confirmed, might lift civilization to new heights.
     "Bringing Star Power to Earth" reads a giant banner that was recently unfurled across a building the size of a football stadium.
     The $3.5 billion site is known as the National Ignition Facility, or NIF. For more than half a century, physicists have dreamed of creating tiny stars that would inaugurate an era of bold science and cheap energy, and NIF is meant to kindle that blaze.
     In theory, the facility's 192 lasers — made of nearly 60 miles of mirrors and fiber optics, crystals and light amplifiers — will fire as one to pulverize a fleck of hydrogen fuel smaller than a match head. Compressed and heated to temperatures hotter than those of the core of a star, the hydrogen atoms will fuse into helium, releasing bursts of thermonuclear energy.
     The project's director, Ed Moses, said that getting to the cusp of ignition (defined as the successful achievement of fusion) had taken some 7,000 workers and 3,000 contractors a dozen years, their labors creating a precision colossus of millions of parts and 60,000 points of control, 30 times as many as on the space shuttle.
     "It's the cathedral story," Dr. Moses said during a tour. "We put together the best physicists, the best engineers, the best of industry and academia. It"s not often you get that opportunity and pull it off."
     In February, NIF fired its 192 beams into its target chamber for the first time, and it now has the world's most powerful laser, as well as the largest optical instrument ever built. But raising its energies still further to the point of ignition could take a year or more of experimentation and might, officials concede, prove daunting and perhaps impossible.
     For that reason, skeptics dismiss NIF as a colossal delusion that is squandering precious resources at a time of economic hardship. Just operating it, officials grant, will cost $140 million a year. Some doubters ridicule it as the National Almost Ignition Facility, or NAIF.
     Even friends of the effort are cautious. "They've made progress," said Roy Schwitters, a University of Texas physicist who leads a federal panel that recently assessed NIF's prospects. "Ignition may eventually be possible. But there's still much to learn."
     Dr. Moses, while offering no guarantees, argued that any great endeavor involved risks and that the gamble was worth it because of the potential rewards.
     He said that NIF, if successful, would help keep the nation's nuclear arms reliable without underground testing, would reveal the hidden life of stars and would prepare the way for radically new kinds of power plants.
     "If fusion energy works," he said, "you'll have, for all intents and purposes, a limitless supply of carbon-free energy that's not geopolitically sensitive. What more would you want? It's a game changer."
     NIF is to fire its lasers for 30 years.
     Like the dedication of a cathedral, the event here on Friday at the Lawrence Livermore National Laboratory is to be a celebration of hope. Officials say some 3,500 people will attend. The big names include Gov. Arnold Schwarzenegger, Energy Secretary Steven Chu (whose agency finances NIF) and Charles Townes, a Nobel Laureate and laser pioneer.
     In preparation, workmen here last Thursday washed windows and planted flowers on the lush campus, the day auspiciously sunny.
     Dr. Moses, who runs science programs for high school students in his spare time, broke from his own preparations to show a visitor the NIF complex.
     In its lobby, he held up a device smaller than a postage stamp. This is where it all starts, he said. From this kind of tiny laser, beams emerge that grow large and bright during their long journey through NIF's maze of mirrors, lenses and amplifiers.

suite:
     The word laser is an acronym for light amplification by stimulated emission of radiation. And each particle of light, or photon, is amplified, Dr. Moses said, to "around 10 to the 25th" photons. Or, "10 million, million, million, million."
     A nearby stand held a thick slab of pink glass about the size of a traffic sign — an example of an amplifier. NIF has 3,200 in all. Dr. Moses said the big step occurred when giant flash tubes — like ones in cameras but six feet long and 7,680 in number — flashed in unison to excite the pink glass. Laser photons then zip through, stimulating cascades of offspring, making the beam much stronger, such amplification happening over and over.
     Photons moving in step with one another is what makes laser light so bright and concentrated and, in some instances, so potent.
     Dr. Moses picked up a mock capsule of hydrogen fuel. It was all of two millimeters wide, or less than a tenth of an inch.
     "It heats up," he said. "It blows in at a million miles an hour, moving that way for about five-billionths of a second. It gets to about the diameter of your hair. When it gets that small, that fast, you hit temperatures where it can start fusing — around 100 million degrees centigrade, or 180 million degrees Fahrenheit."
     Hair nets, hard hats and safety goggles were donned before entering NIF proper. Repeated steps on sticky pads pulled dirt from shoes. Dust is NIF's bane, Dr. Moses said. It can ruin optics and experiments. He said the 33-foot-wide target chamber was evacuated to a near-vacuum, much the same as outer space — a void where light can zip along with almost no impediments.
     Dr. Moses said the team fired the laser only at night and did maintenance and equipment upgrades during the day. "This is a 24/7 facility," he said.
     The previous night, he said, the laser had been fired in an effort to improve coordination and timing. The 192 rays have to strike the target as close to simultaneously as possible.
     The individual beams, he said, have to hit "within a few trillionths of a second" of one another if the fuel is to burn, and be pointed at the target with a precision "within half the diameter of your hair."
     The control room, modeled on NASA's mission control in Houston, was buzzing with activity, even though some consoles sat empty. Phones rang. Walkie-talkies crackled. The countdown to firing the lasers, Dr. Moses said, took three and half hours, with the process "pretty much in the hands of computers."
     The operations plan for NIF, he added, is to conduct 700 to 1,000 laser firings per year, with about 200 of the experiments focused on ignition. There is no danger of a runaway blast, he said. Fusion works by heat and pressure, not chain reactions. Moreover, the fuel is minuscule and the laser flash extraordinarily short. During a year of operations, Dr.
      Moses said, "the facility is on for only three-thousandths of a second," yet will generate a growing cascade of data and insights.
     Next on the tour, after more sticky pads, was the holy of holies, the room surrounding the target chamber. It looked like an engine room out of a science-fiction starship. The beam lines — now welters of silvery metal filled with giant crystals that shifted the concentrated light to higher frequencies — converged on the chamber's blue wall. Its surface was dotted with silvery portholes where complex sensors could be placed to evaluate the tiny blasts.
     "When it's running," Dr. Moses said, "there's a lot of stuff at the chamber's center."
     Despite the giant banner outside and its confident prediction, it is an open question whether NIF's sensors will ever detect the rays of a tiny star, independent scientists say.
     "I personally think it's going to be a close call," said William Happer, a physicist at Princeton University who directed federal energy research for the first President George Bush.
     "It's a very complicated system, and you're dependent on many things working right."
     Dr. Happer said a big issue for NIF was achieving needed symmetries at minute scales. "There's plenty of room," he added, "for nasty surprises."
     Doubters say past troubles may be a prologue. When proposed in 1994, the giant machine was to cost $1.2 billion and be finished by 2002. But costs rose and the completion date kept getting pushed back, so much so that Congress threatened to pull the plug. Today, critics see the delays and the $3.5 billion price tag as signs of overreaching.
    Dr. Moses, who was put in charge of NIF a decade ago in an effort to right the struggling project, said that a decade from now, as NIF opened new frontiers, no one would remember the missteps. He compared the project to feats like going to the Moon, building the atom bomb and inventing the airplane.
     "Stumbles are not unusual when you take on big-risk projects," he said.
. Moses added that the stumble rule applied to cathedrals as well.
     Having grown up in Eastchester, close to New York City, he noted that the Cathedral Church of Saint John the Divine, on the Upper West Side of Manhattan, was still under construction after more than a century. Is it worthwhile, despite the delays?
     "Of course it is." he said. Taking on big projects that challenge the imagination "is who we are as a species."