Part of our weekly "In Focus" seriesstepping back, looking
closer.
A large banner hangs from the front of the stadium-size building that
houses the world's most powerful array of lasers: "Bringing Star Power To
Earth."
For the past four years, physicists at the National Ignition
Facility, or NIF, in Livermore,
California, have been trying to harness nuclear fusion, the same reaction that
powers the sun and the stars. Supporters of the $3.5 billion facility believe
that a successful outcome to the experiments could help usher in an era of
nearly limitless energy. But the ambitious fusion research program at NIF now
faces an uncertain future, both politically and scientifically.
On the political side, President Obama's proposed budget
for fiscal year 2014 would reduce funding for fusion experiments at NIF by more
than $60 million, putting it nearly 14 percent below the 2013 level. Key
committees in both the House
and Senate favor restoring
part of NIF's funding, and a compromise will eventually emerge, but budget
constraints aren't the only challenge facing NIF. Physicists working on the
project expected to have succeeded in their quest for fusion energy by now.
They're currently struggling to figure out what went wrong.
Tiny Stars, Big Lasers
There's an old joke about fusion: It's the energy source of the future, and
always will be. Physicists have been pursuing the dream of controlling fusion
energy for some 60 years now. Unlike nuclear fission, which releases
energy when the nucleus of a heavy atom like uranium splits into two lighter
nuclei, fusion generates energy when two separate light nuclei smash
together to form a single, heavier nucleus. In fission, the energy comes from
breaking the bonds of force that held the original heavy atom together; with
fusion, the energy source is more esotericsome of the mass from each of the two
light nuclei is converted directly into energy when they fuse, in accordance
with Einstein's iconic law, E=mc2.
Both fission and fusion release tremendous amounts of energy. One pound of
enriched uranium used in a conventional nuclear power plant contains about as
much energy as a million gallons of gasoline. Fusion yields even more
energyabout three to four times as much as fission reactions. And while fission
reactions generate waste that remains radioactive for millennia,
fusion's byproducts become harmless within decades. Moreover, the world
possesses a nearly infinite source of fusion fuelthe hydrogen atoms found in
water.
Unfortunately for the world's energy needs, fusion presents far greater
technical challenges than fission, which physicists mastered in the 1940s. It
takes relatively little energy to split a nucleusfission can even happen
spontaneously. But for fusion to occurthat is, to force two nuclei to
joinphysicists must replicate the hellish temperatures and pressures found
inside stars.
Scientists access the NIF target chamber using a service
system lift. "I've dedicated my life to this," says Ed Moses, principal
associate director. "I'm committed to understanding it."
Photograph courtesy Philip Saltonstall, LLNL
NIF seeks to do that with 192 giant lasers, which occupy a space as large as
three football fields. Fired simultaneously, the laser beams blast a
peppercorn-size speck of frozen hydrogen suspended in a 30-foot-wide target
chamber with about 500 trillion watts of powerabout 1,000 times the amount
of energy used by the entire United States during that same few trillionths of a
second. (Because the lasers fire so briefly, NIF uses only about $20 of
electricity for each burst.) Crushed to less than a thousandth of its original
volume, the hydrogen becomes 100 times denser than lead and hotter than the
center of a star; the nuclei fuse and release bursts of energy.
According to NIF's computer simulations, the fused hydrogen should generate
more energy than the lasers put ina process called ignition. Nature,
unfortunately, has stubbornly refused to cooperate. There has been no ignition
at the National Ignition Facility.
When physicists first turned on all the lasers at NIF in February 2009, they
set a goal of reaching ignition by October 1, 2012. NIF's lasers routinely cause
fusion, but the energy pumped in by the lasers still exceeds the energy created
by the fusing hydrogen. The failure to meet that ignition deadline is the main
reason the President, with the support of at least some in Congress, decided to
cut NIF's budget.
"From a back-of-the-envelope calculation, the lasers do deposit enough energy
onto the hydrogen pellet to do the job," said Robert Rosner, a
physicist at the University of Chicago and the former director of Argonne
National Laboratory. "The $64,000 questionactually a lot more than $64,000is,
why is the actual energy captured by the pellet in its implosion so much lower
than that, by close to a factor of ten?"
Like a Leaky Piston
Ed Moses, the photon science principal associate director at NIF, says the
researchers there are focusing on solving two critical problems. For ignition to
occur, the hydrogen pellet must remain perfectly spherical as the lasers
compress it. Using X-ray cameras to track the imploding hydrogen, physicists
have found that the pellet deforms just as fusion starts. It assumes a lumpy,
clover shape, a sign that the hydrogen is losing heat and pressure during its
compression. "It's like a leaky piston, and the pressure doesn't keep going up,"
says Moses. The other problem concerns the thin plastic shell that encases the
hydrogen fuel. Bits of it might be mixing with the hot imploding hydrogen,
cooling it and squashing ignition.
The laser beams blast a peppercorn-size speck of frozen
hydrogen with about 500 trillion watts of power1,000 times the amount of energy
used by the entire United States during that same few trillionths of a
second.
Photograph courtesy LLNL
"We have shown our ability to compress the diameter of the fuel to where it
would ignite if it were round, which is something people would have found
unbelievable a few years ago," says Moses. "What we haven't shown yet is that we
can get the shape we need as we go in, and that we can prevent mixing."
A recent report
by the National Research Council recommended that NIF be given three more years
to solve its problems and determine whether the facility is even capable of
achieving ignition. Some critics argue that NIF needs to adopt a fundamentally
different research strategy, a critique endorsed by the report. David Hammer, a
physicist at Cornell University, says the NIF team treated their fusion
experiments like an engineering project, and assumed that they could achieve
ignition if they tweaked the lasers just right from one "shot" to the next.
"It was misplaced confidence," said Hammer. "They would not accept that the
different stages of the experiments were not well understood, and they went on
to the next step anyway." The NIF researchers should have been more systematic,
he said, starting at lower energies to make sure the computer predictions
matched reality. "If they didn't get it right at some low level, then figure out
what's wrong, because it's a lot easier to figure things out when you're not
driving an experiment to its limits. And once you've understood it at say,
half-energy, then you gradually build up and see how the experiment moves away
from predictions of the computer code. I think if they had started a more
science-oriented program in 2009, when the lasers were finished, they'd be a lot
closer to ignition now."
NIF isn't the only fusion project competing for federal dollars. The United
States is also investing in an international collaboration that plans to harness
fusion using a completely different strategy from NIF's. Now under construction
in France, ITER, short for International
Thermonuclear Experimental Reactor, will use powerful magnetic fields to
compress a plasmaessentially hydrogen gas heated to such high temperatures that
the electrons and protons in the hydrogen fly apartuntil the protons fuse. The
$20 billion project, which is scheduled to begin its first experiments in
November 2020, aims to produce ten times the amount of energy needed to run it.
But that 2020 deadline is likely to recede, given that President Obama's budget
would cap future United States contributions to ITER at $225 million. The budget
would also cut funds for a fusion laboratory at MIT, one of the three American
projects conducting experiments related to ITER.
A final decision on NIF's funding is months away, as budget wrangling
continues on Capitol Hill.
"Right now we're in the era of incremental government," said Representative
Eric Swalwell, a Democrat, whose district includes the facility at Livermore.
"We govern by crisis these days, which is really unfortunate, because while
science is very unpredictable, when it comes to funding, scientists need
certainty."
Nuclear Weapons and Getting to the World Series
Even with the proposed budget cuts, NIF will continue to operate for decades.
Achieving ignition is only one aspect of the lab's mission. Its primary
purposeone that will most likely overshadow fusion research in the years
aheadis to enable the United States to maintain its stockpile of nuclear
weapons. The country has observed a ban on explosive testing since
1992, and classified work at NIF tests components of nuclear weapons without the
need to blow anything up. That aspect of NIF's research has broad bipartisan
support, and the President's budget for 2014 would increase funding for the
lab's weapons-testing program.
But ignition is the game-changing research that inspires most of the
physicists who work at NIF.
"I've dedicated my life to this," said Moses. "I'm committed to understanding
it. I think it's likely we'll work through all these issues. We have this
three-year time line we've agreed to. If we're funded and can do our
experiments, we think we can explore this phenomenon pretty completely in that
time period. In sports, over a long season, some things go well, sometimes you
boot the ball. The question is, how do you get to the World Series? And that's
what we're trying to do."
This story is part of a special
series that explores energy issues. For more, visit The Great Energy
Challenge.