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By Jason Palmer
Science and technology reporter, BBC News
A major hurdle to producing fusion energy using lasers has been swept aside,
results in a new report show.
The controlled fusion of atoms - creating conditions like those in our Sun -
has long been touted as a possible revolutionary energy source.
However, there have been doubts about the use of powerful lasers for fusion
energy because the "plasma" they create could interrupt the fusion.
An article in Science showed the plasma is far less of a problem than expected.
The report is based on the first experiments from the National Ignition
Facility (Nif) in the US that used all 192 of its laser beams.
Along the way, the experiments smashed the record for the highest energy from a
laser - by a factor of 20.
Star power
Construction of the National Ignition Facility began at Lawrence Livermore
National Laboratory in 1997, and was formally completed in May 2009.
The goal, as its name implies, is to harness the power of the largest laser
ever built to start "ignition" - effectively a carefully controlled
thermonuclear explosion.
INERTIAL CONFINEMENT FUSION
hohlraum
mixture of hydrogen isotopes
to millions of degrees
fusion can result
It is markedly different from current nuclear power, which operates through
splitting atoms - fission - rather than squashing them together in fusion.
Proving that such a lab-based fusion reaction can release more energy than is
required to start it - rising above the so-called breakeven point - could
herald a new era in large-scale energy production.
In the approach Nif takes, called inertial confinement fusion, the target is a
centimetre-scale cylinder of gold called a hohlraum.
It contains a tiny pellet of fuel made from an isotope of hydrogen called
deuterium.
During 30 years of the laser fusion debate, one significant potential hurdle to
the process has been the "plasma" that the lasers will create in the hohlraum.
The fear has been that the plasma, a roiling soup of charged particles, would
interrupt the target's ability to absorb the lasers' energy and funnel it
uniformly into the fuel, compressing it and causing ignition.
Siegfried Glenzer, the Nif plasma scientist, led a team to test that theory,
smashing records along the way.
"We hit it with 669 kilojoules - 20 times more than any previous laser
facility," Nif's Siegfried Glenzer told BBC News.
That isn't that much total energy; it's about enough to boil a one-litre kettle
twice over.
However, the beams delivered their energy in pulses lasting a little more than
10 billionths of a second.
By way of comparison, if that power could be maintained, it would boil the
contents of more than 50 Olympic-sized swimming pools in a second.
'Dramatic step'
Crucially, the recent experiments provided proof that the plasma did not reduce
the hohlraum's ability to absorb the incident laser light; it absorbed about
95%.
But more than that, Dr Glenzer's team discovered that the plasma can actually
be carefully manipulated to increase the uniformity of the compression.
"For the first time ever in the 50-year journey of laser fusion, these
laser-plasma interactions have been shown to be less of a problem than
predicted, not more," said Mike Dunne, director of the UK's Central Laser
Facility and leader of the European laser fusion effort known as HiPER.
"I can't overstate how dramatic a step that is," he told BBC News. "Many people
a year ago were saying the project would be dead by now."
Adding momentum to the ignition quest, Lawrence Livermore National Laboratory
announced on Wednesday that, since the Science results were first obtained, the
pulse energy record had been smashed again.
They now report an energy of one megajoule on target - 50% higher than the
amount reported in Science.
The current calculations show that about 1.2 megajoules of energy will be
enough for ignition, and currently Nif can run as high as 1.8 megajoules.
Dr Glenzer said that experiments using slightly larger hohlraums with
fusion-ready fuel pellets - including a mix of the hydrogen isotopes deuterium
as well as tritium - should begin before May, slowly ramping up to the 1.2
megajoule mark.
"The bottom line is that we can extrapolate those data to the experiments we
are planning this year and the results show that we will be able to drive the
capsule towards ignition," said Dr Glenzer.
Before those experiments can even begin, however, the target chamber must be
prepared with shields that can block the copious neutrons that a fusion
reaction would produce.
But Dr Glenzer is confident that with everything in place, ignition is on the
horizon.
He added, quite simply, "It's going to happen this year."