lasers could spark fusion
Technology Research News
Finding ways of producing useful energy
is an age-old problem, and the ways humans have come up with often seem
to involve some type of pollution.
Scientists have been working for more than 50 years to harness nuclear
fusion, the reaction that powers both the sun and hydrogen bombs, as a
clean, controllable energy source.
When a pair of atoms are forced together they fuse to create a new type
of atom; this reaction gives off a tremendous amount of energy. The trick
to setting off the reaction is compressing the fuel atoms enough, generating
enough ignition energy, and coordinating the process.
A second challenge is containing the reaction, which, at one hundred million
degrees Celsius, is hot enough to burn through any type of material. And
to be a useful energy source, the reaction must produce more energy than
is used to ignite it.
An international team of researchers has shown that it is possible to
use very high-powered lasers to isolate and heat very dense fuel to the
incredibly high temperatures necessary to set off nuclear fusion.
The method could eventually be used to generate power on an industrial
scale, according to Peter Norreys, physics group leader at Rutherford
Appleton Laboratory in England. "We can now seriously consider the construction
of full-scale fast ignition facilities... that bring the commercial realization
of fusion energy a lot closer," he said.
The effort is one of several aimed at developing fusion as a power source.
Hydrogen, the first element, has a nucleus made up of just one proton.
When a pair of hydrogen atoms fuse they become one helium atom, which
contains two protons.
It takes a lot of energy to meld two atoms because the positively charged
protons contained in atomic nuclei repel each other. The force keeping
atomic nuclei apart -- the Coloumb force -- is proportional to the product
of the two charges divided by the square of the distance between them,
meaning the closer two nuclei are, the more difficult it is to bring them
Teams of scientists across the globe are working on an approach that involves
heating and compressing a large pool of the hydrogen isotope tritium using
powerful electromagnetic fields, which also isolate the hot plasma.
The two hydrogen isotopes, deuterium and tritium, contain one proton plus
one and two neutrons, respectively. Neutrons have no charge. This makes
deuterium twice as heavy as hydrogen, and tritium three times as heavy
and also more reactive.
One of the challenges of the electromagnetic field approach is precisely
synchronizing the heating and compressing to occur at same time.
The laser method involves a small amount of deuterium fuel, and ignites
the fuel more quickly than the usual approach, according to Norreys. "The
idea of laser fusion is to compress the matter to ultra-high density so
that the material does not have time to respond to the increase in temperature
generated by the spark before the fusion process is complete," Norreys
The concept was proposed by scientists from the Lawrence Livermore National
Laboratory eight years ago. "Their idea was to separate the two processes
of compression to high density and heating to thermonuclear temperatures,"
and to use an ultra-fast laser pulse to ignite fusion, he said.
The general set-up is similar to the internal combustion engines cars
use, said Norreys. "Fuel is periodically compressed, and a spark is administered
to ignite the fuel. The fuel explodes and releases energy," he said.
The challenge to proving the scheme possible was finding a way to allow
the ignition laser beam to come close to the compressed fuel, or plasma,
without being deflected, said Norreys.
The researchers got around the problem by using a hollow gold cone to
insure that exhaust from the plasma did not interact with the laser pulse,
allowing the laser energy to be deposited as close as possible to the
The researchers inserted the guide cone into a polystyrene-deuterium shell
seven microns thick and 500 microns in diameter, and used nine laser beams
to implode the shell and hold the plasma in place. A micron is one thousandth
of a millimeter.
The implosion compressed the fuel to about 100 grams, or just under a
quarter pound, per cubic centimeter. They shot a petawatt laser pulse
into the cone, depositing a large number of energetic electrons at its
tip, which was about 50 microns from the center of the fuel, said Norreys.
One petawatt is one million billion watts, which is more than a billion
times the watts used by the 500-odd high-power lights that illuminate
Syracuse University's 50,000-seat Carrier Dome.
The energetic electrons slowed down in the compressed material, Norreys
said. And "as they slowed down they transferred their energy to the plasma
in the form of heat," he said.
The experiments confirmed that at least 20 percent of the petawatt laser
energy was transferred to the plasma in the form of heat, proving the
method viable, said Norreys.
One of the advantages of using lasers rather than magnetic fields to spark
and contain fusion is the compression process does not have to be as precise,
according to Norreys. Strict implosion symmetry, where the fuel compresses
into an exactly round ball, is not necessary under this scheme, he said.
The research efforts into magnetic-field contained fusion, however, are
more advanced. The laser experiment was possible only because scientists
now have access to petawatt lasers capable of depositing enough energy
into the fuel, said Norreys.
Now that the laser scheme has been proven possible "we can... seriously
consider the construction of full-scale fast ignition facilities that
will greatly reduce the size of the drive laser needed for ignition,"
said Norreys. Refining the method so that less power is needed to ignite
the fuel is a step toward making fusion energy generation commercially
viable, he said.
The researchers plan to perform the same experiments on a similar laser
in order to confirm that the results can be reproduced, said Norreys.
"We're starting these experiments in the next three weeks at... the Rutherford
Appleton laboratory," said Norreys.
Longer-term, the researchers are aiming to demonstrate that it is possible
to fully ignite a fusion reaction using this method, and that it is possible
to produce a net gain of energy from such a reaction, said Norreys.
The research is a valid demonstration, and an experiment well done, said
Hector Baldis, director of the Institute for Laser Science and Applications
at Lawrence Livermore National Laboratory and a professor of applied science
at the University of California at Davis. "It does... give credibility
to the idea of using what we call fast ignition," he said.
It is one thing to achieve ignition, and another to generate power from
laser fusion, Baldis added.
It will take ten years of work to demonstrate ignition, and 20 years before
the method could be ready for commercial use in a laser-fusion power plant,
In the near term, the method can be used to study fusion in the laboratory,
Norreys said. "Applications in nuclear physics and laboratory-based astrophysics
are much closer," he said.
Norreys' 23 research colleagues were Ryosuke Kodama and his fast-igniter
Consortium team at Osaka University in Japan, Thomas Hall from the University
of Essex in England, Hideaki Habara from Rutherford Appleton Laboratory,
Karl Krushelnick from Imperial College in England, Kate Lancaster from
Rutherford Appleton Laboratory and Imperial college, and Matthew Zepf
from Queens University in Northern Ireland.
They published the research in the August 29, 2002 issue of the journal
Nature. The research was funded by the Japanese Ministry of Education
(Monbusho), the Japan Society for the Promotion of Science, and the UK
Engineering and Physical Sciences Research Council.
Timeline: < 1 year, 10 years, 20 years
TRN Categories: Energy; Physics
Story Type: News
Related Elements: Technical paper, "Fast Heating Scalable
to Laser Fusion Ignition," Nature, August 29, 2002.
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