Huge
lasers could spark fusion
By
Kimberly Patch,
Technology Research NewsFinding 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 closer still.
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 said.
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 compressed fuel.
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, said Norreys.
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
Funding: Government
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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September 18/25, 2002
Page One
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