OXFORDSHIRE, England, Sept. 20, 2008 -- Laser fusion is a step closer after intense laser pulses from the Vulcan petawatt laser facility heated significant volumes of solid-density matter to 10 million K -- hotter than the sun.
The milestone in high-energy density science was reached by an international team of physicists from Japan, the EU and the US. Previously only ultrathin layers of matter (less than 1-µm thick) had been heated to similar temperatures.
The Vulcan petawatt laser facility, at the UK Science and Technology Facilities Council Rutherford Appleton Laboratory, provides staggeringly powerful pulses of energy to target. One petawatt (1015 W) is 100 times the entire world’s electricity production, and the laser beam is focused to a spot of a few µm across -- about one-tenth the diameter of a human hair. It only lasts for less than 1 picosecond (10-12 of a second), but during that time it is possible to heat materials above their normal melting point, creating conditions that are found in exotic astrophysical objects such as supernova explosions, white dwarfs and neutron star atmospheres, the lab said in a statment. A reasonable volume of matter is needed to initiate the fusion process to enable energy gain (to get more energy out than the energy needed to produce it).
A view of the Vulcan petawatt target chamber. Daniel Hey (right), a researcher with Lawrence Livermore National Laboratory, is part of an international team of researchers reviewing data extracted from the instruments. (Photo: STFC)
"Previously only ultrathin layers of matter (less than 1-µm in thickness) had been heated to similar temperatures," the lab said. "This made them interesting but of limited value for applications, since the expansion of the material inevitably introduces density variations. This new work confirms that the heated material stays at this temperature and at solid density for a least 20 picoseconds -- which is more than enough time for high speed instruments, such as x-ray spectrometers, to probe the heated material."
Peter Norreys, physics group leader at the lab, a professor at Imperial College London and the principal investigator for the experiments, said, “This is an exciting development. We now have a new tool with which to study really hot, dense matter. Careful selection of the target parameters allows access to this new regime."
The measurement was made possible by the deployment of an innovative optical diagnostic, Hisac (high-speed sampling camera), that provided both spatial and temporal resolution needed for the measurements. The scientists measured the black-body radiation from the reverse surface of irradiated and heated foil targets and compared them with sophisticated computer modelling.
Ryosuke Kodama, a professor at Osaka University, Japan, said, “Hisac was developed in my laboratory at Osaka University and is a powerful tool for study of ultrahigh speed phenomena in extreme conditions."
The temperatures reached are only one-tenth of those needed for ignition of fusion capsules with only 300 J of energy on target. The team found that at least 15 percent of the laser energy was transferred to the fast electron beam. That transfer fraction informs designs for ignition of fusion targets on the High Power Laser Energy Research (Hiper) laser facility, the lab said. Hiper is a proposed European high-power laser energy research facility. Hiper will be a civilian facility that will study the feasibility of laser fusion energy as a future energy source and will enable new science research including extreme material studies, astrophysics in the laboratory and miniaturized particle accelerators.
Demonstration of energy production from laser driven fusion is expected in the period 2010-2012 from the National Ignition Facility, an extremely large laser nearing completion in California. A similar facility is under construction in Bordeaux, France, called Laser MegaJoule, by the French nuclear science directorate, CEA.
"Efficient coupling of the laser energy to the target is crucial for fast-ignition fusion and is one of the main questions on which the design of the European laser fusion laboratory, Hiper, depends," said Jonathan Davies, a researcher from Instituto Superior Technico, Lisbon, who performed the modeling of the experiment. (See also: Hiper Enters Prep Phase)
"What is now needed is to move from the scientific proof of principle stage to a commercial reactor, and Hiper will provide the critical next step along that path," the lab said.
A description of the team's work appears in the online New Journal of Physics published by the Institute of Physics.
For more information, visit: www.scitech.ac.uk
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