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Hot Graphite Shines New Light on Laser-driven Fusion

COVENTRY and OXFORD, England, Dec. 14, 2012 — A new strongly heated graphite experiment has left an international team of researchers with some unexpected results that may shine new light on giant planets, white dwarfs and laser-driven fusion.

In an attempt to learn more about how energy is shared between the different species of matter — especially how it is transferred from strongly heated electrons to the heavy ionic cores of atoms that have been left to cool — scientists from the University of Warwick and Oxford University were left with some puzzling new results. The findings could lead to a revised understanding of the life cycle of giant planets and stars.

The difference in temperatures between the hot electrons and cooler ions should level out quickly as the electrons interact with the ions; the time it takes to reach a common temperature is a good measure of the interaction strength between the two. This interaction also defines, for instance, how heat or radiation is transported from the inside of a planet or star to its surface and, thus, planetary and stellar evolution. The process is also essential for nuclear fusion where the electrons are heated by fusion products, but the ions need to be hot for more fusion to occur.

Previous experiments using direct laser heating have been plagued by uncertainties in target preparation and heating processes that complicate observations and analysis. In addition, theoretical models struggled to explain the long temperature equilibration time found experimentally.


Graphite experiments conducted by researchers from the University of Warwick and Oxford University suggest that white dwarfs may fade faster than we thought. Courtesy of Mark Garlick space-art.co.uk, University of Warwick.

To resolve this difference, the Warwick and Oxford team devised a more precise experiment that employed intense proton beams created via a novel laser-driven acceleration scheme, rather than direct heating by a laser. Heating by the protons resulted in better-defined conditions because the protons heat only the electrons but for the entire sample.

As a result, the researchers obtained a clean sample with electrons at 17,000 K while the ions remained at about 300 K (around room temperature). Rather than eliminating the gap between the model and the observed results, the scientists discovered that this method significantly increased the difference.

The more precise experiment showed that the equilibration of the temperatures for hot electrons and cool ions was actually three times slower than previous measurements and more than 10 times slower than the mathematical model predicted.

“I think the results [will] send theoreticians back to the drawing board when modeling the interactions between particles in dense matter,” said Dr. Gianluca Gregori of Oxford University. “The wide range of implications and the huge range in temperature, where these issues were found, make the results so important.”

The same process also governs many other material properties, with wide implications from material processing to inertial confinement fusion to understanding astrophysical objects.

“This is an intriguing result which will require us to look again at the plasma physics models, but it will also have significant implications for researchers studying planets and white dwarf stars,” said Dr. Dirk Gericke of the University of Warwick. “My laser-fusion colleagues who depend on their lasers delivering a lot of energy simultaneously to both ions and electrons will certainly be interested in our findings as well.”

The findings were reported in Scientific Reports (doi: 10.1038/srep00889). 

For more information, visit: www2.warwick.ac.uk


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