November 29, 2012
Experiment May Lead To Revised Understanding Of Life Cycle Of Giant Planets
Lee Rannals for redOrbit.com - Your Universe Online
Scientists from the University of Warwick and Oxford University have come up with unexpected results in an experiment with strongly heated graphite, posing a new problem for physicists working in laser-driven nuclear fusion.
The team was attempting to gain a better understanding about how energy is shared between the different species of matter, looking into how it is transferred from strongly heated electrons to the heavy ionic cores of atoms.
They wrote in the journal Scientific Reports that the difference in temperatures between the hot electrons and cooler ions should level out quickly as the electrons interact with the ions.
This interaction defines how heat or radiation is transported from the inside of a planet or star to its surface. The process is essential for nuclear fusion where the electrons are heated by fusion products but the ions need to be hot for more fusion to take place.
Previous experiments found uncertainties in target preparation and heating processes complicating observations and analysis. Also, theoretical models struggled to explain the long temperature equilibration time found experimentally.
The researchers hoped they could resolve the difference by devising a more precise experiment. Instead of direct heating by a laser, they used intense proton beams to create a novel scheme of laser-driven acceleration.
They found that heating by the protons resulted in much better defined conditions as the protons heat only the electrons for the entire sample. The team was able to use this method to obtain a clean sample with electrons at 17,000 degrees Kelvin while the ions remain at around room temperature of 300 degrees Kelvin.
The researchers found that rather than eliminating the gap between the model and the results the difference significantly increased. Their experiment shows that the equilibrium of the temperatures for hot electrons and cool ions is actually three times slower than previous measurements have shown, and more than ten times slower than the mathematical model predicts.
The results have implications from material processing to inertial confinement fusion to our understanding of astrophysical objects. This result becomes more important if combined with previous indications for much hotter systems.
“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 from 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.”
Dr Gianluca Gregori from the University of Oxford said that he believe the results send theoreticians back to the drawing board when modeling the interactions between particles in dense matter.
"The wide range of implications and the huge range in temperature, where these issues were found, make the results so important," Gregori said.