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How elements mix deep inside giant planets?

It could offer insights into the evolution of planetary systems and guide scientists hoping to harness nuclear fusion as a new source of energy.

An international team that includes scientists from the Department of Energy’s SLAC National Accelerator Laboratory has devised a new way to quantify how chemical elements behave and mix deep inside icy giants. This new experimental setup is expected to reveal detailed insights into the formation and evolution of planetary systems.

Past experiments made use of SLAC’s Linac Coherent Light Source (LCLS) X-ray laser to obtain details on the creation of “warm dense matter,” a superhot, supercompressed mixture believed to be at the heart of these enormous planets. Through these experiments, scientists were able to gather evidence for “diamond rain.” Diamond rain is exotic precipitation predicted to form from mixtures of elements deep inside icy giants.

Until now, the X-ray diffraction technique was being used to determine this phenomenon. However, there is a drawback of this technique: This technique works well for crystal samples but is less effective for non-crystal samples whose molecules and atoms are arranged more randomly, which limits the depth of understanding scientists can reach.

Hence, in this new study, scientists used X-ray Thomson scattering technique. X-ray Thomson scattering technique accurately generates previous diffraction results while also allowing them to study how elements mix in non-crystal samples at extreme conditions.

LCLS Director Mike Dunne says, “This research provides data on a phenomenon that is very difficult to model computationally: the ‘miscibility’ of two elements, or how they combine when mixed. Here they see how two elements separate, like getting mayonnaise to separate back into oil and vinegar. What they learn could offer insight into a critical way fusion fails, in which the inert shell of a capsule mixes in with the fusion fuel and contaminates it so that it doesn’t burn.”

During the experiment, optical laser beams launched a shock wave in a plastic sample made up of carbon and hydrogen. Scientists observed the shock waved by hitting the shocked regions with X-ray photons from LCLS that scattered both backward and forwards off electrons in the sample, while it was moving through the material. 

SLAC scientist and co-author Eric Galtier said, “One set of scattered photons revealed the extreme temperatures and pressures reached in the sample, which mimic those found 10,000 kilometers beneath the surface of Uranus and Neptune. The other revealed how the hydrogen and carbon atoms separated in response to these conditions.”

According to scientists, the technique will help them in measuring the microscopic mix of materials used in fusion experiments at large, high-energy lasers such as the National Ignition Facility at DOE’s Lawrence Livermore National Laboratory (LLNL).

Tilo Doeppner, LLNL physicist, said, “We want to understand if this process could occur in inertial confinement fusion implosions with plastic ablator capsules, as it would generate fluctuations that could grow and degrade the implosion performance.”

Dominik Kraus, a scientist at Helmholtz-Zentrum Dresden-Rossendorf, who led the study, said“This technique will allow us to measure interesting processes that are otherwise difficult to recreate. For example, we’ll be able to see how hydrogen and helium, elements found in the interior of gas giants like Jupiter and Saturn, mix and separate under these extreme conditions. It’s a new way to study the evolutionary history of planets and planetary systems, as well as supporting experiments towards potential future forms of energy from fusion.”

Journal Reference:
  1. S. Frydrych et al. Demonstration of X-ray Thomson scattering as diagnostics for miscibility in warm dense matter. DOI: 10.1038/s41467-020-16426-y

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