The world's largest laser has applied 50 million times atmospheric pressure to samples to tiny diamonds to replicate the conditions inside planets like Jupiter.
Diamonds are famously formed under exceptional pressures deep within the Earth. Not surprisingly, their capacity to survive pressure is exceptional. Diamonds have the highest rating on the Mohs scale of any macroscopic material, although responses to pressures applied in these scratch hardness tests is different from the sort applied in this case.
Nevertheless, the forces applied using 176 lasers at the National Igniton Facility are about 50 times those that shatter natural diamonds. Not surprisingly the diamonds vaporized in ten billionths of a second, according to the paper in Nature.
Nevertheless, the researchers were able to compress the 3mm by 0.2mm diamond embedded in a hohlraum by a factor of 3.7, to a density greater than lead, before this occurred. Sound waves in the material did not change speed abruptly as the pressure increased, indicating that while it got more dense, there was no fundamental change in structure.
“The experimental techniques developed here provide a new capability to experimentally reproduce pressure-temperature conditions deep in planetary interiors," said lead author Ray Smith of the Lawrence Livermore National Laboratory.
Pressures of the order of terapascals have been applied before, but only when accompanied by temperatures of hundreds of thousands of degrees, more in keeping with the center of stars than planets. For Smith the hard part was getting the pressure this high while keeping temperatures low, by which he means a thousand degrees. Smith achieved this through careful tuning of the way the laser's intensity stepped up with time.
Eighty years ago the then emerging field of quantum mechanics was used to predict the state of matter in the cores of planets and stars. However, these predictions have been hard to test experimentally. Smith says that the results generally fit well with the predictions, but have some important differences, which he is keen to explore further. The Ignition Facility, so futuristic it featured in the latest Star Trek film, still has 16 more lasers that can be added to the ones used for this experiment.
Besides placing our understanding of quantum behavior under such extreme conditions on a firmer footing, the work could help us understand how giant planets form, a field of increasing interest now that we are finding so many more of them. We are also starting to think that many smaller planets will turn out to be made up almost entirely of diamonds and this work could help us recognize such objects by, in the paper's words “provid[ing] new constraints on mass–radius relationships for carbon-rich planets.”
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