A view of the interior of the world's biggest laser, which researchers used to simulate the cores of giant planets.
Credit: Damien Jemison/Lawrence Livermore National Laboratory
The biggest laser in the world was used to crush a diamond, offering insights into how the hardest known material behaves when it is exposed to extremely high pressures. The experiment could also reveal new clues about what happens at the cores of giant planets, where conditions of intense atmospheric pressures exist.
Researchers at the Lawrence Livermore National Laboratory in Livermore, California, led by physicist Raymond Smith, blasted a sliver of diamond with a laser beam at a pressure of 725 million pounds per square inch (51 million kilograms per square centimeter). This is the kind of pressure found near the core of giant planets, such as Jupiter or huge, rocky bodies known as "super-Earths."
The entire experiment took only 25 billionths of a second. The researchers fired 176 laser beams at a small cylinder of gold, called a hohlraum, with a tiny chip of synthetic diamond embedded in it. When the laser beams hit the cylinder, the energy was converted to X-rays. The hohlraum was vaporized, and in the process, the diamond was exposed to pressures tens of millions of times Earth's atmospheric pressure. [Shine On: Photos of Dazzling Mineral Specimens]
Theoretical calculations predicted that such high pressures should cause a diamond to change its crystal structure. One way to test if this is true is to measure the speed of sound waves in a material. If this speed changes abruptly as the pressure goes up, then the diamond structure has rearranged itself.
But that didn't happen — the velocity of sound waves changed smoothly.
"If there was a phase transformation you'd expect a discontinuity," Smith said.
The rate of change in the diamond's density also didn't match up with earlier theoretical models. Materials typically become denser at high pressures, and diamond is no exception. But how fast its density changed was a surprise, the researchers said.
The experiment was a breakthrough, in that instead of smacking the diamond with high pressure in a stepwise fashion, such as hitting it with successively heavier hammers, the researchers were able to boost the pressure smoothly. This enabled them to crush the diamond and expose it to intense pressure without the substance becoming too hot and melting. (Diamonds can and do melt at sufficiently high temperatures).
Since diamonds are made of carbon, understanding how this material behaves at high pressures can be important in the study of planets around other stars, said Nikku (Madhu) Madhusudhan, a professor of astrophysics at the University of Cambridge.
"The pressure regime they report is similar to pressures in the deep interiors of large planets, super-Earths and larger," Madhusudhan told Live Science in an email. "The findings are relevant to understanding the interior structure of potential carbon-rich super-Earths, like 55 Cancri e, which could have diamond in their interiors at high pressure."
Until now, he said, scientists had only theoretical models to describe what happened to carbon at such pressures. Smith's team has now provided real experimental data.
Natalia Dubrovinskaia, a professor of material physics at the University of Bayreuth in Germany, who has worked with some of Smith's team on other experiments, said the laser technique in itself offers new possibilities.
"To a great extent this paper is about the new experimental techniques rather than about diamond," she told Live Science in an email. "Important is a new capability to experimentally reach really extreme pressure-temperature conditions."
In fact, the laser used in the experiment is so powerful that it made a cameo in the film "Star Trek: Into Darkness," standing in for the starship Enterprise's warp engine core.
"Even if the interpretation will need to be corrected or reconsidered in the future… one must start to explore the new capability. So the presented work is one step forward on this way," Dubrovinskaia said.