Secrets of a diamond as big as a planet

Scientists have flattened a diamond with the atom-crushing pressure of the most powerful laser in the world. They hope it will lead to the discovery of new materials and possibly an answer to viable nuclear fusion. Cathal O'Connell reports.

The interior of the target chamber at the National Ignition Facility at Lawrence Livermore National Laboratory. The object entering from the left is the target positioner, on which a millimeter-scale target is mounted. Researchers recently used NIF to study the interior state of giant planets. – Damien Jemison/LLNL

Two years ago an Australian-led collaboration announced the discovery of a planet, 40 light years from Earth, which is made of solid diamond. But is this gigantic gemstone, twice the size of our own planet, really diamond to the core? Or would the crushing pressures at the planet’s centre create an even more exotic form of carbon? In an unprecedented feat of high pressure physics scientists have blasted a sliver of diamond with the world’s most powerful laser to recreate in the lab the crushing conditions at the diamond planet’s core.

They hope that eventually such experiments will lead to a viable way to harness the energy from nuclear fusion.

“This is, by a long way, a record in controlled high pressure experimentation in a laboratory,” says Damien Hicks, an expert in high pressure physics at Swinburne University of Technology in Melbourne.

When it comes to weird science, physics takes the cake. From the 5.5 trillion kelvin temperatures inside the Large Hadron Collider to the vacuous emptiness of the furthest reaches of the universe, physicists focus on extremes. Why? Because that’s where things get interesting.

Now, reporting in Nature, physicists have pushed the horizon of a new extreme – pressure. The atom-crushing five terapascals they achieved is 14 times greater than at the centre of the Earth and greater even than in the core of Saturn. It’s the equivalent of 40,000 Asian elephants standing on your toe. At that pressure even diamond – the most incompressible material known to man gets flattened like a penny on a railway track.

High pressure physics has an explosive history. Much of the early work was performed during the Cold War arms race and involved igniting thermonuclear weapons deep underground. Though these “experiments” could yield phenomenal pressures they were uncontrolled. The major advance in recent decades has been an instrument that squeezes a sample between two diamonds, but even a material as hard as diamond cannot withstand the pressures physicists want to study. “The trouble is diamonds break,” says Hicks.

The new work, led by Gilbert “Rip” Collins at the Lawrence Livermore Laboratory in California, has trail-blazed a new idea. Their approach, reported in Nature this month, boils down to shining an extremely powerful light on a Barbie-sized engagement ring.

'We have no idea what form carbon takes at such extreme pressure.'

The engagement ring was actually a gold cylinder, a few millimetres across, into which a sliver of artificial diamond was set. The powerful light was a suite of 176 laser beams with the combined power of a bolt of lightning. Collins and his team trained the lasers at the gold from all angles and fired. The laser beams vaporised the gold and this generated enough pressure to momentarily squeeze the diamond to just a quarter of its original size. This gave it, for 25 billionths of a second, the same density as lead.

One of the major achievements of the work, Hicks explains, is that they achieved such high pressure without overheating the diamond. “If you whack something hard it’s very difficult to keep it at low temperature,” he says. By carefully controlling the laser blast the Livermore team avoided creating too much heat and thus could study the diamond in solid form throughout. “That’s the big trick here,” he says.

Keeping the diamond cool is vital for mimicking the cores of the diamond planet and other large exoplanets. From astronomical observations we know the masses and densities of these planets and we can usually make a good estimate of their composition. We know, for example, that the diamond planet is very dense and that it is made of carbon. But we have no idea what form the carbon takes at the extreme pressure inside the planet. Is the structure similar to diamond only more condensed? Or, deep within the planet's core, does the diamond change into an entirely new and exotic structure? Collins’ experiments are the first steps to finding out.

So far the Lawrence Livermore group has only measured the physical properties of the diamond during compression, but not its chemical structure. Over the next year the team plans to take the next step and observe the crystal structure itself at the instant of peak pressure. “We’ve answered a few questions,” says Collins, “and we’ve opened up a whole bushel of new ones.”

The work could also lead to the discovery of exotic new materials. The pressures Collins and his team are creating have never existed naturally on Earth before (except, perhaps, during large meteorite impacts). So what happens to diamond under this new extreme of pressure? “Is there some sort of exotic material at an extreme condition that we can create in the lab?” says Hicks. “That’s the big potential technological discovery that’s waiting out there.”

But the overarching technological goal driving these kinds of experiments is not to create new materials or study distant planets but to work towards generating power from nuclear fusion, the process that powers the Sun. The lasers Collins used form part of the National Ignition Facility, the US attempt to kick-start a self-sustaining fusion reaction by crushing tiny pellets of nuclear fuel.

It is essential to the fusion program, Collins explains, to understand materials at very high pressures. Recreating the core of the diamond planet in the lab is one step towards recreating the cores of stars inside nuclear power plants.

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