New measurement of diamond phase change could mean increased ICF energy gain

Researchers at Lawrence Livermore National Laboratory have measured how diamond melts under extreme pressures, resolving a long-standing discrepancy between experiment and theory. In inertial confinement fusion experiments, where a diamond capsule is used to hold fuel, this refined understanding of diamond's phase change has the potential to triple energy gain, provided that other degradation mechanisms can be controlled.

"We were able to take tiny diamond samples and shock compress them to temperatures hotter than the surface of the sun and to pressures higher than the center of Neptune and Uranus-and still measure atomic structure, temperature, density, and optical reflectivity," said LLNL scientist Marius Millot.

The experiment: The team conducted laser-driven dynamic compression experiments at the University of Rochester's Laboratory for Laser Energetics (LLE), where they compressed a tiny diamond sample and captured information such as X-ray diffraction data that illuminates atomic structure, all in a billionth of a second. The results were published yesterday in Nature Physics.

"This was the first time that shock-compressed diamond was probed with X-ray diffraction all the way up to melting," said Millot. "These measurements are extremely difficult because carbon is a small and lightweight atom. It scatters very few X-rays, so the signal we needed to measure was quite faint."

Resolving an open question: Theorists have spent the past 20 years trying to reproduce a measurement of the melting temperature of diamond conducted by LLNL lab scientist Jon Eggert and colleagues. No matter what they tried, a roughly 20 percent discrepancy remained. The LLE measurement, however, agrees almost perfectly with simulations.

"While it was frustrating to discover that our original temperature measurements were off by more than 1,000 [Kelvin], it is exciting to see such a dramatic improvement in data quality with our new diagnostics," said Eggert.

Boosting ICF energy gain: At LLNL, home to the National Ignition Facility, understanding diamond under these extreme conditions is highly intertwined with exploring inertial confinement fusion, where tiny diamond capsules are used to hold a deuterium-tritium fuel mixture.

Lasers are used to send a series of shockwaves into the capsule, which then compresses the fuel. With enough compression, a fusion-driven implosion will occur. But it's a precise operation: If the diamond melts unevenly it changes how pressure is applied to the fuel, and those distortions can be amplified by hydrodynamic instabilities.

"If the non-uniformity becomes too important, the implosion will be disrupted and the fuel will not be compressed and heated enough to achieve ignition," Millot told Nuclear News.

To avoid this, NIF has been using a strong first shock that is guaranteed to melt the diamond, avoiding this degradation mechanism. Millot said studies in the early 2000s found that a first shock near 12 Mbar could achieve this, with some margin. A strong shock also allows the laser pulse to be shorter and more controlled, hitting the target before the hohlraum fills with plasma, after which it becomes more difficult to direct the laser energy along the equator of the capsule.

According to Millot, the downside is that a stronger first shock raises the entropy, reducing the maximum theoretical compression, which in turn reduces the maximum theoretical energy yield. If a slower first shock is used, the fuel would be more compressible, allowing a larger fraction of it to burn before it disassembles.

Millot said NIF has not extensively explored diamond ablator implosions with slower first shocks, but with their new understanding of diamond's phase change, the team may try it soon. According to the paper's supplementary information, the work supports a reduction in the first shock from 33-34 km/s to 24.5 km/s.

"We have started to design experiments to test this. Tuning the first shock is very easy to achieve with the exquisite capabilities of the NIF laser system. The challenge is mostly associated with making sure we understand the flows in the hohlraum and the laser-plasma interactions to maintain the spherical symmetry of the implosion even if the laser pulse is about 1 nanosecond longer," he said.