New measurement of diamond phase change could mean increased ICF energy gain – ANS/Nuclear Newswire

“We were able to take small samples of diamond and shock-pressure them to temperatures higher than the surface of the Sun and to pressures higher than the centers of Neptune and Uranus, and still measure atomic structure, temperature, density and optical reflectivity,” said LLNL scientist Marius Milot.

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

“This was the first time that impact-pressed diamonds had been examined using X-ray diffraction until melting,” Melott said. “These measurements are very difficult because carbon is a small, lightweight atom. It scatters very few X-rays, so the signal we needed to measure was very faint.”

Solution to an open question: Theorists have spent the past 20 years trying to reproduce a measurement of diamond’s melting temperature made by LLNL laboratory scientist John Eggert and his colleagues. No matter how hard they tried, there was still a discrepancy of about 20%. However, the LLE measurement agrees almost perfectly with the simulations.

“Although it was disappointing to discover that our original temperature measurements varied by more than 1,000 degrees Celsius, [Kelvin]“It is exciting to see such a significant improvement in data quality with our new diagnostics,” Eggert said.

Enhanced ICF power gains: At LLNL, home of the National Ignition Facility, understanding diamonds under these extreme conditions is closely intertwined with exploring inertial confinement fusion, where tiny diamond capsules are used to hold a deuterium-tritium fuel mixture.

A laser is used to send a series of shock waves into the capsule, which then compresses the fuel. With enough pressure, a fusion-induced implosion will occur. But it’s a delicate process: If the diamond melts unevenly, it changes how pressure is applied to the fuel, and these distortions can be amplified by hydrodynamic instability.

“If the asymmetry becomes too significant, the implosion will be disrupted and the fuel will not be compressed and heated enough to achieve ignition,” Mellott said. Nuclear news.

To avoid this, NIF used a powerful first shock that ensured the diamond was dissolved, avoiding this dissolution mechanism. Studies in the early 2000s found that a first shock near 12 millibars could achieve this, with some margin, Melott said. The stronger shock also allows the laser pulse to be shorter and more controlled, reaching the target before the holoraum fills with plasma, after which it becomes difficult to direct the laser energy along the equator of the capsule.

According to Milo, the downside is that a stronger first shock raises the entropy, which reduces the maximum theoretical pressure, which in turn reduces the maximum theoretical energy output. If a slower first shock is used, the fuel will be more compressible, allowing more of it to be burned before it is broken down.

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

“We have begun to design experiments to test this. Tuning the first shock is very easy to achieve using the remarkable capabilities of the NIF laser system. The challenge is mostly related to making sure we understand the flows in the holoradium and the laser-plasma interactions to maintain the spherical symmetry of the implosion even if the laser pulse is about 1 nanosecond longer.”