New Findings on Diamond Melting Could Transform ICF Targets

A new study of how diamond behaves under extreme pressure could open a path toward higher energy gains in inertial confinement fusion (ICF), offering researchers a potentially important way to improve the performance of fusion targets.

Scientists at Lawrence Livermore National Laboratory (LLNL) have made new measurements of diamond as it undergoes melting at pressures and temperatures far beyond ordinary laboratory conditions. The findings resolve a long-standing disagreement between experimental measurements and theoretical simulations and could allow researchers to rethink how diamond capsules are used in fusion experiments.

Diamond is a critical material in some ICF experiments because tiny diamond capsules, known as ablators, are used to contain a deuterium-tritium fuel mixture. At the National Ignition Facility (NIF), powerful lasers generate a series of shock waves that compress the capsule and its fuel until conditions suitable for fusion are reached.

The process requires extremely precise control. If the diamond melts unevenly during the implosion, variations in the way pressure is transferred to the fuel can create hydrodynamic instabilities. These distortions can prevent the fuel from reaching the temperatures and pressures needed for ignition.

Measuring Diamond at Extreme Conditions

The research team performed laser-driven dynamic-compression experiments at the University of Rochester’s Laboratory for Laser Energetics. Using powerful lasers, researchers compressed tiny diamond samples for approximately a billionth of a second while collecting information about their atomic structure, temperature, density and optical properties.

The experiments provided X-ray diffraction measurements of shock-compressed diamond all the way through its melting process. Researchers noted that the measurements were particularly challenging because carbon atoms are light and scatter relatively few X-rays.

The new measurements also help settle a scientific question that has remained unresolved for roughly two decades. Earlier measurements of diamond’s melting temperature differed from theoretical predictions by about 20 percent. The new experimental results, however, closely match modern simulations.

LLNL scientist Jon Eggert said the improved diagnostics revealed that earlier temperature measurements had been off by more than 1,000 kelvin, while also demonstrating how significantly experimental data quality has improved.

Why Diamond Melting Matters for Fusion

The findings could have a direct impact on the design of ICF experiments.

To ensure that diamond melts uniformly, NIF has traditionally used a relatively strong initial shock. Earlier studies indicated that a first shock of around 12 megabars could reliably melt the diamond and prevent nonuniformities from damaging the implosion.

However, there is a trade-off.

A stronger initial shock increases the entropy of the fuel, reducing its maximum possible compression and therefore limiting the theoretical amount of fusion energy that can be produced. A slower initial shock could allow the fuel to remain more compressible and potentially enable a greater fraction of the fuel to undergo fusion before the implosion disassembles.

The new understanding of diamond’s phase change could therefore allow researchers to use a weaker initial shock while still controlling the effects of diamond melting.

According to the research team’s analysis, the first shock could potentially be reduced from approximately 33–34 kilometers per second to about 24.5 kilometers per second.

Potential for Higher Energy Gain

The implications could be significant. According to the American Nuclear Society’s report on the research, the improved understanding of diamond melting has the potential to triple ICF energy gain, provided that other sources of performance degradation can also be controlled.

Researchers are already beginning to design experiments to test the concept. The NIF laser system provides considerable flexibility in adjusting the timing and strength of the initial shock.

The challenge will be maintaining the symmetry of the implosion. A slower first shock would require a laser pulse roughly one nanosecond longer, creating additional challenges involving flows inside the hohlraum—the small enclosure surrounding the fusion capsule—and interactions between the laser and plasma.

A Step Toward More Efficient Fusion Targets

The study does not by itself guarantee a threefold increase in fusion energy output. Instead, it provides researchers with a more accurate understanding of one of the key physical processes affecting diamond-ablator targets.

By resolving the uncertainty surrounding diamond’s melting behavior, scientists can refine simulations, adjust laser pulse designs and explore new approaches to compressing fusion fuel.

If upcoming experiments confirm that weaker initial shocks can be used without introducing unacceptable instabilities, the results could provide a relatively straightforward route to improving the performance of existing ICF target designs.

The research therefore represents more than a better measurement of diamond’s melting point. It gives fusion scientists a clearer picture of how the material behaves under some of the most extreme conditions achievable in the laboratory—and potentially a new way to extract more energy from laser-driven fusion experiments.

Source: ans