Key Takeaways
1. Researchers captured detailed X-ray diffraction images of liquid carbon at extreme pressures, revealing a temporary fluid with tetrahedral bonds and fewer nearest neighbors than expected.
2. A transition from amorphous carbon to diamond occurs around 80 GPa, followed by melting into liquid at approximately 160 GPa, with experimental estimates aligning with first-principles calculations.
3. The study provides crucial insights for inertial-confinement fusion (ICF), emphasizing the importance of understanding liquid carbon’s properties for improving fusion ablator designs.
4. The findings highlight potential differences in performance between crystalline and amorphous carbon coatings, leading to new ICF strategies that utilize lower-density, hydrogen-rich films.
5. The research offers valuable data for training machine-learning interatomic potentials, enhancing molecular-dynamics simulations of carbon during shock and enabling exploration of larger systems and longer timescales.
Researchers have successfully captured the first ever detailed X-ray diffraction images of liquid carbon at pressures close to one million atmospheres. Instead of finding a densely packed atomic mixture, they discovered a temporary fluid with tetrahedral bonds. Using the DiPOLE 100-X laser on glassy carbon and examining the shocked material with 18 keV pulses from the European XFEL, they found about four nearest neighbors per atom. This is significantly less than the expected dozen in typical liquids and creates a strong baseline for quantum-molecular-dynamics simulations of carbon under extreme conditions.
Transition to Diamond
The research team noticed a change from amorphous carbon to diamond around 80 GPa, which was then followed by total melting into liquid at about 160 GPa. By analyzing the diffraction data with Fourier methods, they calculated a first-shell coordination number of 3.78 ± 0.15 and observed a 7 percent volume increase at melting, both of which are in line with recent first-principles calculations. This information also provided an experimental estimate of the latent heat of fusion at roughly 130 kJ mol-1, confirming the positive slope of carbon’s melt curve at 11 K GPa-1 in this pressure region.
Importance for Fusion
This microscopic understanding is crucial for inertial-confinement fusion (ICF). Current ignition designs, such as the National Ignition Facility’s record-breaking attempt, depend on a high-density carbon (diamond) shell that uniformly compresses a deuterium-tritium target. This shell is pushed close to its melting point during the initial shock, and its properties—like strength, opacity, and heat capacity—play a vital role in the implosion process. A thorough understanding of the structure and equation of state of liquid carbon is essential for future fusion ablator designs and accurate hydrodynamic modeling.
The findings also point to the performance difference between crystalline and amorphous carbon coatings. New ICF approaches are looking into lower-density, hydrogen-rich amorphous films to reduce pre-heating and enhance implosion symmetry. The newly acquired data on liquid states provide a method to customize these films by adjusting porosity, optimizing optical depth, and choosing compositions that retain desirable melting properties under shock conditions.
Machine Learning Applications
In addition to aiding in target fabrication, the findings serve as an excellent training set for machine-learning interatomic potentials. This can significantly accelerate molecular-dynamics simulations of carbon during shock, enabling researchers to explore larger system sizes and longer timescales more easily than before.
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