Researchers measure the structure of liquid carbon – an unprecedented achievement in fusion energy research

Liquid carbon, which may exist in the interiors of giant planets, plays a key role in the development of future nuclear fusion technologies – a clean and virtually limitless energy source. However, until now, very little was known about carbon in its liquid form, as it was practically impossible to study it in the laboratory. At atmospheric pressure, carbon does not melt but transitions directly to a gaseous state, becoming liquid only under extremely high pressures and temperatures.

It is known that laser compression can transform solid carbon into liquid within fractions of a second. “The challenge lies in using these fractions of a second to make measurements – and, in a previously unimaginable way, this has now become a reality using the DIPOLE 100-X, a high-energy laser capable of creating conditions with pressures of up to 10,000 tonnes per square centimetre and temperatures exceeding 10,000 degrees in a very short time”, explains Daniel Errandonea, professor of applied physics, researcher at the Institute of Materials Science of the University of Valencia (ICMUV) and co-author of the paper published in Nature.

Thanks to the capabilities of the DIPOLE 100-X, installed at the European X-ray Free Electron Laser (EuXFEL), the research team has, for the first time, combined powerful laser compression with ultrafast X-ray analysis and large-area X-ray detectors.

According to the article, in the experiment, high-energy laser pulses generate compression waves through a solid carbon sample, liquefying the material for just a billionth of a second. During this brief interval, the sample is irradiated with X-rays from the EuXFEL and the carbon atoms scatter the X-ray light in a way similar to how light diffracts through a grating. The resulting diffraction pattern enables researchers to extract valuable information about the distribution of the atoms in liquid carbon, how they are organised and their interactions.

The results reveal that the structure of liquid carbon resembles that of solid diamond, in which each carbon atom is surrounded by four other carbon atoms. This arrangement is also adopted in many organic compounds and is fundamental to the structure of numerous materials. The study also allows the determination of the melting point – crucial information for use in environments where temperature is a critical factor, such as fusion reactors. “Until now, theoretical predictions about the structure of liquid carbon and its melting point had diverged significantly. Accurate knowledge of these properties is essential for planetary modelling and the design of improved materials for nuclear fusion power generation”, concludes David Santamaría-Pérez, senior lecturer in applied physics, member of the ICMUV team, co-author of the article and member, alongside D. Errandonea, of the Advanced Materials Programme of the Complementary R&D&i Plan.

The study is an international collaboration led by researchers from the University of Rostock and the Helmholtz-Zentrum Dresden-Rossendorf. In addition to the Institute of Materials Science of the University of Valencia, contributors include the universities of Edinburgh and Oxford, the Los Alamos and Livermore national laboratories, the Sorbonne University, the Paul Scherrer Institute, and another 35 scientific institutions across Europe, the United States and Asia.

Reference:
D. Kraus, et al.: The structure of liquid carbon elucidated by in situ X-ray diffraction, in Nature, 2025. (DOI: 10.1038/s41586-025-09035-6)