Single X-ray pulse reveals plasma nanostructures across scales with one camera

Single X-ray pulse reveals plasma nanostructures across scales with one camera Sadie Harley Scientific Editor Robert Egan Senior Editor Understanding instabilities and turbulence in plasmas is one of the key challenges across a wide range of plasma research fields, including nuclear fusion, astrophysics and laser-plasma physics. In plasmas, slight disturbances can grow due to instabilities, and energy is transferred between structures of various sizes due to turbulence. These phenomena give rise to structures ranging from very small to large, which significantly influence the overall motion and energy flow of the plasma. Since structures generated by instabilities and turbulence vary with even slight differences in conditions, it is not easy to reproduce the same state in every experiment. Therefore, there is a need for "spatially multiscale single-shot measurements" that can simultaneously capture structures of a wide range of sizes in a single measurement. In particular, in high-energy-density plasmas, instabilities grow in an extremely short time, so high temporal and spatial resolution is also required to capture their initial stages. Capturing multiple scales in one X-ray pulse By combining the newly developed "beamstop-free X-ray small-angle scattering method" with an X-ray free-electron laser and a microscale rod assembly target, an international research group, led by Assistant Professor Masato Ota of the National Institute for Fusion Science (then a graduate student at the Graduate School of Science, Osaka University), Associate Professor Yoichi Sakawa of the Institute of Laser Engineering, Osaka University, succeeded in measuring the structural collapse and rarefaction processes of high-energy-density plasma across a wide range of spatial scales—from 10 nm to 500 nm—with sub-picosecond temporal resolution. The findings are published in the journal Communications Physics. This is the first time in the world that structures across such a wide range of spatial scales have been measured simultaneously using a single X-ray pulse and a single camera. Revealing signals hidden by the beamstop In conventional small-angle X-ray scattering, the central portion of the beam is shielded by a beam stop to protect the camera from the intense X-ray beam, and the small-angle diffraction patterns appearing around it are measured. However, with this method, it is fundamentally impossible to obtain signals from the ultra-small-angle region hidden by the beamstop. In the newly developed beamstop-free X-ray small-angle scattering method, a light-attenuation filter system—composed of stacked metal foils optimized for thickness, size and shape—was employed to expand the intensity range measurable by a single X-ray camera to more than 100 times that of conventional methods. This allows the intense X-ray beam at the center to be attenuated to the camera's measurable range, enabling the simultaneous acquisition of a wide range of diffraction signals—including those from the ultra-small-angle region—from the target, thereby revealing microstructures that could not previously be measured. Tracking laser-driven collapse at SACLA The experiments were conducted using a high-brightness, short-pulse X-ray beam generated by the SACLA (SPring-8 Angstrom Compact free electron LAser) X-ray free-electron laser in Harima, Hyogo Prefecture, and a high-intensity femtosecond pulsed laser installed at the same facility. The researchers used a high-intensity laser to instantaneously transform a silicon microscale rod assembly target—consisting of submicrometer-scale cylinders arranged in a regular pattern—from a solid to a plasma state, and observed the process of structural collapse and rarefaction as changes in the X-ray small-angle scattering signal on picosecond time scales and nanometer spatial scales. The X-ray beam directed at the rod assembly is diffracted by the individual rods, and the diffracted waves interfere, forming characteristic periodic peak signals on a distant detector. The intensity of the diffraction peaks decreases by several orders of magnitude from low to high order. Therefore, to selectively attenuate the relatively high-intensity low-order diffraction peaks, the team installed an attenuation filter system in front of the detector, in which the filters were made thicker toward the center to reduce transmittance. This allowed the team to detect a wide range of diffraction peaks, from low to high order, within the detector's dynamic range and measure them simultaneously. The researchers were able to access low-order signals such as m = 0, ±1 and ±2—which could not be measured using conventional methods employing beam stops—and captured information on the macroscopic structure of the plasma. As a result, compared to conventional control experiments, the team succeeded in expanding the measurable spatial and temporal domains by approximately one order of magnitude each. Nanometer-scale changes affect laser energy absorption A comparison of the small-angle X-ray scattering results for the solid-state and plasma-state rod assembly targets reveals that the intensities of higher-order diffraction peaks are reduced in the plasma state. This change in signal contains information about the plasma size. Therefore, by performing measurements while varying the time difference between the arrival of the high-intensity femtosecond laser and the X-ray free-electron laser at the microscale rod assembly target, the team elucidated the picosecond-scale dynamics and derived the plasma rarefaction velocity and electron temperature. Furthermore, by comparing experimental results with numerical simulations, they demonstrated that even an increase of a few nanometers in the size of the initial plasma formed on a microstructured solid surface reduces the absorption rate of laser energy by the solid to approximately one-fourth. Applications from fusion reactors to living cells The developed beamstop-free small-angle X-ray scattering method is a measurement technique for elucidating the formation and evolution of structures of various sizes arising from plasma instabilities and turbulence. These results have made it possible to use this method to investigate in detail the fine structure of the plasma that governs laser energy absorption. These findings also contribute to our understanding of laser ion acceleration, which requires efficient energy transfer to the target, and are expected to lead to further advancements in laser-plasma research. Furthermore, this technique has the potential to serve as a new measurement platform for investigating the fundamental processes by which solids disintegrate and transform into plasma; in the future, it is expected to find applications in areas such as the interaction between plasma and reactor walls in fusion reactors. This measurement technique can also be applied to fields beyond plasma physics. For example, even for samples—such as living cells—where repeated measurements are difficult and the state changes over time, it becomes possible to capture structures across a wide range of spatial scales with a minimal number of X-ray exposures. By leveraging these characteristics, the technique is expected to find applications in a variety of fields. Publication details Masato Ota et al., Beamstop-free small-angle X-ray scattering for multiscale structure measurement of high-energy-density plasma, Communications Physics (2026). DOI: 10.1038/s42005-026-02872-5 Journal information: Communications Physics Key concepts Nonlinear phenomena in plasmasPlasma production & heatingPlasma diagnostic techniquesX-ray techniquesProvided by National Institutes of Natural Sciences