World’s first simultaneous measurement of nanometer- to submicrometer-scale structures with a single camera using a single X-ray pulse

Background

 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 of various sizes, ranging from very small to large, which significantly influence the overall motion and energy flow of the plasma.

 On the other hand, 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.

Results

 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, our research group 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.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.

 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.

 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. A conceptual diagram of the experiment is shown in Figure 1. We 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 and nanometer time-space scales.

 Figure 2 shows a schematic diagram of the beamstop-free X-ray small-angle scattering method. The X-ray beam irradiated onto the rod assembly is diffracted and interfered by the individual rods, 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, we 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 us to successfully detect a wide range of diffraction peaks, from low to high order, within the detector’s dynamic range and measure them simultaneously. In particular, we 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, we succeeded in expanding the measurable spatial and temporal domains by approximately one order of magnitude each.

 Figure 3 shows the experimental results obtained using this method. A comparison of the small-angle X-ray scattering results for the solid-state and plasma-state rod assembly targets reveal that the intensities of higher-order diffraction peaks are reduced in the plasma state. In fact, 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, we elucidated the picosecond-scale dynamics and derived the plasma rarefaction velocity and electron temperature. Furthermore, by comparing experimental results with numerical simulations, we 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.

Significance and Outlook

 The developed beamstop-free small-angle X-ray scattering method is a crucial 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.

 Furthermore, this measurement technique can 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.

Glossary

・Plasma Instability

A phenomenon in which slight disturbances in density or electromagnetic fields within a plasma grow over time. Initially very small disturbances are amplified by electromagnetic interactions with the electrons and ions that make up the plasma, eventually developing into structures with characteristic sizes and shapes.

・Turbulence

A state in which vortices of various sizes, as well as disturbances in density and electromagnetic fields, arise within a plasma and interact with one another in complex ways. As energy is transferred between structures of different sizes, a wide range of structures—from very small to very large—emerge.

・Plasma

Known as the “fourth state of matter” following solids, liquids, and gases, this is a state in which, for example, by further heating a gas, electrons are separated from atoms, allowing electrons and ions to move freely.

・Nuclear Fusion

A reaction in which light atomic nuclei, such as those of hydrogen, fuse to form heavier atomic nuclei. A large amount of energy is released during this process. This is the source of the Sun’s energy, and research toward fusion power generation based on this principle is being conducted on a global scale.

・Laser-Plasma Physics

A field of physics that studies the plasma generated when high-intensity, high-power lasers are directed at matter, as well as the interactions between lasers and plasma. It investigates how the laser’s energy is transferred to the plasma, how the plasma is heated and expanded by that energy, and how electrons and ions move within it.

・High-Energy-Density Plasma

A plasma characterized by extremely high temperatures and pressures, with a large amount of energy concentrated within a very small space.

・X-ray Free-Electron Laser

A device that accelerates electrons to near the speed of light and generates extremely intense X-rays from those electrons. It is characterized by its ability to produce X-ray pulses that are extremely bright and have very short durations compared to conventional X-ray sources.

・Microscale Rod Assembly Target

An assembly created through the integration of nanoengineering and materials science, in which the diameter, height, and spacing of the rods are controlled and designed on the nanometer scale. As a bulk structure with a large specific surface area, it exhibits unique linear and nonlinear optical dispersion, such as the penetration of the laser field into the bulk beyond the average cutoff density.

・Diffraction

A phenomenon in which waves spread out upon striking a structure, and these waves interfere with one another to create characteristic patterns such as fringes.