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

In a groundbreaking discovery, an international research team led by Assistant Professor Masato Ota from the National Institute for Fusion Science has successfully captured plasma nanostructures across a vast spectrum of scales using a single X-ray pulse and a single camera. This achievement marks the first time that structures spanning from 10 nm to 500 nm have been measured simultaneously, offering unprecedented insight into the complex world of plasma instabilities and turbulence.

Traditional small-angle X-ray scattering methods utilize a beam stop to shield the central portion of the X-ray beam, preventing damage to the camera. However, this setup inherently obstructs the detection of ultra-small-angle signals, which are crucial for detailed analysis. To overcome this limitation, the researchers employed a novel beamstop-free X-ray small-angle scattering method.

This innovation involves a sophisticated light-attenuation filter system, consisting of stacked metal foils precisely engineered for optimal performance.

By utilizing a high-brightness, short-pulse X-ray beam from the SACLA X-ray free-electron laser in Harima, Japan, and a femtosecond pulsed laser at the facility, the team transformed a silicon microscale rod assembly target—a series of submicrometer-scale cylinders—into a plasma state in mere femtoseconds. The resulting structural collapse and rarefaction processes were meticulously observed, with the X-ray beam diffracted by the individual rods and the diffracted waves interfering to form periodic peak signals detectable on a distant detector.

The key to their success lay in the strategically designed attenuation filter system, which progressively thickened toward the center of the detector to selectively attenuate high-intensity low-order diffraction peaks. This allowed the researchers to capture and measure a broad spectrum of diffraction peaks—from low to high order—within the detector's dynamic range, revealing critical insights into the macroscopic structure of the plasma.

The study's findings indicate a significant reduction in the intensities of higher-order diffraction peaks when comparing solid-state and plasma-state rod assembly targets. This change in signal intensity contains vital information about the plasma's size. By varying the time difference between the arrival of the femtosecond laser pulse and the X-ray free-electron laser at the microscale rod assembly, the researchers were able to elucidate picosecond-scale dynamics, determine plasma rarefaction velocity, and electron temperature.

Moreover, the experimental results, corroborated by numerical simulations, demonstrated that even a slight increase in the size of the initial plasma—just a few nanometers—can drastically reduce the laser energy absorption rate by the solid to approximately one-fourth. These findings not only advance our understanding of plasma instabilities and turbulence but also pave the way for more precise measurements and control in high-energy-density plasma research.