One X-ray Pulse Captures Plasma Structures From 10 to 500 Nanometres
A single X-ray pulse and a single camera have captured how a high-energy-density plasma breaks down across spatial scales from 10 to 500 nanometres, with a time resolution below a picosecond. The international team, led by Masato Ota of Japan's National Institute for Fusion Science, reported the result on October 16 in Communications Physics.
According to the research group, this is the first time structures across such a wide range of sizes have been measured simultaneously with a single X-ray pulse and a single camera. Ota did the work as a graduate student at Osaka University, alongside Yoichi Sakawa of the university's Institute of Laser Engineering.
Why plasmas are hard to measure
Understanding instabilities and turbulence in plasmas is a key challenge in fields from nuclear fusion to astrophysics and laser-plasma physics. In plasmas, slight disturbances can grow because of instabilities, and turbulence transfers energy between structures of various sizes. Those structures, from very small to large, strongly influence the overall motion and energy flow of the plasma.
Because the structures vary with even slight differences in conditions, it is hard to reproduce the same state in every experiment. That creates a need for "spatially multiscale single-shot measurements" that capture a wide range of sizes in one measurement. In high-energy-density plasmas, instabilities grow in an extremely short time, so both high temporal and high spatial resolution are required to catch their initial stages.
A scattering method without a beam stop
The team combined a newly developed "beamstop-free X-ray small-angle scattering method" with an X-ray free-electron laser and a target made of an assembly of microscale rods, which was hit by a high-intensity femtosecond laser. In small-angle X-ray scattering, the pattern of X-rays scattered at small angles reveals the size of structures in the sample.
In the conventional version, the central part of the beam is shielded by a beam stop to protect the camera from the intense X-ray beam, and only the diffraction patterns around it are measured. This makes it fundamentally impossible to obtain signals from the ultra-small-angle region hidden behind the beam stop.
In the new method, a light-attenuation filter system made of stacked metal foils, optimised for thickness, size and shape, was used instead. It expanded the intensity range that a single X-ray camera can measure to more than 100 times that of conventional methods. The intense central beam is thus weakened to a level the camera can handle, so signals from a wide range of angles can be recorded at once.
What was measured
With this approach, the researchers measured the structural collapse and rarefaction (thinning out) of the plasma across spatial scales from 10 nm to 500 nm, with sub-picosecond time resolution. The experiment, which used a microscale rod assembly as the target, was a demonstration of the measurement method, and the paper's title refers to it as a measurement of the multiscale structure of high-energy-density plasma.
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