One X-Ray Pulse Captures Plasma Structure From 10 to 500 Nanometers at Once
Japanese researchers used a stacked-foil filter to let a single camera see signals that normally require a beam blocker, opening a sharper view of laser-heated matter.
Physicists in Japan have shown that one X-ray pulse and one camera can capture how a laser-heated plasma changes across a wide range of sizes, a measurement that normally takes several instruments and many shots.
The work was led by Assistant Professor Masato Ota of the National Institute for Fusion Science, who did it as a graduate student at Osaka University, with Associate Professor Yoichi Sakawa of Osaka University's Institute of Laser Engineering. It was published in Communications Physics under the title "Beamstop-free small-angle X-ray scattering for multiscale structure measurement of high-energy-density plasma." Phys.org reported the results on October 24.
Small-angle X-ray scattering reveals structure by watching how X-rays bend as they pass through a sample. The trouble is that the main beam is vastly brighter than the faint scattered signal. Conventional setups put a small metal block, a beamstop, in front of the detector to stop the main beam, and that block also hides the lowest-angle signals that carry information about the largest features.
The team replaced the beamstop with a stack of thin metal foils that dims the light in steps. That widened the range of brightness one camera can record to more than 100 times that of conventional methods. It also let the researchers capture the low-order diffraction signals, labeled m = 0, ±1 and ±2, which beamstop-based methods cannot measure. Compared with control experiments, the measurable spatial and time ranges grew by about an order of magnitude each.
The experiment ran at the SACLA X-ray free-electron laser in Harima, Hyogo Prefecture, where an intense femtosecond laser heated the target. The target was an array of submicrometer silicon cylinders arranged in a regular pattern. The team tracked the collapse and spreading of the structure across sizes from 10 to 500 nanometers with a time resolution below a picosecond. The data shown were taken 7.5 picoseconds after the heating pulse arrived. From them, the researchers extracted the speed at which the plasma thinned out and its electron temperature.
Simulations added a practical finding. A few nanometers' increase in a target's initial size can cut the amount of laser energy the solid absorbs to about one-fourth.
The authors point to several uses: laser-driven ion acceleration, the interaction between plasma and walls in fusion reactors, and measurements on living cells, where repeating a shot on the same sample is not possible. Because the method needs only a single pulse, it suits situations in which the sample is destroyed by the measurement.
For fusion research in particular, understanding how tiny structures absorb and shed energy in the first trillionths of a second is a basic input to designing targets and wall materials, and a tool that sees all the relevant scales at once makes that job easier.
