One X-ray Pulse, One Camera: Tracking Plasma Collapse Across 10 nm to 500 nm

At SACLA, the X-ray free-electron laser in Harima, Hyogo Prefecture, a team of researchers fired an intense femtosecond laser at a tiny silicon target and watched it turn from solid to plasma. The target was an assembly of submicrometer-scale cylinders arranged in a regular pattern. The measurement that followed, described in Communications Physics, captured structures from 10 nm to 500 nm in a single X-ray pulse on a single camera. The researchers say this is a first.

The problem they set out to solve is an old one in plasma research. Instabilities let slight disturbances grow, and turbulence moves energy between structures of different sizes. Together they shape how a plasma moves and how its energy flows, which matters in nuclear fusion, astrophysics and laser-plasma physics. Because these structures shift with even small differences in conditions, no two experiments reproduce the same state. That makes it necessary to measure many scales at once, in one shot. In high-energy-density plasmas, instabilities also grow extremely fast, so catching their early stages demands high resolution in both time and space.

The group was led by Assistant Professor Masato Ota of the National Institute for Fusion Science, who was a graduate student at Osaka University's Graduate School of Science when the work was done, together with Associate Professor Yoichi Sakawa of Osaka University's Institute of Laser Engineering. Their key advance is a beamstop-free method of small-angle X-ray scattering.

Conventional small-angle scattering places a beamstop at the center of the beam to shield the camera from the intense X-rays, and records the diffraction pattern around it. The cost is that signals from the ultra-small-angle region, hidden behind the beamstop, cannot be obtained. The team replaced the beamstop with a light-attenuation filter system built from stacked metal foils, with thickness, size and shape optimized. It widens the range of intensities a single X-ray camera can handle to more than 100 times that of conventional methods.

The design follows from how the target scatters light. X-rays diffracted by the individual rods interfere and form periodic peaks on a distant detector, and the peaks weaken by several orders of magnitude from low order to high order. The filters are thicker toward the center, cutting transmission where the bright low-order peaks land. As a result, low and high orders fit within the detector's range together. The team reached low-order signals, m = 0, ±1 and ±2, that beamstop methods cannot record, and these carry information on the plasma's macroscopic structure. Compared with conventional control experiments, the measurable spatial and temporal domains each grew by about one order of magnitude.

Comparing solid and plasma targets showed that higher-order diffraction peaks weaken in the plasma state, a change that encodes the plasma's size. The researchers varied the delay between the femtosecond laser and the X-ray pulse arriving at the target, which let them follow the dynamics on picosecond scales and derive the plasma's rarefaction velocity and electron temperature. Comparing experiment with numerical simulation, they found that an increase of just a few nanometers in the initial plasma formed on a microstructured solid surface cuts the solid's absorption of laser energy to roughly one-fourth.

That result bears on laser ion acceleration, which depends on efficient energy transfer to the target. The team also sees the method as a platform for studying how solids break down into plasma, with possible use in examining plasma interactions with reactor walls in fusion devices. Beyond plasma physics, it could suit samples such as living cells, where repeated measurements are hard and the state changes over time, since it captures many length scales with very few X-ray exposures.