A specialized instrument called the flexible imaging diffraction diagnostic for laser experiments (FIDDLE) has been designed to capture split-second, atomic-level changes in material properties.

This technology was developed specifically for the National Ignition Facility (NIF) experimental platform. 

FIDDLE is designed for laser-driven compression experiments. It subjects non-fusing materials to extreme temperatures and pressures (1 to 10 million times Earth’s atmospheric pressure) to capture atomic-scale changes as materials change phases.

Developed by Lawrence Livermore National Laboratory (LLNL) alongside Sandia National Laboratories and Advanced hCMOS Systems, the device recently captured a prestigious R&D 100 Award. 

Extreme precision snapshots

Carbon atoms can become a soft smudge of graphite or a diamond hard enough to cut steel. The difference is a matter of layout. At the subatomic level, a small shift in how atoms arrange themselves changes everything about how a material behaves.

Experts have long sought to observe these structural transformations in real time under extreme conditions. They couldn’t. The changes occur too fast, under pressures too violent for standard cameras to survive. But now, FIDDLE could solve this problem. 

FIDDLE is used for “dynamic compression” tests. Giant laser beams slam into a non-fusing target material, pushing it to pressures up to 10 million times greater than Earth’s atmosphere. 

This all happens in tens of nanoseconds. Under this crushing weight, the target’s internal crystal structure warps and shifts.

“A common way to appreciate the differences between material phases is to consider the physical differences between diamond and graphite,” explained LLNL physicist Cara Vennari. “Drastic differences in macroscopic material behavior are linked to shifts that take place on the order of an angstrom.”

To see these angstrom-scale shifts, FIDDLE uses a highly specialized technique. A secondary set of laser beams hits a nearby metal foil, generating a brief flash of X-rays. As these X-rays pass through the compressed sample, they bend and scatter. This creates an X-ray diffraction pattern.

Capturing that pattern is where older instruments faced challenges. Previous diagnostics could only manage one or two snapshots before the experiment ended or the equipment failed.

FIDDLE can do more. It packs up to eight custom hybrid CMOS sensors into a single, tightly clustered array. Perched just 50 millimeters from the target, these sensors capture four to eight distinct images in rapid succession. Interestingly, the timing between frames is two nanoseconds.

The result is a time-resolved sequence showing exactly how a material’s atomic grid evolves under stress.

Signal optimization

Building the instrument wasn’t easy. The inside of NIF’s target chamber during a laser shot is a chaotic nightmare of shrapnel, flying metal debris, and blinding background radiation. It also produces intense electromagnetic pulses capable of frying some electronics.

The engineering team had to build heavy shielding to isolate the 130-kilogram instrument from this hostile environment. Forced-air and water-cooling loops were added to prevent the densely packed sensor chips from overheating.

Early tests faced a frustrating obstacle: background X-rays were bleeding into the data and muddying the images. It was solved by focusing on the target housing itself. The team managed to shadow the sensors from rogue radiation simply by subtly shaving and angling the outer edge of the target body. The signal cleared up.

So far, the system has been tested on lead samples, comparing the captured data against known phase diagrams to perfect the instrument’s calibration. Next, the plan is to turn FIDDLE toward highly classified stockpile stewardship materials and elements critical to astrophysics.

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