This year’s list of R&D 100 Award winners from Lawrence Livermore includes a bespoke x-ray diffraction (XRD) diagnostic built to reveal split-second changes to material properties at the atomic level. The flexible imaging diffraction diagnostic for laser experiments (FIDDLE) is built exclusively for use at the National Ignition Facility (NIF)—not for fusion ignition shots but for laser-driven compression experiments that subject different materials to extreme pressures and temperatures. Co-developers Sandia National Laboratories and Advanced hCMOS Systems were included in the award for this technology.
Observations from compression experiments add to fundamental physics knowledge and enable researchers to update computational physics models with new measurements and insights. However, compression events are challenging to analyze because scientific instruments must discern atomic-level structural changes unfolding at imperceptibly brief intervals. Moreover, these subtle signals must be distinguishable amid the damaging, energetic environment created by NIF lasers. The FIDDLE team integrated state-of-the-art sensors into a custom-built XRD diagnostic capable of taking rapid snapshots of diffraction patterns that reveal atomic-scale changes—all the while withstanding the destructive environment inside NIF’s target chamber.
Tracking Material Phase
When energetic pulses of light from NIF lasers strike a target designed for laser-driven compression of non-fusing material, they dynamically compress the target material to extremely high pressures—typically 1 million to 10 million times the Earth’s atmospheric pressure—in tens of nanoseconds. Over this brief period, the increasing pressure and density causes atoms to rearrange into different material phases; different phases, in turn, can modify the material’s macroscopic properties such as strength, compressibility, and thermal and electrical conductivity. For example, researchers compressing samples of lead usually look for signatures of the transition between its hexagonal close-packed phase and body-centered cubic phase. “A common way to appreciate the differences between material phases is to consider the physical differences between diamond and graphite,” says physicist Cara Vennari. “Both materials are made exclusively of carbon atoms, but their unique atomic arrangement makes one phase incredibly strong while the other phase is brittle. Drastic differences in macroscopic material behavior are linked to shifts that take place on the order of an angstrom (10-10 meters). The measurements we obtain using FIDDLE enable researchers to address fundamental science questions about how rapid phase changes occur in different materials.”
Using XRD measurements to map material density and phases in a sample, scientists can better predict how non-fusing and stockpile-relevant materials alike will react to extreme conditions at a macroscopic level. Yet, a single snapshot of a material phase can only reveal so much information about how the compression experiment transpired. When researchers obtain multiple snapshots of the experiment, they can analyze how pressure and material phase evolve in time over the course of compression. Gathering more data from each compression event reduces the risk of shot-to-shot variation, which cannot be avoided across multiple experiments. However, until FIDDLE, the XRD diagnostics capable of operating in this combination of spatial, temporal, and energy regimes could capture no more than two snapshots before the experiment’s end. FIDDLE accomplishes the extraordinary feat of capturing a standard of four, but up to eight, snapshots of each experiment with a temporal resolution of 2 nanoseconds, yielding an XRD “movie” of material phase during the compression event. Its snapshots reveal displacements 100 times smaller than the diameter of an atom.
Building a New Experimental Platform
At the heart of FIDDLE’s impressive speed and resolution is the instrument’s array of hybrid complementary metal-oxide-semiconductor (hCMOS) sensors. FIDDLE may have as many as eight custom-made Icarus2 hCMOS sensors, whereas no previous instrument had ever fielded more than two. These sensors were initially developed by Sandia National Laboratories to capture ultrafast XRD data, but exactly how the sensors should be integrated into a capable diagnostic platform presented significant engineering challenges for the Livermore team to resolve.
For time-resolved XRD measurements at NIF, x-ray light originates from a backlighter foil below the target and diffracts through the sample at different angles depending on the material’s density and phase during each sensor collection window. FIDDLE captures this pattern of diffracted x-ray light by holding its closely packed array of hCMOS sensors approximately 50 millimeters from the top of the target. Such close proximity is necessary to cover a wide range of diffraction angles while maintaining a sufficient signal-to-noise ratio. Supporting this large number of sensors are custom circuitry, logic, communications, and software control throughout the instrument. The hCMOS sensors required extensive multiplexing of trigger and monitor signals without degrading the precision of the timing, and the densely packed electronics also required a robust cooling system using both water and forced air.
Even if its sensors performed flawlessly during development, FIDDLE required shielding against the extreme conditions within the NIF target chamber during an actual experiment. Laser-driven compression experiments generate showers of fine metal debris and shrapnel as well as strong electromagnetic pulses that can upset nearby electronics. The research team met the need to design multiple physical and electromagnetic shields to protect the sensors from debris, electromagnetic pulses, and stray laser light, all the while accounting for the 130-kilogram instrument’s mass and its compatibility with the diagnostic instrument manipulator at NIF. (See S&TR, December 2010, Precision Diagnostics Tell All.) A parallel design effort with the NIF target fabrication team has focused on reducing sources of background x-rays, which initially muddied and obscured the sought-after diffraction signal on FIDDLE’s sensors. “We had to make adjustments across multiple NIF experiments to overcome these challenges,” says Vennari. While the central physics package that contains the compression sample was already well optimized, the target body holding the physics package underwent numerous design changes to further shield FIDDLE from background x-rays. Alterations as subtle as reshaping the outer edge of the target body brought noticeable improvements. The result has been steady improvement in the XRD signal-to-background ratio over time. Vennari says, “We overcame much of the signal-to-noise optimization issue by changing the target’s geometry. A simple change to the angle of the target’s outside edge allowed for increased shadowing of the high background to the sensors.” So far, the team has used lead as the compression sample while adjusting their designs and optimizing the signal-to-background ratio because they can compare their measurements to lead’s well-established phase diagram, but they will soon begin exploring phase transitions in other materials of interest for high-energy-density science.
By enabling researchers to obtain multiple, high-resolution XRD snapshots of split-second materials response, the team behind FIDDLE is equipping Livermore scientists—and the scientific community at large—with a valuable tool to extract more information from each NIF compression experiment. With more data to analyze and integrate into computational models, FIDDLE will go on to perform research supporting stockpile stewardship as well as fundamental science studies such as better understanding the cores of stars and planets.
—Elliot Jaffe
For further information contact Cara Vennari (925) 423-2111 (vennari1 [at] llnl.gov (vennari1[at]llnl[dot]gov)).
