Reducing the Time to Grow Good Cryogenic Layers
One of the most demanding aspects of preparing targets for NIF ignition experiments is growing the wafer-thin layer of solid deuterium-tritium (DT) fuel in the target capsule. Only 69 microns wide–about two-thirds the average width of a human hair–and frozen to 18.5 kelvins (minus-426 degrees Fahrenheit), the "DT ice" layer must be extremely smooth and free of grooves or other defects before the target can be approved for use in an experiment.
Cryogenically cooling the fuel to just below the freezing point of hydrogen, which changes hydrogen's phase from a gas to a liquid and finally to a solid, enhances the density of the fuel and improves the chances of achieving the required density for ignition. The ice layer takes an average of 14 hours of "script time" to grow, so rejection of a completed layer means at least a half-day delay in the experiment while a new layer is grown.
A large effort is under way by NIF target fabrication and cryogenic fielding teams to achieve a higher NIF shot rate through improved engineering, automation, training, and early detection of defective layers. Among the techniques being employed by NIF researchers to shorten the time between experiments is the use of x-ray images called "space-time plots" to evaluate the quality of a layer as it grows. The technique, developed by members of the Target Systems and Optics Technology (TSOT) Group, is "a kind of forensics," said Science & Technology Group Leader Tayyab Suratwala.
"The space-time plots make it easier to visualize the signature of the growing layer and decide early on whether it will be a good layer based on a series of metrics, such as early groove detection and the rate of meniscus (DT layer) height change during early freeze," he said. "The time to yield a good layer can be shortened by detecting a bad layer earlier rather than later. Since February, the space-time plots have been employed as dashboards, allowing for real-time decision-making and increased efficiency."