Working to Tame Disturbances in the NIF Force

To fuel NIF’s powerful target-capsule implosions, miniscule syringes called fill tubes inject hydrogen gas into the capsules that NIF’s lasers then obliterate in a flash of energy hotter than the Sun.

In a notable shift from previous practice, a research team led by physicist Vladimir Smalyuk is deliberately bending the tiny fill tubes that, until now, have been as straight as steel pipes. The team is experimenting with this diversionary bend in the target fabrication process to better diagnose and understand the importance of the fill tube to implosion performance, with the ultimate goal of increasing NIF’s neutron yield.

Smalyuk is experimental co-leader of the Inertial Confinement Fusion (ICF) Capsule Science Group in the NIF & Photon Science Directorate along with Harry Robey of the Lab’s Weapons and Complex Integration Directorate.

The bent fill tube is among a number of approaches researchers are taking to try to reduce the impact of fill tubes and other sources of hydrodynamic instabilities that interfere with NIF implosions. They have experimented with reducing the size of the fill tubes by as much as half, from 10 microns to 5 microns in diameter, in experiments that have been shown to boost NIF’s neutron yield significantly. Minimizing the effects of the gossamer-thin plastic membranes, or "tents," that suspend the fuel capsules in the hohlraum is another avenue to improve NIF implosions.

To put the size of these microscopic fill tubes in perspective, the size of a human hair ranges from about 30 to 100 microns thick. Additionally, the fill tubes are made of glass, making the tiny fuel lines extremely fragile.

The tubes always have been considered a necessary intrusion in ICF implosions, like a weld seam on a pipe. But tiny as fill tubes are, research shows they can cause significant hydrodynamic perturbations that can interfere with NIF’s performance.

The Capsule Science Group has set out to measure just how much of an impact fill tubes make on NIF conditions, and then modify the angles in the tube to minimize the perturbations. The fill tube campaign "will give a better understanding of what this fill tube does to performance," Smalyuk explained. "And if it (bending the tube) really mitigates the perturbation, it will give us improved performance. We don’t know without trying it."

The team includes members from the Capsule Science, Target Fabrication, and Integrated Implosions groups. They completed their first major experiment in the campaign in March to measure perturbations of the bent fill tube through a hohlraum diagnostic port. The team members are now developing their second experimental campaign, with the next experiment scheduled for March of next year.

Normally the fill tube exits the target capsule in a straight line and doesn’t line up with any diagnostic instruments, target engineer Steve Johnson explained. But to measure the perturbations caused by the fill tube, the fill tube hole must be aligned with a diagnostic port in the hohlraum so the diagnostics have direct access to the fill tube’s effects. The hohlraum’s diagnostic ports are covered by high-density carbon windows, and the fill tube can’t penetrate the windows, so the tubes must bend around the windows to exit the hohlraum.

Johnson’s team worked on the problem of how to actually bend the extremely delicate fill tubes, and were able to manipulate the needle-thin tubes to apply an approximately 14-degree bend angle and maneuver them around the diagnostic port windows.

The team’s main challenge was bending the glass tube while maintaining capsule orientation to allow measurements of the perturbations caused by the fill tube. Early test build attempts showed that, after bending, the fill tube would tend to relax and shift the capsule’s orientation, making the perturbations less visible during the experiment. The researchers evaluated and tested variations of tent designs until they observed no bend relaxation or capsule shift.

"We’ve never intentionally bent a fill tube before," Johnson said. "We wondered whether it was going to be like spaghetti, or a spring, and go back to straight shape; (or) is it going to affect the centering of the capsule, or do something weird?"

"It’s not a simple thing, getting these measurements," Smalyuk added. "That’s basic experimental science. You have your hypothesis, (and) you want to come up with experiments that test your hypothesis."

Working on the bent fill tube campaigns along with Smalyuk, Robey, and Johnson are: in Capsule Science: Chris Weber, Dan Clark, Bruce Hammel, Jose Milovich, Andrew MacPhee, and David Martinez; in Integrated Implosions: Laura Berzak Hopkins and Sebastien Le Pape; in Target Fabrication: Michael Stadermann, Sean Felker, Abbas Nikroo, John Bigelow, Scott Vonhof, Hans Truax, Silverio Diaz, Cresenciano Alday, Jaime Cortez, Steve Gross, and Thomas Bunn; and from General Atomics: Karina Kangas, Javier Jaquez, Neal Rice, and Jay Crippen.