Target Fabrication Steps Up to the Challenges

Every NIF experiment needs a target. That might seem obvious, but it’s far from routine. Fabricating targets for NIF is a multi-faceted process requiring constant adjustments to meet the changing demands of experimenters and to deal with new engineering and material science issues that have a way of cropping up unexpectedly.

And despite the challenges involved in designing, manufacturing, and testing a constant stream of tiny, custom-made, precision-engineered targets, the NIF & Photon Science Target Fabrication Team, in partnership with colleagues at General Atomics (GA) in San Diego, has been able to keep up with NIF’s steadily increasing shot rate (see "NIF Lasers Continue to Fire at a Record Rate") through several efficiency improvements of its own.

Following the example of the automobile industry and many other mass-production facilities, the LLNL Target Fab team is turning to robotics to automate such time-consuming processes as installing the ultrathin membranes called "tents" that suspend target capsules inside NIF hohlraums, and mounting the hohlraums in the cryogenic target positioners that hold them in the center of the Target Chamber. The team also has been using a precision robotic assembly machine to automate the assembly of NIF hohlraums since experiments began in 2009.

"The tent robot saves us about four hours a target," said NIF target development and fabrication manager Alex Hamza. "It takes us (an average of) 70 hours to build a target, so that’s about a five percent savings. Maybe five percent doesn’t sound like much, but when you’re talking 400 targets (a year), that’s another 20 targets, which is another 20 shots.

"The hohlraum insertion also saves us about four hours a target." Hamza said. "But the biggest thing is (a new) automatic proofing station" used to test cryogenically layered deuterium-tritium and tritium-hydrogen-deuterium targets before sending them to NIF. "We’re still running it through its paces and commissioning it, but when that comes on-line it should save us an additional eight hours per target, a more than 10 percent savings—or another 40 targets."

"When applied judiciously, bringing robotics into the picture can be very beneficial," said NIF Target Fabrication program manager Abbas Nikroo, who joined LLNL last month after leading the target fabrication effort at GA for the last six years. Nikroo said GA engineer Lane Carlson has led the automation of a number of assembly and other processes to speed the fabrication of capsules and other target components.

GA uses phase-shifting diffraction interferometer (PSDI) technology to make digital 3-D holographic images that allow "spheremapping" the exterior surface features of target capsules with nanometer (billionths of a meter)-scale precision. "That’s a process that an operator could do maybe six to eight times a day a number of years ago," Nikroo said. "By simplifying the procedure, GA got it up to about 20 per day with a trained operator. With the robotic system they can do up to 120 with the robot running overnight. So that was a big bottleneck that was solved."

Ignition capsules also go through a process similar to that used in Lasik eye surgery to remove tiny mounds, or "pimples," on the capsule shell. "If there are any pimples on the shell that can"t be polished, ahead of the polishing they cut down those pimples so that in the polishing process it can be completely planarized," Nikroo said. "That was a manual effort—someone had to go find the pimple, position it, take it to the laser, do the operation.

"Now it’s all on a robotic system where the shell can be moved around and mapped, and then the 4pi Laser-Polishing Station automatically determines if there are mounds on the shell that are too high. Based on the measurements the system aligns it, moves the microscope head out of the way and brings the laser in, cuts down the mound, and then brings the microscope back in, measures and confirms that it’s been trimmed, and then moves on. So now the operator is basically out of the loop."

These and other efficiencies are predicted to reduce the average fabrication time from 70 hours per target to about 55 hours by the end of this year. "At the beginning of Fiscal Year 2014 (October 2013) we (produced) about three and a half targets per week," Hamza said. "When we’re done we expect to be at five and a half targets a week by Christmas of this year." Annual production of targets for the wide variety of experiments conducted at NIF is predicted to grow from 380 now to 480 by the end of FY16.

Along with maintaining a high production rate, the Target Fabrication Team also must find ways to address several vexing issues that have arisen during NIF’s campaign to achieve ignition (see "Climbing the Mountain of Fusion Ignition"). "Currently there’s a big push to eliminate the tent," which is thought to cause instabilities in NIF implosions, Hamza said. "Figuring out how you can position the capsule to within ten microns in the center of the hohlraum without the thin polymer membrane supporting it is quite a challenge." Ideas include suspending the capsule with spider silk, or levitating it with magnets. "Levitation with magnets is very promising," he said, "but you’ll have to add some kind of a magnetic material to the targets, which may or may not be OK. I don’t know how thick the layer has to be to have the magnetic properties that you need."

Other challenges include fabricating depleted uranium hohlraums that are sturdy enough to use in experiments without the gold liners that normally support them; adding a silicon dopant to high-density carbon (diamond) capsules without creating excessive levels of silicon carbide; and avoiding non-uniform oxygen uptake in the plastic capsules used in the majority of current ignition experiments.

"We just learned in the last year or so that any kind of light in an oxygen environment will create oxidation of the plastic," Hamza said. "If (the capsule) would be taking up oxygen uniformly, that’d be one issue and we’d be worried about it, but it wouldn’t be so bad. We don’t know that this is true, but if it would be taking up oxygen azimuthally—non-uniformly—that could create a seed for instabilities."

NIF targets must be built to extremely rigid specifications (see "Targets"), but if they were all the same or similar, producing more than 400 a year would be relatively straightforward. But the team is asked to create 40 to 50 different types of targets, many consisting of entirely new designs with new serial numbers that are used in only a handful of experiments. "Experiments are one-off or a couple-off, and then you change the design a little bit based on the results of the experiment, and the serial number starts over again," Hamza said. "We (currently) produce 380 targets and at least 160 of them are (new) serial numbers" with unique requirements.

The target fabrication process starts with the physicists, who establish the target physics requirements for inertial confinement fusion, high energy density science, and Discovery Science experiments on NIF. Target science & technology experts and material scientists from LLNL and GA work closely with Livermore engineers to design the target, and the specifications are then provided to the GA technicians who fabricate the components. The final stage is assembly of the components in a 3,000-square-foot "class 100" cleanroom in LLNL’s Bldg. 381.

GA has held the contract to produce targets for LLNL and the other National Nuclear Security Administration laboratories since 1991. "The relationship between GA and LLNL has been beneficial, it’s been very close, and it’s been a good partnership," Nikroo said. "In various areas it’s very much seamless, where you could think of the GA operation as an extension of what Livermore is doing. Over the years we’ve tried to keep it away from becoming a supplier-and-vendor type relationship because understanding what is needed, what’s the experiment, why the part is being made this way, is just as crucial as getting the drawing—it’s important that that’s part of the communication."

Nikroo noted that while some target components are "off-the-shelf" items from other sources, GA, like LLNL, produces many specialty items—"components and parts that you just can’t get elsewhere." GA also puts "a lot of effort into internal R&D investments that look for targets of the future," he said. "That’s something I’m hoping to foster even more—having them do some of the R&D that we may not have enough resources and funding to do at Livermore."

Recently Schafer Corp. of Arlington, Virginia, has re-engaged in target fabrication for the NNSA effort, including work on NIF targets through on-site assembly support at LLNL as well as fabrication of components at its facility in Livermore. Schafer brings additional capabilities which will be utilized as target demand for NIF experiments continues to grow in the coming years.