Trying to see what happens during the last few fractions of a second of a National Ignition Facility implosion is a lot like trying to look at the sun with the naked eye. The intense x-ray glare from NIF’s “mini-sun,” as hydrogen atoms are compressed and heated to hundreds of millions of degrees, can wash out images needed to fully understand implosion performance.
“But imagine you could put on a pair of sunglasses that allow you to see just one super-specific color,” said LLNL physicist Gareth Hall. “It could mean instead of looking at the sun and being blinded, you could see all the beautiful detail—the sunspots, or the corona, or a solar flare—but only in that specific color. You’d be able to see that detail by looking at just that color, so that you didn't get overwhelmed by the light from all the other colors.”
To view the details of NIF capsule implosions at or near the peak of compression, NIF will use “sunglasses” called the Crystal Backlighter Imager (CBI) to pick out the specific color, or x-ray wavelength, of a backlighter placed behind the capsule. The CBI wraps around a NIF target perched on a target positioner so it’s able to capture a backlit radiograph of the target as it implodes. The CBI’s curved crystal filters out all but a narrow wavelength of light and reflects the single-color, or monochromatic, image back past the target and onto an x-ray imager called a framing camera.
“The CBI is essentially like a very narrow-band filter,” Hall said. “It enables us to throw away nearly all of the self-emission and put our bandpass filter right over the backlighter atomic line that we’re using to take the radiograph.”
The atomic line, Hall explained, is the characteristic energy of emissions from an atom of a particular substance. Atoms and ions don’t emit light of every color, they do so at very specific energies,” he said. “And when you put an ion in a helium-like state (as happens when a backlighter is hit by NIF’s high-energy laser beams), it emits very strongly at a particular energy. We’ve coupled that very bright, unique emission energy to a very specific crystal” to produce an almost single-wavelength radiographic image while eliminating much of the background radiation.
Suppressing self-emission is critical because current imaging techniques are unable to fully resolve the last few hundreds of picoseconds (trillionths of a second) of an implosion known as the stagnation phase, when the target capsule is rapidly decelerating just before “bang time”—the instant of peak x-ray emissions (see “How NIF Targets Work”).
“NIF has other diagnostics which image the self-emission from the center of the capsule very nicely,” Hall said, “but the thing we can’t do at the moment is radiograph the shape and structure and integrity of the capsule shell right in its last stages, because that self-emission just completely overwhelms our current backlighter diagnostics. It’s really important that we know what’s happening there because something might be going very wrong in that last stage that we can’t see.”
A crucial element of implosion performance is symmetry: the shape of the imploding fusion fuel must remain as spherical as possible to maximize compression and form a central hot spot. Hall cited examples in which images of the capsule shell taken just before the self-emission washes out the data show an asymmetrical, pancake-shaped capsule—squashed horizontally—while later images of the hot spot in the same experiment are more sausage-shaped—squashed vertically.
“You Wonder What’s Going On”
“You would expect, if everything kept on going the same way, that the hot spot would look like it was squashed in the same direction as the capsule shell,” he said. “But sometimes it isn’t. Sometimes one looks like a sausage and one looks like a pancake. So you wonder what’s going on. Are we getting a sudden change of shape in that last moment that is really ruining things?”
Researchers also are studying the effects on implosion performance of two engineering features—the gossamer-thin “tent” that suspends the capsule in the hohlraum, and the tiny fill tube that injects the fusion fuel into the capsule. Both structures are believed to cause perturbations that adversely affect the implosion, but their full effects are masked by the self-emission flash, Hall said. More data could inform the research now under way to find alternatives to both features.
Fielding the CBI for its first tests earlier this year posed a number of engineering challenges, not least of which was maneuvering the unique structure into the center of the Target Chamber without risking damage to the target, backlighter, and target positioner. Since all the elements are on the same plane, the CBI uses a pivoted robotic arm that folds down to clear the target assembly as the CBI is inserted, then moves up and is locked into position for the shot.
Rapid yet precise alignment was another major challenge, Hall said. “You have to align this crystal very carefully,” he said. “You need to have the crystal at just the right place at just the right angle to make sure that you really have your bandpass right over your atomic line. And you need a straight line between your backlighter source, your experiment (the target), and the center of the crystal. If the crystal is off to one side you could miss half your image, or it’ll be blurred, or it could be much dimmer because you’re missing some of the radiation from the source. We need the crystal to be in position within the chamber to better than 200 microns in all directions.”
Aligning the crystal inside the Target Chamber would be too time-consuming for NIF’s demanding shot schedule, so the CBI was designed to allow the crystal to be aligned in an offline alignment station using optical lasers. “Once we’re happy that it’s aligned,” Hall said, “all we have to do is put the diagnostic snout in the right place” in the Target Chamber, “and the crystal automatically goes to the right place. This structure allows us to use this diagnostic in a time scale which is appropriate for the NIF facility.”
The early tests of the CBI are being conducted at a backlighter energy that is already a record high for this kind of crystal-based diagnostic. For now, a standard NIF framing camera, which can capture only a single image for each shot, is used. As the diagnostic evolves it will operate at even higher energies, using different backlighter and crystal materials, to provide clearer images at later stages of the implosion and eliminate more lower-energy self-emission. New framing cameras are under development that will be able to take multiple images so that CBI can record various stages of the implosion on a single NIF shot.
Development of the CBI took less than two years. Hall laid down the physics foundations toward the end of 2015, and engineering design and fabrication began early last year. Toward the end of 2016, scientist Christine Krauland of General Atomics joined the team. The project required modifications to the diagnostic instrument manipulator as well as software additions to NIF’s computerized control system. “Many different teams of people came together to make this work,” Hall said. “The fact that it worked so beautifully on the first shot, when we did it so quickly and under such pressure, was through the efforts of the engineers and alignment team. They really pulled it out of the bag for this one.”
NIF experiments support the National Nuclear Security Administration’s Stockpile Stewardship Program to ensure the safety, security and reliability of the nation’s nuclear deterrent, while also providing scientists from around the world with unique conditions of heat and pressure for fundamental science studies.