At the National Ignition Facility (NIF), researchers direct the energy from 192 lasers to initiate nuclear reactions at higher energy and density than possible elsewhere. Study of these reactions will advance understanding of the workings of stars and giant planets, fusion energy, and nuclear weapon stockpiles. At the American Chemical Society national meeting in Philadelphia last month, researchers described some of the equipment and tests they are using to analyze reactions at NIF.
Located at Lawrence Livermore National Laboratory (LLNL) in California, NIF was completed in 2009. Since then, NIF staff has slowly ramped up the energy delivered by the lasers; the highest-energy “shot” to date reached 1.89 MJ earlier this year.
To set off a reaction, NIF lasers are directed at a small gold capsule, called the hohlraum, that surrounds a fuel pellet containing deuterium and tritium. The hohlraum capsule is a cylinder 9 mm long and 3 mm wide; the fuel pellet is about 2 mm in diameter. The laser beams heat up the hohlraum, turning the inner walls into plasma that radiates X-rays. The X-rays in turn heat the outer layers of the pellet, which implodes and ignites, combining the deuterium and tritium into helium. If researchers at NIF can get the fuel to produce more energy than went into it, then fusion could become a viable technology for commercial power plants.
But first, NIF researchers have to understand what’s happening inside the hohlraum.
One way they’re doing so is by tracking neutrons produced by deuterium-tritium fusion. At the meeting, LLNL researcher Darren L. Bleuel discussed a tracking strategy that involves placing zirconium disks around the reaction chamber. One stable isotope of zirconium, 90Zr, absorbs neutrons at energies greater than 12.1 MeV; neutrons created in deuterium-tritium fusion have energies around 14.1 MeV. A 90Zr nucleus absorbs one high-energy neutron and then kicks out two lower-energy neutrons. That leaves 89Zr, which has a half-life of about 3.2 days and can be detected by measuring gamma ray emission as it decays to stable yttrium-89.
After a perfect laser shot at a perfectly prepared target, Bleuel and colleagues expect to detect neutrons spread evenly in all directions around the reaction chamber. Analyzing actual shots at NIF so far, Bleuel and colleagues have picked up asymmetric neutron emission patterns. That result indicates that some of the fuel pellets themselves are asymmetric. “It was kind of surprising to everybody, seeing double or even triple the amount of fuel on one side compared to the other,” Bleuel said. To achieve ignition, pellets must be perfectly symmetric. Other NIF researchers are working to improve the consistency of target preparation.
Other diagnostic tests in the reaction chamber look for the reactions of neutrons with material inside the hohlraum. LLNL chemist Carol A. Velsko described using xenon tracers such as 124Xe, 126Xe, and 128Xe implanted in the pellet. The xenon isotopes may either capture a neutron or capture a neutron and release two, just like Bleuel’s zirconium. “The different reactions have different probabilities as a function of energy,” Velsko said, giving scientists more information on what exactly is happening within the deuterium-tritium fuel.
After an NIF shot, xenon released from the pellet winds up as a gas in the reaction chamber. Velsko and colleagues pump gaseous material out of the chamber, separating unwanted material such as hydrogen and its isotopes, nitrogen, and oxygen from the xenon. Once the xenon is collected, the researchers use gamma ray detection to measure the isotopes produced in the reaction. They’ve demonstrated that they can collect as much as 50% of the expected xenon products. “We were hoping that we would get something greater than 25%, so actually getting 50% was quite gratifying,” Velsko said.
Now her team wants to use just 124Xe and implant it in the pellet at different depths to see what additional information they can obtain. The researchers also hope to add in krypton, which reacts with neutrons similarly to xenon. Planting the different tracers in different places would provide additional sensitivity to the diagnostic approach, Velsko said.
The team would also like to add iodine-127 to the pellet to see if material from the shell is falling off and mixing into the fuel plasma. Such mixing would cool the plasma and reduce the nuclear yield. In these tests, 127I mixing with the fuel would react with deuterium or tritium to form 127Xe. Likewise, bromine-79 or -81 would react to produce 79Kr or 81Kr.
Another method for analyzing what happens in NIF shots is what LLNL chemist Julie Gostic called “trash to treasure”: collecting and analyzing solid debris in the chamber. Much of the equipment in the reaction chamber is protected by blast shields, so Gostic and colleagues started by analyzing what landed on the shields, either by reacting the material with neutrons and detecting resulting gamma emissions or by dissolving the material and using mass spectrometry to identify the elements. They found that they could quantitatively account for the gold that went into the hohlraum capsules. “There’s only on the order of 100 mg of gold, spread over a 10-meter-diameter chamber,” Gostic said. “It’s amazing that the blast shields get enough material to analyze.”
But using blast shields isn’t ideal, because they contain contaminating elements that complicate analysis. So Gostic and coworkers developed their own collection materials, such as niobium or tantalum foils, that they now attach to the blast shields or other equipment in the reaction chamber. They continue to look for gold and can also analyze for germanium or yttrium if hohlraums are doped with it. The elements from the hohlraum react with neutrons, alpha particles, deuterium, and tritium, giving clues to what exactly is going on in the fuel. For example, comparing the number of gold atoms that added a neutron to those that released two neutrons yields information on the density of the plasma and how much hohlraum material mixed with the fuel.
Gostic and colleagues also look at the physical effects on the foil surface, such as whether it melted or how deeply it ablated. Like Bleuel’s team, Gostic’s has also seen asymmetric spread of shot debris. Their analysis shows more gold debris hitting foils facing the middle of the hohlraum than the ends. At the same time, they see more shrapnel from the target holder and other chamber structural material on foils facing the hohlraum ends. “We don’t know if it’s from the capsule design or how it breaks apart during the explosion,” Gostic said. “We’ve started modeling it.”
Los Alamos National Laboratory chemist Robert S. Rundberg, who was one of the symposium organizers, described another approach to collecting solid debris. It involves reflecting plasma off a sacrificial material to cool it down, then collecting the matter on another foil. A detector placed behind the collection foil can look for short-lived products from reactions.
The method has not yet been tried at NIF, but Rundberg and colleagues use it at the Omega Laser Facility, part of the University of Rochester’s Laboratory for Laser Energetics. Omega shots work differently from NIF shots in that the lasers directly compress and ignite a fuel pellet that is coated in some sort of shell. Rundberg spoke about experiments involving a fuel pellet contained in beryllium-9. 9Be reacts with a neutron to produce a proton and lithium-9, which has a half-life of 178 milliseconds and is detected by beta decay. If the fuel burns as the shell is driven in by the lasers, the beryllium-neutron reaction rate increases. If the fuel burns late, when the shell is expanding again, then the reaction doesn’t happen.
Solid debris collection, as well as the other techniques for analyzing what happens in NIF-driven reactions, all provide different insight into how the nuclear reactions proceed. As work at NIF continues and expands, these techniques will also help provide deeper understanding of a variety of nuclear reactions, from the fusion of deuterium and tritium to the stellar reactions that produce the heavy elements found in our solar system.