Engineering divisions provide expertise and support fusion energy’s future

In the 2000s, Lawrence Livermore National Laboratory (LLNL) engineer Steve Hunter was asked to work on the concept for an Inertial Fusion Energy (IFE) power plant, not because of any laser or electronics knowledge that he has, but due to his firearms expertise. He needed to figure out how to inject a stream of targets into the target chamber, so that a constant source of fuel was available. “The targets were a bit delicate, so they could only withstand a certain amount of acceleration. I calculated that we would need a 10-meter-long barrel to keep the acceleration within limits, and I designed an air gun based on the Gatling gun with a special rotary valve. Then I built a Plexiglas prototype in my garage and powered it with a shop vacuum.”

National security and energy needs go hand-in-hand, but typically not this directly.

While the scientific basis for achieving fusion energy was recently bolstered by NIF’s recent ignition achievement, LLNL’s National Security Engineering Division (NSED) and Defense Technologies Engineering Division (DTED), both part of the Engineering Directorate, have long histories of providing infrastructure for NIF’s operations and for the pursuit of fusion for stockpile stewardship as well as energy applications. This history is partly due to the expertise of these divisions’ engineers in handling volatile and rare materials. But it’s also attributable to the link between national security and energy production: energy dependencies make countries susceptible to each other’s priorities, and climate change threatens national ecosystems and economics.

Before his work on IFE, Hunter — who has been embedded at NIF off and on since 2004 — worked on a system that projects dark spots into each laser beam to minimize optics damage. Since defects in the optics absorb more energy, thereby causing damage sites to expand during subsequent full power shots, projecting dark spots into the beams blocks the defects from absorbing their energy. Optics need to be replaced when the damage grows too large, and since there are five final optics in each of 192 beams, with each costing approximately $50,000, it can be an expensive problem. The team had to develop a special liquid crystal using a material that was in such short supply that they bought nearly the entire world’s stock, and Hunter was responsible for the electronics that projected an image of the dark spots onto this liquid crystal. Hunter said, “this was a very difficult project, and NIF wouldn’t work without it. But one thing I learned while working on this project was that there are many groups working on difficult problems, and all of them are required for NIF to function. I came to appreciate what an incredible science and engineering achievement NIF is.”

Another example of rarefied engineering expertise that quietly keeps the facility running is that of the team mitigating electromagnetic interference (EMI). Charlie Brown has been in NSED for the 20 years he’s been at the Lab, working mainly at NIF in the context of EMI.

Brown consults with NIF teams to characterize and help mitigate the EMI that occurs in NIF due to the motion of charged particles when the lasers hit the target and when ionizing radiation strikes objects in and around the target chamber. In such interactions, charged particles like electrons are spewed everywhere, and when they’re violently put in motion, electromagnetic fields are generated. Even some of the diagnostics generate their own EMI. As Brown said: “That's a bad thing in a facility where many diagnostics rely on electrical cables — you get interference, and it obscures the actual signal that you’re looking for, damages your instruments, or maybe worse: it perhaps gives you physics that aren’t real.”

The diagnostics he’s referring to include X-ray streak and framing cameras that look at the target, and which are the eyes of the physicists. These diagnostics are crucial, because they give the physicists the feedback that allows them to tune their models and get NIF to ignition.  

Since EMI is very sneaky — high frequency EMI in particular is very hard to defend against — Brown is highly alert to gaps or seams where metal surfaces are bolted together, and he is keenly attuned to the engineering tradeoffs that come with designing to defend against EMI. No matter how well designed an aspect of NIF is, it may require additional shielding based on simulations that show how much interference to expect from different leakage points.

While Brown and his team attempt to mitigate EMI in the existing fusion setup, NSED engineers led by John Moody are contributing to an LDRD (Lab Directed Research and Development Program)-funded project that harnesses the power of magnetic fields, called MagNIF. MagNIF, as the name suggests, involves magnetizing the fusion fuel at NIF to reduce heat loss from the compressed fuel core by constraining the motion of electrons and fusion-generated alpha particles. The capability, when completed, could be one tool to help increase fusion yields by a factor of two or more, and increase the types of fusion experiments that can be done on NIF. In addition to potential yield enhancements, magnetic fields may also reduce the effect of key implosion degradations such as ablator-fuel mix and hot spot asymmetries, leading to a more robust implosion design.

During a NIF shot, the 192 lasers are focused on the hohlraum, generating X-rays that cause the target capsule to implode and causing the deuterium and tritium atoms inside to fuse. When this fusion takes place, helium nuclei (AKA alpha particles) are released into the surrounding fuel, and their energy — during ignition — results in rapid heating of the surrounding fuel and a cascade of fusion events. The initial spark of fusion in the imploded hot spot needs to be sufficiently strong to cause ignition, and a higher temperature hot spot can increase this initial fusion spark. As Moody noted at the start of the MagNIF project — prior to the December 2022 ignition milestone: “When you apply the magnetic field, the alpha particles remain longer in the hot-spot and cooling from electron thermal conduction heat loss decreases, making the hot-spot hotter, pushing the system closer to the self-heating regime….This is the advantage of using a B-field.”

DTED, the Laboratory’s Defense Technologies Engineering Division, also is in the business of enhancing the power of reactions and viability of shots. The Tritium Team, supervised by Clint Byington, calibrates and delivers the tritium-deuterium gas that surrounds the target and which has long been used in nuclear experimentation and design.

"Tritium is a constant in fusion experiments because of its reaction with deuterium,” Byington said. “Combined, the two gases produce a large amount of energy, amplifying the fusion potential between nuclei in inertial confinement fusion reactions.”

Because of the gases’ volatility, the Tritium Team does its work at LLNL’s Tritium Facility, a hazard category 3 radioactive nuclear space located within the Superblock compound. 

While some of the team’s requested fills get quite exotic and held to very tight tolerances, the gas fill that Byington’s team delivered to NIF for the December fusion shot was considered a “standard fill” at 50% tritium and 50% deuterium. Standard or not, each fill involves a dynamic process whereby extreme, repeatable precision is challenging because of constant fluctuations in the percentages of the product maintained on the team’s storage beds. Byington said: “It is incredibly satisfying to have played a role in this milestone, and we are increasingly motivated to continue providing precision gas mixtures and to ensuring that our contribution is consistently excellent.” 

-- Aimee Fountain