Innovations that enabled LLNL’s fusion ignition breakthrough

In December 2022, the Laboratory celebrated a first-of-its kind accomplishment—fusion ignition at LLNL’s National Ignition Facility (NIF), the world’s most energetic laser system. While the experiment that achieved this historic breakthrough took only a fraction of a second, it is the result of work performed over several decades, by numerous LLNL staff, including dozens of scientists from LLNL’s Physical and Life Sciences Directorate (PLS).

PLS staff played key roles in enabling LLNL to achieve this first-ever demonstration of fusion ignition, the moment when the energy output is greater than the laser energy used to generate the fusion reaction. Building on more than 60 years of foundational research in physics, laser science, materials science, and nuclear science at LLNL, they developed innovative solutions in areas such as target design and fabrication, optics, experimental design, and diagnostics.

One example involves work done by a specialized team of PLS experts, who have spent the last two decades focusing on refining the design of NIF targets. The targets are composed of more than 100 specialized components, including the tiny, fuel-filled capsules at their core, which each measure only 2 millimeters in diameter. The group’s recent accomplishments include analyzing and refining design of:

• Hollow capsules specifically for fusion experiments. The team continually revamps capsule design in response to experimental results, identifying ways to mitigate imperfections that can cause implosion instabilities.
• The complex micro-assembly process used to fabricate targets, which focuses on creating capsules that can achieve precision performance under cryogenic conditions, as well as continually reducing the time needed to produce the targets.
• Materials that are strong enough to suspend the capsule inside a slim tube, yet they cause minimal experimental interference.
• Glass tubes used to inject hydrogen fuel into target capsules through a tiny hole drilled into the capsule’s shell. Reducing the size of fill tubes to just 2 microns in diameter, much smaller than a human hair, minimizes damage to the capsule.
• The cryogenic hydrogen fuel used in LLNL’s fusion experiments, including efforts to tune the fuel’s chemistry so that it forms a smooth, uniform layer on the capsule’s inside surface and remains frozen, at the density required for ignition.

PLS employees also made noteworthy contributions to the recent fusion ignition breakthrough by developing computational models of matter under extreme conditions and sophisticated diagnostics that enable scientists to analyze experimental data, refine the models, and improve experimental design. For example, they developed:

• Opacity and equation-of-state models capable of simulating ultrafast processes that occur during an ignition experiment, such as conversion of laser light to x rays, and the compression of capsule walls, which ultimately causes the fuel to ignite.
• An optical diagnostic known as VISAR, which is used to measure velocities and tune the timing of shocks produced by NIF lasers that compress capsules to a very high density.
• X-ray diagnostics that measure how capsules respond to pressure, including whether capsules can maintain their shape under extreme pressure.
• X-ray spectroscopy techniques used to measure the amount of ablator material that mixes with the fuel—which can make it more difficult to compress the capsule.
• Narrowband radiography that uses a crystal backlighter imager coupled to a special camera, enabling scientists to capture a backlit radiograph of the target as it implodes.

For more than a decade, experts at LLNL’s Nuclear Counting Facility (NCF) have used neutron yield diagnostics to assess NIF shots. Following each fusion experiment, NCF staff analyze coupons retrieved from NIF’s target chamber, using gamma spectrometry to quantify the number of neutrons emitted by the target. This reliable benchmark diagnostic was deployed the day after the recent ignition experiment, with NIF leaders waiting only an hour after handing off the coupons to NCF to obtain an initial assessment of the shot’s yield.

In addition, PLS materials scientists and engineers helped develop a strategy to ensure that LLNL has ongoing access to high-quality optics, capable of withstanding the increasing laser energy used in fusion experiments—including delivering 2.05 megajoules of energy to the target in the recent ignition experiment. Even the tiniest flaws, defects, and contaminants can absorb the laser light and initiate damage that can degrade the optic’s performance. For example, this multidisciplinary team developed:

• An optics recycling process, where experts inspect, clean, and repair damage to lenses and debris shields, avoiding the more expensive and time-consuming process of fabricating and installing new optics. Over the last 12 years, they have recycled more than 10,000 optics.
• Fused silica debris shields that reduce damage to optics during NIF experiments. A relatively new technique uses cone-shaped surface features to produce shadows that inhibit the growth of damage on the optic’s exit surface.

We are fortunate to have such a talented, dedicated team of experts in PLS who have explored fusion ignition from a variety of angles and contributed to the recent fusion breakthrough.