For 75 years, it has been a dream to reproduce solar-like fusion on Earth. Teams of scientists and engineers around the world have spent tens of billions of dollars on various fusion methods, but have long struggled to reach the "net energy gain" milestone.
Until a year ago, however, that all changed.
On December 5, 2022, at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL), the world's largest and highest-energy laser fired 192 laser beams and pointed them at a peppercorn-sized target, creating a tiny "sun" on Earth. After firing 2.05 megajoules of laser energy at the target, the experiment produced more fusion energy than it takes to ignite fusion fuel by generating 3.15 megajoules of energy output - a major scientific breakthrough in decades.
Breaking the Laser Energy Limit Again
Happily, the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) in the United States recently set another new record for laser energy - emitting 2.2 megajoules (MJ) for the first time on an ignition target.
The newly reported experiment, conducted on October 30, produced 3.4 MJ of fusion energy, achieved ignition, and produced the second highest neutron yield ever at NIF.
Gordon Brunton, director of the National Ignition Facility (NIF), said, "This record level of laser energy is an incredible accomplishment that took years of hard work to achieve. And it marks our fourth successful demonstration of fusion ignition at NIF. This work is fundamental to the Laboratory's mission, with new capabilities that can support the National Nuclear Security Administration's Stockpile Stewardship Program and hopefully bring us closer to the future of fusion energy."
On December 5, 2022, LLNL achieved fusion ignition for the first time. The second time was on July 30, 2023, when the NIF laser delivered 2.05 megajoules of energy to the target in a controlled fusion experiment, producing a fusion energy output of 3.88 megajoules, the highest energy gain achieved to date.The third time that the NIF laser achieved fusion ignition was on October 8, 2023, with a laser energy of 1.9 MJ and a fusion energy output of 2.4 MJ.
Key advances in nuclear fusion
"We are on a steep performance growth curve," said Jean-Michel Di Nicola, joint project director of the NIF and Photon Science Laser Science and Systems Engineering Organization, "Increasing the laser energy gives us more more leeway to address issues such as fuel capsule defects or fuel hotspot asymmetries. Higher laser energies help to achieve more stable implosions, which in turn lead to higher energy yields."
There is no doubt about the laser's ability to deliver this much energy. And the challenge is to protect NIF's precious optics from debris damage, says Bruno Van Wonterghem, operations manager at NIF: "The laser itself is capable of generating higher energies without making fundamental changes to the laser. We do all this to maximize damage control. After all, if there's too much energy without proper protection, your optics can be blown to bits."
NIF manages the only laser system in the world that operates above the damage threshold, a feat made possible in part by what's known as the Optical Recycling Loop.
Stronger lasers, better performance
Two major mitigation measures, completed in June 2023, were critical to delivering 2.2 MJ of laser energy to the target - the use of fused silica debris shielding on two-thirds of the NIF's beamlines and the installation of metal shielding on 32 lower-hemisphere beamlines, which, depending on the beamline, has reduced the rate of debris-induced damage by a factor of reduced by a factor of 10-100. These lower beamline optics receive the most debris from the target chamber due to gravity.
Other improvements include new anti-reflective coatings, vapor hexamethyldiazepane (HMDS) treatment, and increased capacity of the optical recovery loop. A new mitigating agent - a gray-edge blocker - solves a problem that scientists had not yet identified.
"There is a subset of beams that don't perform as well as others," says Di Nicola, "and we found that if we radically reduce the laser energy density by casting a shadow on one edge of the beamline, those beamlines perform better. We're still not quite sure what the root cause of the problem is, but we'll be actively investigating this in the future."
Solving the mystery came naturally to the scientists and engineers working on the world's most energetic laser system, the NIF, and OMST Chief Representative Tayyab Suratwala said, "We have scrutinized the laser damage and identified the mitigation measures were modeled and tested. However, each time we increase the laser energy, we enter unprecedented territory and reveal new mechanisms of damage."
More energy alone is not enough to sustain the National Institute of Science's incredible record of scientific breakthroughs.Di Nicola: "You need to swing that bigger hammer with control and skill. The laser pulse lasts only a billionth of a second, so you need to be very precise to get it just right."
To that end, the team recently completed deployment of the High Fidelity Pulse Shaping (HiFiPS) system, which enables more precise and accurate pulse shaping.HiFiPS is a project years in the making that enables better power balance and symmetrical control in implosions.
In another improvement, the team refurbished the device's optical fibers to make them more resilient to repeated neutron exposure. These fibers are used to accurately measure the laser pulses transmitted to the target. The refurbishment increased the signal strength by a factor of 10-100, allowing researchers to continue to "see" the laser's performance.
What are the hopes for the future?
Currently, the laser has delivered 2.2 megajoules of laser energy. The team went back to the research stage and carried out the same process after the first experiment that produced fusion ignition.
We're looking at the optics, assessing the damage, and understanding how often we can use this new capability," Suratwala said. In the meantime, we are celebrating this major achievement. It is the result of years of hard work by a large team within LLNL and many external partners."
