Image Courtesy of Alex Dong.
Fusion – the ambitious goal of energy research and science fiction material from Iron Man to Star Wars. Though touted as the ultimate solution to our search for a clean and cheap energy source, practical fusion energy has eluded our grasp for decades since its theoretical conception in the late 1920s.
During fusion, two isotopes of hydrogen–deuterium and tritium–are subjected to extreme heat and pressure until they form a plasma and subsequently coalesce into helium. During this process, a small fraction of the mass is converted into astronomical amounts of thermal energy. The process is far more sustainable, productive, and safe than any current energy source, but a series of physics and engineering challenges have long prevented retrieving a net gain in energy. However, recent developments at the National Ignition Facility (NIF), such as having the fusion fuel heat itself, are rapidly changing that narrative.
Just east of San Francisco, the Lawrence Livermore National Laboratory, which hosts the NIF, has been tackling fusion since the 1960s. The multi-billion-dollar campus was created to engineer a particular path towards fusion: inertial confinement fusion (ICF). In this method, the inertia of the fuel keeps itself stationary for less than a billionth of a second, during which intense heat and pressure force its compression.
Inside the apparatus, deuterium-tritium fuel is contained within a diamond capsule, hovering at the center of a cylinder called a hohlraum. Lasers surround the hohlraum and inject light into openings on the hohlraum’s ends at various angles. Some lasers–outers–strike the hohlraum at a greater angle and farther from the capsule than others called inners, and both cause X-ray emissions. These X-rays converge on the capsule like hammers on hot metal, carrying energy that ionizes the diamond surface, producing an explosion. This explosion generates pressure that initiates an implosion, compressing and heating the capsule and fuel to the point of fusion.
These reactions operate on a small scale. Only two hundred micrograms of fuel are used, the energy entering the hohlraum–1.9 megajoules–is only enough to run a computer for a few hours, and the fusion yield is currently less than that. However, the power involved is on a massive petawatt scale due to the speed at which the energy is consumed and produced. Burning more fuel requires extra control, a future goal for the team.
Fusion research proceeds through a stairway of milestones. “[Each milestone] slowly tips the balance in the fusion plasma between [energy] losses and gains,” said Omar Hurricane, the co-lead author of two breakthrough papers recently published in Nature. After the first milestone—initial fusion with some self-heating—comes fuel gain, where fusion yield surpasses the energy input. Next is the burning plasma state, where self-heating by helium nuclei produced during fusion eclipses external heating. Finally, ignition. Much like a rocket, ignition allows fusion reactions to take off. Ignition overcomes all cooling processes, and the fusion reactions become self-sustaining.
Following the launch of ICF experiments in 2009, the NIF achieved fuel gain in 2014. Subsequently, hundreds of scientists at the NIF set their eyes on the next milestone of the burning plasma state. However, they encountered one central problem: asymmetric implosions.
“We’d like this nice spherical compression to maximize the transfer of kinetic energy. We put energy into the shell, it flies inwards, and it carries the fusion fuel on the inside, and at some point, it runs out of any place to go, converting that kinetic energy into internal energy. That’s what heats the fusion fuel up,” Hurricane said.
In an asymmetric implosion, the pressure is unevenly distributed around the capsule and fuel, inducing movement of the capsule’s center of mass during the implosion. The kinetic energy of that movement is siphoned from the input energy, representing a major leakage in the conversion of the fuel’s kinetic energy into internal energy. Asymmetry can be produced by imperfections in the capsule, in the lasers and their resultant X-rays, and in the inward movement of plasma generated when the lasers strike the hohlraum.
The scientists approached these problems through a combination of theoretical physics, experimentation, and iterative development of simulations. They soon discovered a problem: previous simulations weren’t accurate in predicting the interference to the laser beams from laser-generated plasma. For example, a plasma cloud generated by the outers, which strike the hohlraum nearest to where the lasers enter, can expand rapidly and interfere with the inners, producing asymmetries.
“It’s been a very iterative set of steps where we go from doing experiments, seeing something wrong, figuring out what’s wrong, how to fix it, implementing the fix, doing another experiment, and the whole cycle starts over and over again,” Hurricane said. “That’s what a lot of science is … but you do steadily make progress, and that’s what we’ve done over the last decade.”
Of the many proposed solutions, only two could be selected for testing due to limited resources. The first solution implements cross beam energy transfer (CBET), allowing one laser to transfer its energy to the other laser, given correct engineering of the laser wavelengths. CBET permits energy transfer to the inners, producing a more symmetric implosion. The second solution addresses the issue of the outer-laser-generated plasma interference. Creating pockets in the hohlraum at the site where outers hit increases the distance the plasma must travel toward the center. This gives inners more opportunity to travel without interference, decreasing laser asymmetry.
Now that asymmetry is less of a limiting factor for energy transfer, the researchers have enlarged the fuel-capsule target, which increases the heating tendency of the fusion fuel relative to its cooling tendency. As further solutions to fuel-capsule asymmetry are developed, the fuel load can be increased, producing more efficient fusion reactions.
The NIF tested these three innovations in combination and found that they had achieved a burning plasma state, making substantial energy gains in the process. Their maximum fusion yield was 0.17 MJ, about ten times smaller than the input laser energy but ten times greater than the energy input into the fuel, far surpassing the fuel gain milestone.
While the significance of burning plasma for energy research is undeniable, the limited yield and scalability of the reaction hinder its applicability. A viable power source requires ignition, the efficient capture and conversion of released thermal energy into electric energy, and a yield surpassing the hundreds of megajoules required to operate the fusion reactor. Yet, the burning plasma milestone provides a strong foundation for the future of fusion. “It’s an existence proof… that maybe this is possible, that we’re not all crazy, and we’re not wasting our time (entirely) working on this stuff. That being said, it’s also showing it’s actually much harder than people originally expected,” Hurricane said.
The first of these requirements, and the last milestone from the physics end of fusion research, has already been achieved by the NIF – ignition! In unpublished results, the team achieved an energy yield of 1.35 MJ, approximately 70% of the input energy. Further research will focus on overcoming engineering hurdles to improve energy yield, increasing fuel volume while avoiding asymmetry, and better simulating compression dynamics.
Unlike what science fiction often leads us to believe, these developments are not straightforward solutions designed by a few notable people but are decades-long projects requiring the input of generations of scientists and engineers repeatedly redesigning, testing, and troubleshooting. As the hundreds of scientists and staff at the NIF have demonstrated, the future of fusion is bright. Although we are still far from having miniature Suns powering our everyday devices, we now have experimental confirmation of a self-heating and even igniting plasma.