For the last six decades, LLNL researchers and their colleagues have been working to achieve one of the most challenging goals in all of science and a primary objective of NIF: fusion ignition. This effort drives excellence in every aspect of NIF operations—the laser, targets, optics, diagnostics, and the people who make it all possible.
“Nature does not give up her secrets easily.”
—NIF Senior Scientist John Lindl
In “indirect drive” inertial confinement fusion (ICF) experiments on NIF, up to 192 laser beams are fired into a centimeter-sized hollow cylinder called a hohlraum. This generates a “bath” of soft x rays that ablate, or blow off, the surface of a peppercorn-sized capsule suspended in the hohlraum.
The result is a rocket-like implosion that compresses and heats partially frozen hydrogen isotopes inside the capsule to conditions of pressure and temperature found only in the cores of stars and giant planets and in exploding nuclear weapons. The speed of the implosion—more than 400 kilometers per second—allows the fusion reactions to take place before the fuel can disassemble; the fuel is trapped by its own inertia (hence the term inertial confinement fusion).
Fusion describes what happens when the nuclei of light atoms such as hydrogen overcome the repulsive electrostatic force that keeps them apart. When the nuclei get close enough, the force that binds protons and neutrons together, the strong force, takes over and pulls the nuclei even closer together so they “fuse” into a new, heavier helium nucleus with two neutrons and two protons.
The helium nucleus, also known as an alpha particle, has a slightly smaller mass than the sum of the masses of the two hydrogen nuclei, and the difference in mass is released as energy according to Albert Einstein’s famous formula E=mc2. The energy is released in the form of the alpha particles, high-energy neutrons, and other forms of energy such as electromagnetic radiation.
Nuclear fusion is different from nuclear fission, where the nuclei of heavy elements like uranium are split, forming two lighter elements—the process used in today’s nuclear power plants. In both nuclear reactions, the elements themselves change and become new elements—and in the process, a small amount of mass is converted to a large amount of energy.
Fusion ignition refers to the moment when the alpha-particle energy deposited in the hot spot at the center of the target capsule is equal to the energy losses due to emitted x-rays and electron heat conduction: as much or more energy “out” than “in”. NIF’s goal is to create a “burning plasma,” in which a burn wave of fusion reactions propagates into the cold fuel surrounding the hot spot. In this process, known as alpha heating, the alpha particles generated by the fusion reactions in the hot spot spread throughout the cold fuel, depositing their energy, stimulating additional fusion reactions, and greatly increasing the yield.
If enough alpha particles are “stopped,” or absorbed, in the high-density fuel layer, the dense fuel temperature will become high enough to launch a self-sustaining thermonuclear reaction leading to ignition. Achieving ignition would be an unprecedented, game-changing breakthrough for science and could lead to a new source of boundless clean energy for the world.
While reaching ignition on NIF has proven more challenging than first expected, we gain new understanding of the process with every experiment. We have learned important lessons about the limitations of our simulations and have used that knowledge to address issues and continually improve implosion performance.
In particular, the advent of high-resolution 3D modeling and simulations has contributed to a better understanding of the perturbation sources—including such “engineering features” as the thin membranes that suspend the target capsule inside the hohlraum and the fill tubes used to inject fuel into the capsule—that interfere with implosion performance. Other factors inhibiting energy yield are laser-plasma and hydrodynamic instabilities, asymmetries in the hohlraum x-ray flux that drives the implosion, and the mixing of capsule material with the fuel.
The addition of new and enhanced diagnostics, such as multiple line-of-sight neutron detectors, has also led to improved target performance. The ability of new high-performance supercomputers and powerful “deep learning” techniques to process, analyze, and simulate the mountain of data produced by these diagnostics in 3D over a broad range of perturbation sources—capsule surface imperfections, engineering features, drive asymmetries— has significantly improved the ability of simulations to match experimental results.
At the same time, each ignition experiment advances the science of ensuring the reliability of the nation’s nuclear stockpile (see “NIF and Stockpile Stewardship”) as well as the eventual use of fusion as a safe, clean, and virtually unlimited energy source (see “Exploring Energy Security”).
Since the first NIF laser shots, scientists have set records for neutron yield and the amount of energy generated. NIF made history in May, 2018, when it fired a record 2.15 megajoules (MJ) of ultraviolet energy to the Target Chamber—a 15 percent improvement over NIF’s design specification of 1.8 MJ and more than 10 percent higher than NIF’s previous 1.9 MJ energy record set in March, 2012.
Neutron yields have grown by a factor of 21 since NIF experiments began; more energy is being coupled to the target capsule; implosion velocities have increased; and the pressures in the center of the implosion are a factor of three or four higher.
And, for the first time ever in a laboratory setting, we have seen the initial signs of alpha heating—when the energy generated by alpha particles stimulating additional fusion reactions in the cold fuel exceeds the kinetic energy delivered by the implosion. A burning plasma is achieved when the energy deposition by the fusion-produced alpha particles contributes more than 50 percent of the heating of the fuel.
If better contained, this process would eventually lead, as the other issues are resolved, to fusion ignition.
Researchers are now poised to further improve NIF’s performance by coupling still more laser energy to the capsule while maintaining symmetry control. We also are pursuing strategies like novel hohlraum designs; larger capsules; magnetized targets; new methods for finishing, mounting, and filling capsules; and increased laser energy.
We owe our progress to the decades of remarkable work by previous generations of LLNL scientists and engineers who advanced the field of high energy density and ICF science to where it is today. A new generation of LLNL researchers, some of whom are pictured below, are carrying on this tradition of excellence.
Following the completion of NIF construction in March 2009, scientists focused on installing, qualifying, and integrating the facility’s many systems and the required scientific platforms to support a wide variety of experiments. Precision experiments devoted to ignition began in May 2011 and have since produced unprecedented high-energy-density environments.
Here are some examples of recent ignition experiments:
“Icarus Camera Soars Close to NIF’s Sun—and Thrives!,” NIF & Photon Science News, August 12, 2020
“The Shape of Things to Come,” Science & Technology Review, July, 2020
“On the Threshold of a Critical Milestone,” Science & Technology Review, April, 2019
“Experimentally trained statistical models boost nuclear-fusion performance,” Nature, January 30, 2019
“Why Ignition? NIF Experiments and Stockpile Stewardship,” NIF & Photon Science News, June, 2018
“The long road to ignition,” Physics World, April 20, 2017
“Climbing the Mountain of Fusion Ignition: An Interview with Omar Hurricane,” NIF & Photon Science News, May, 2015
“A Significant Achievement on the Path to Ignition,” Science & Technology Review, June, 2014
“On the Path to Ignition,” Science & Technology Review, March, 2013