Fifty years ago in December 1974, Lawrence Livermore National Laboratory (LLNL)'s inertial confinement fusion (ICF) program blasted a glass ball so tiny it was hardly visible to the human eye. The experiment was small, but the result was enormous.
"It was the simplest possible experiment to achieve thermonuclear fusion," said LLNL senior scientist John Lindl.
This experiment and a similar one carried out a few months earlier at KMS Fusion were the first laboratory-scale ICF experiments to reach the extreme temperatures and pressures that allow atomic nuclei to fuse together. KMS Fusion was a private company founded by Keith M. Siegel that closed in 1990. LLNL scientists used the single-beam Janus laser to deliver 15 joules of energy to a ball-on-plate laser fusion target. The neutron yield was 1.2 x 104, or 12,000 neutrons.
The Janus laser was the first of a series of record-setting lasers at LLNL. Janus was followed by Cyclops, Argus, Shiva, Novette and Nova. Each advance in the laser systems enabled increasingly sophisticated experiments.
"This simple experiment started us on the road to ignition at LLNL's National Ignition Facility (NIF)," said Richard Town, associate program director of ICF sciences. Ignition occurs when the energy yield from a fusion experiment equals or exceeds the laser energy delivered to the target.
The "ball" of the target was a tiny glass microballoon about 55 microns in diameter - smaller than the width of a human hair - less than a micron in thickness and filled with a mixture of deuterium and tritium gas. The single laser hit the sphere directly, causing the thin shell to explode and accelerate towards the center of the microballoon, a so-called "exploding pusher" target. Reflected light off the glass plate helped direct laser light to the back side of the target.
"We hit the sphere with high enough intensity that the explosion velocity heated the fuel to a few thousand electron volts [one thousand electron volts is about 10 million degrees kelvin]. This got the temperature range needed for fusion reactions to occur," Lindl said.
The achievement caused a stir in the scientific community. Magnetic fusion experiments had already achieved thermonuclear conditions, but the December 1974 experiment on Janus and the similar KMS Fusion experiment were the first time fusion conditions were achieved using a laser.
Achieving thermonuclear conditions in a laser-driven implosion validated a key concept in the ICF vision laid out by former LLNL director John Nuckolls and his coauthors in their seminal 1972 Nature paper. LLNL researchers presented the results at the 1975 IEEE International Conference on Plasma Science and in an October 1975 Physical Review Letters paper, "Measurement of the Ion Temperature in Laser-Driven Fusion."
Critical indirect-design decision soon followed
In the two years following the first ICF experiment, LLNL scientists experimented with direct- and indirect-drive experiments. In indirect drive, the lasers do not impact the fuel capsule directly. Rather, the lasers hit the walls of a small cylinder, called a hohlraum, that creates an X-ray bath. These X-rays ablate material from the surface of the capsule containing the fuel. This causes the capsule to implode inward at high velocity, much like a rocket, compressing and heating the fuel inside the capsule.
By 1976, LLNL had decided to pursue indirect drive. This decision ran counter to the predominant view in the ICF physics community, which held that direct drive would provide the best path to ignition.
"Hydrodynamic instabilities are far more stabilized in indirect drive, and the hohlraum provides a critical smoothing effect on nonuniformities that can be seeded by the laser beams," Lindl said. "I designed the first radiation-drive experiment on Cyclops, the single-beamline Shiva prototype."
The hohlraum was produced by electroplating gold onto a copper wire, which was then leached out. Holes were drilled for laser entry and a diagnostic for viewing the imploded capsule. This was a significant advance in the complexity of targets.
The laser hit the inside of the hohlraum, creating X-rays that bounced around, being absorbed and reemitted several times. Even with only a single laser beam, this process helped produce a relatively uniform X-ray bath on the capsule.
The initial experiments worked very well, generating "exploding pusher" type implosions, similar to the first ICF direct-drive experiments done on Janus. Yields ranged from 10,000 to 200,000 neutrons and were correctly predicted.
Some 48 years after the first Janus experiment, LLNL achieved fusion ignition on the NIF. On Dec. 5, 2022, the NIF laser delivered 2.05 megajoules (MJ) of laser energy, resulting in 3.15 MJ of fusion energy output and a neutron yield of about 1018 (one quintillion). From December 1974 to December 2022, LLNL scientists increased the laser energy by five orders of magnitude and the neutron yield by 14 orders of magnitude.
Since then, LLNL has achieved ignition on NIF five more times, with the highest yield of 5.2 MJ occurring on Feb. 12, 2024.
"Having demonstrated the repeatability of ignition, we are now conducting experiments in realms of high-energy-density physics that have not been explored in a laboratory," Town said. "This is critical for our stockpile stewardship mission and the continued pursuit of still-higher fusion gain."
Some critics of indirect drive continued to voice skepticism right up until LLNL achieved ignition on NIF.
"That's the nature of this kind of science," Lind saidl. "We were trying to do something no one had ever done. You only have partial knowledge. The art is determining how to integrate that partial knowledge with what you hope to learn in upcoming experiments and facilities. You must be willing to make decisions and take risks."
