Net energy gain marks breakthrough for fusion research

Fusion ignition has been achieved at the US Department of Energy’s Lawrence Livermore National Laboratory (LLNL), an advance with the potential to herald a new era in clean energy.

On December 5, 2022, a team at LLNL’s National Ignition Facility (NIF) conducted the first controlled fusion experiment in history to reach this milestone, producing more energy from fusion than the laser energy used to drive it.

As well as clean fusion energy, the achievement at NIF is expected to provide unprecedented capability to support National Nuclear Security Administration (NNSA)’s Stockpile Stewardship Program.

The target chamber of LLNL’s National Ignition Facility, where 192 laser beams delivered over two million joules of ultraviolet energy to a tiny fuel pellet to create fusion ignition on Dec. 5, 2022 - LLNL

“The pursuit of fusion ignition in the laboratory is one of the most significant scientific challenges ever tackled by humanity, and achieving it is a triumph of science, engineering, and most of all, people,” LLNL director Dr Kim Budil said in a statement. “Crossing this threshold is the vision that has driven 60 years of dedicated pursuit - a continual process of learning, building, expanding knowledge and capability, and then finding ways to overcome the new challenges that emerged. These are the problems that the US national laboratories were created to solve.”

LLNL’s experiment surpassed the fusion threshold by delivering 2.05MJ of energy to the target, resulting in 3.15MJ of fusion energy output, demonstrating for the first time a most fundamental science basis for inertial fusion energy (IFE).

Many advanced science and technology developments are still needed to achieve simple, affordable IFE to power homes and businesses, and DOE said it is restarting a broad-based, coordinated IFE program in the United States.

The hohlraum that houses the type of cryogenic target used to achieve ignition on Dec. 5, 2022, at LLNL’s National Ignition Facility - LLNL

Fusion is the process by which two light nuclei combine to form a single heavier nucleus, releasing a large amount of energy. In the 1960s, scientists at LLNL hypothesised that lasers could be used to induce fusion in a laboratory setting. Led by physicist John Nuckolls, this idea became inertial confinement fusion, kickstarting over 60 years of research and development in lasers, optics, diagnostics, target fabrication, computer modelling and simulation and experimental design.

To pursue this concept, LLNL built a series of increasingly powerful laser systems, leading to the creation of NIF, the world’s largest and most energetic laser system. NIF - located at LLNL in Livermore, California  is the size of a sports stadium and uses powerful laser beams to create temperatures and pressures like those in the cores of stars and giant planets, and inside exploding nuclear weapons.

Commenting on NIF’s breakthrough, Prof Justin Wark, Professor of Physics at Oxford University and director of the Oxford Centre for High Energy Density Science, said: “The Lawrence Livermore National Laboratory uses the largest laser in the world to compress heavy hydrogen to conditions similar to those in the centre of the sun.

“The lasers enter the ends of a centimetre-scale cylinder, hitting its inner walls, making them glow x-ray hot.  These x-rays then heat a sphere at the centre that contains the nuclear fuel. The outside of the sphere vaporises and becomes a plasma that rushes off the surface creating an imploding ’spherical rocket’ which in a few billionths of a second reaches velocities of order 400km per second.”

To create fusion ignition, the National Ignition Facility’s laser energy is converted into X-rays inside the hohlraum, which then compress a fuel capsule until it implodes, creating a high temperature, high pressure plasma - LLNL

Prof Wark continued: “The subsequent ‘crunch’ at the centre is tailored in a specific way to make a hot spark in the middle, and the density of the compressed ‘fuel’ surrounding the spark is so great that the nuclear fusion reaction takes place in about a tenth of a billionth of a second - faster than the tiny hot sphere can fly apart.  It is thus confined by its own inertia, and thus this method of fusion is called inertial confinement fusion.” 

“Although positive news, this result is still a long way from the actual energy gain required for the production of electricity,” said Tony Roulstone, lecturer in nuclear energy at Cambridge University. “That’s because they had to use 500MJ of energy into the lasers to deliver 1.8 MJ to the target – so even though they got 2.5MJ out, it’s still far less than the energy they needed for the lasers in the first place. In other words, the energy output - largely heat energy - was still only 0.5 per cent of the input. An engineering target for fusion would be to recover much of the energy used in the process and get an energy gain of double the energy that went into the lasers – it needs to be double because the heat must be converted to electricity and you lose energy that way.”