Scientists at Lawrence Livermore National Laboratory and their international collaborators, including colleagues from the UK’s Central Laser Facility, have achieved the ‘Holy Grail’ of fusion research for the first time in a laboratory – producing more fusion energy output than energy input.
The National Ignition Facility (NIF) in California is a huge laser facility roughly the size of a football stadium. The laser facility uses 192 laser beams to initiate fusion reactions using laser driven Inertial Confinement Fusion (Laser Fusion) experiments.
On this record experiment NIF achieved a gain of 1.5, or one and a half times the energy input. While significantly higher energy gain will be required for power production, this represents a huge step forward in laser fusion science.
The hollow spherical target contains fusion fuel – deuterium and tritium – and is roughly the size of a peppercorn. The 192 laser beams then switch on for a few billionths of a second and deliver ~175 times the average global electricity power to the target. The target then implodes inwards on itself at over 1 million miles per hour and heats and compresses the fusion fuel creating the same conditions at the centre of the Sun.
Fusion has the potential to provide a near-limitless, safe, clean, source of carbon-free baseload energy. It could complement renewables by filling supply-gaps and provide the increase in generation-capacity required to move away from fossil-fuels. In comparison to fission, it is safer, creates limited radioactive waste, and the near-limitless fuel is readily available. Fusion energy could supplant a considerable fraction of the $3.5 Tn/annum fossil fuel industry, so given this, fusion’s economic potential is huge. However, until now, scientists have not been able to generate more fusion energy than was used to hold the fuel in place.
Dr Robbie Scott, Senior Plasma Physicist, Science and Technology Facilities Council (STFC) Central Laser Facility says:
“It cannot be understated what a huge breakthrough this is for laser fusion research. More importantly however, is that fact that it paves the way for the rapid development of Laser Inertial Fusion Energy – power generation by Laser Fusion.”
Director of the UK’s Central Laser Facility (CLF), Professor John Collier, said of the results:
“We welcome this milestone result from NIF which is fantastic news. It is great that the CLF and UK academic community have been part of this journey. We now look forward to translating this result into what has real potential to be a long-term green energy solution.”
Fusion is the process of fusing two light atoms together to form a heavier one, e.g., fusing deuterium and tritium fuel atoms by slamming them together to form the neutron and the alpha particle. To understand why fusion works, we turn to Einstein’s most famous equation, E = mc2, which tells us that if the mass of the reaction products is less than the mass of the two fusing atoms, then energy will be released during the nuclear reaction. Fusion turns matter into energy! In doing so, fusion yields a tremendous amount of energy, millions of times greater than other sources that are currently used today.
In fact, this process already occurs naturally, and we depend on a nuclear fusion reactor to bring about and sustain life: the Sun. At the centre of our local star, and all stars for that matter, you’ll find extreme plasma conditions that achieve fusion. In reality, except for tidal power, fusion is the original source of pretty much all energy on earth!
The ‘holy grail’ of fusion energy research
Now scientists at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory in the USA have finally achieved the ‘Holy Grail’ of fusion energy research – where more fusion energy is output than input by the laser beams. In the recent NIF experiment 3.15MJ of fusion energy was produced while the lasers input 2.05MJ of laser energy into the target chamber. Physicists measure fusion progress in terms of fusion energy-gain.
Dr Robbie Scott who works closely with the NIF team stated, “The experiment demonstrates unambiguously that the physics of Laser Fusion works. To transform NIF’s result into power production a lot of work remains, but this is a key step along the path. Next steps include the demonstration of even higher fusion energy-gain and the further development of more efficient methods to drive the implosion. We have very well-developed plans for how to go about this and hope this result will help stimulate the funding required to drive our work forward at the pace required to make an impact on global warming.”
The result from NIF is the culmination of the work of hundreds, perhaps thousands, of scientists and engineers working for decades starting in the early 1970s – in fact the concept was developed 3 days after the invention of the laser! Since then, huge advances have been made in the development of high energy lasers, the ultra-precise targets, experimental diagnostics, muti-dimensional simulation codes running on some of the world’s largest supercomputers.
In the years leading up to achieving ignition, Dr Robbie Scott made a crucial discovery related to the implosions on NIF at the time, which would improve the fusion yield towards ignition. Dr Robbie Scott continued to say:
“This fantastic result was only possible due to the work of hundreds of scientists over decades. My own contribution was to discover that if NIF’s implosions were not spherical, this would reduce the efficiency of the implosion and so the number of fusion reactions. Importantly, I also showed that certain non-spherical implosion shapes would appear to be perfectly spherical using NIF’s X-ray imaging diagnostics. This led to the development of new diagnostics for NIF which confirmed the implosions were non-spherical, just as predicted. This resulted in a multiyear effort at NIF to make the implosions as spherical as possible, improving NIF’s fusion yield. I’d like to congratulate the NIF team on this massive success – while it took longer than originally hoped, I never doubted they’d get there in the end.”
The laser fusion approach
There are two extensively investigated approaches to fusion; Magnetic Confinement Fusion (MCF) and Laser driven Inertial Confinement Fusion (Laser Fusion). MCF uses Tokamaks which are donut shaped vessels surrounded by powerful magnetic coils which makes a trap for fusion fuel to produce plasma. The aim of Tokamaks is to hold this position long enough for fusion reactions to take place.
The Laser Fusion approach, as taken by NIF, uses lasers to heat the surface of a spherical shell containing deuterium and tritium fuel. This can either be done indirectly, like on NIF where the laser energy is first converted into x-rays, or with the lasers directly incident on the shell. Either way, the outside of the shell heats up and expands outwards very quickly. This outward expansion creates an equal and opposite inward force which accelerates the fuel-shell inwards at 1 million miles per hour! As the implosion proceeds towards its centre, eventually it can go no further. This causes the fusion fuel to be heated and compressed so much that it creates similar densities and temperatures to those at the centre of the Sun. Now fusion reactions start to occur. These reactions release both neutrons and alpha particles. The neutrons escape, but the alpha particles deposit their energy in the dense fuel, heating it even more. This causes even more fusion reactions, and more heating, and more fusion reactions… For a few tens of picoseconds the fuels’ own inertia holds it in place while the fuel burns, giving rise to the name ‘Inertial Confinement Fusion’. Eventually the fuels’ own internal pressure pushes it apart, stopping the fusion reactions.
The National Ignition Facility
The National Ignition Facility (NIF) is a huge laser by any standard. Each of its 192 laser beams have an aperture of 40cm x 40cm and are roughly 100 meters long. The building itself is about the size of a football stadium.
Within the UK they are a number of smaller scale laser facilities which are also used to investigate the physics of laser fusion. At the Central Laser Facility (CLF), STFC Rutherford Appleton Laboratory the Vulcan laser has been a workhorse for Laser Fusion research since its inception in the early 1980s. A number of upgrades have kept it at the cutting edge with the planned Vulcan 20-20 upgrades pencilled to significantly enhance its laser fusion research capabilities.
Next Steps Towards Fusion Energy
While NIF has taken a massive step forward, a lot more work is required to transform NIF’s result into power production. The principle of power production by laser fusion is like that of an internal combustion engine such as a diesel engine: the fuel is injected, compressed, ignited, then energy is released. Then the process repeats. In the case of laser fusion, the shell containing the fusion fuel must be fired into the target chamber at high speed then shot by the lasers. This needs to be repeated approximately 10 times per second.
In terms of developments required for laser fusion power production, while huge progress has been made, a lot of work remains to be done. The key areas include further increasing the fusion energy-gain to cover losses in any power-production system – there are plans for how to increase the energy output by NIF. A lot of research has also gone into the direct drive approach to Laser Fusion which has great potential to reduce the amount of laser energy which needs to be input and may also be more practical for power production due to the simpler targets required. The laser efficiency also needs to be improved – NIF’s lasers are inefficient, but thankfully technologies have been developed which can do this. The CLF’s DiPOLE laser was developed with laser fusion in mind and has far higher energy efficiency that the laser of NIF. The third main area is the rapid economic manufacturing of the targets, and their injection into the target chamber. While concepts exist for this, more work is required.