Nuclear fusion: What does the latest breakthrough actually mean?

15 Dec 2022

The NIF Target Bay. NIF’s 192 laser beams converge at the centre of this giant sphere to make the tiny hydrogen fuel pellet implode. Image: Damien Jemison/LLNL

Dr Nathan Garland of Griffith University explains what nuclear net gain is, why it’s a big deal, and where we go from here.

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A version of this article was originally published by The Conversation (CC BY-ND 4.0)

For the first time, scientists in the US have confirmed that a fusion energy experiment achieved net gain. This means releasing more energy than it takes to initiate, demonstrating the physical basis for producing fusion energy in a controlled way.

This historic feat took place at Lawrence Livermore National Laboratory (LLNL) in California, using the National Ignition Facility (NIF) experiment after decades of planning and research.

The milestone used a process called inertial confinement fusion. It involves bombarding a tiny gold cylinder containing a pellet of hydrogen fuel – about the size of a pencil eraser – with the world’s most powerful laser system comprised of 192 laser beams.

This produces hot plasma and x-rays that trigger an implosion, compressing the fuel pellet and kicking off a fusion reaction that unleashes energy.

The team at LLNL reports they delivered “2.05 megajoules of energy to the target, resulting in 3.15 megajoules of fusion energy output”.

A key-shaped object with dimples across the surface and a green outline. It is an object used in nuclear fusion experiments.

NIF’s 192 lasers focus on a tiny fuel capsule suspended inside a cylindrical x-ray oven called a hohlraum (left, in the circle). The dimpled surface scatters stray light to prevent damage to the laser. Image: Jason Laurea/LLNL

To quickly review, the goal of fusion energy is exactly that – to heat up and compress fuel particles so they undergo fusion: merging together to create a heavier atomic particle, unleashing energy in the process.

Fusion is what powers stars like our sun, but it can only occur under specific conditions. Atoms must be subjected to immense heat and pressure to overcome tremendous physical forces and fuse. It is the opposite of nuclear fission used in current nuclear power plants.

The lofty end goal for harnessing fusion for power production is to generate vast amounts of clean, sustainable electricity.

What is net gain and why is it a big deal?

This week’s announcement is akin to other historic milestones such as the four-minute mile or the Wright brothers’ first controlled, sustained flight of a powered aircraft.

People had run races before, or experienced temporary flight through gliders, but these milestones had been thought of as impossible pipe dreams. They were eventually made reality through long-term effort – and it feels like this is one of those historic moments for fusion science.

A net gain result in a fusion experiment essentially means producing more energy through fusion reactions than the amount of energy put into the system to start said reaction.

This is usually measured as a Q factor, which is the ratio of energy out to energy in. For decades, the holy grail in fusion science has been achieving Q > 1.

A Q factor of more than 1 means you got out more energy than what you put into the fuel. This is generally known as ‘scientific breakeven’. The result announced translates to a Q factor of about 1.5.

The complication around measuring your experiment in this way is that it does not account for energy inefficiencies in how you power the laser from the electricity grid, or how you may generate electricity from the energetic particles created from the fusion reactions.

Just like current electricity generation methods, no process is 100pc efficient and we lose bits of energy along the way. When you account for these effects, you then start to talk about an ‘engineering breakeven’.

Practically, most people seem to think higher Q factors of 10, 100 or even 1,000 might be needed to achieve a viable product for electricity generation from fusion. This is what the fusion community will be striving towards with future efforts.

How does this result compare to other fusion experiments?

The most popular global approach for confining fusion reactions is not with lasers – as done at LLNL – but with magnetic fields.

This method is called magnetic confinement fusion. In the process, the fuel is heated up to kick out electrons from the atoms to create a plasma of electrons and positively charged nuclei. These nuclei are then fused together.

Since the plasma in these fusion devices is hotter than the core of our sun, strong magnetic fields are used to control the shape and direction of the plasma so it doesn’t damage the walls of the machine.

Magnetic confinement devices have repeatedly reached plasma temperatures of over 100m degrees Celsius – but to date, net gain has not been reached in such a device. There are plans afoot to hopefully achieve this with the International Thermonuclear Experimental Reactor (ITER) under construction in France.

ITER is the world’s biggest science experiment and has been designed to reach a Q factor of 10, producing 500 megawatts of fusion power from 50 megawatts of injected power.

Where to from here?

For now, this result means a lot more to the scientific community than it probably does for folks waiting for a new commercial electricity alternative. We’ll need to be a bit more patient for those prospects.

Whether using lasers or magnetic confinement, fusion scientists around the world need to continue along their path if we’re going to achieve commercially viable fusion power production. As we continue on this journey we’ll likely see continued development of the technologies needed and keep climbing up the ladder towards higher values of Q.

In the short term, this result will likely lead to more concrete plans and funding from government and private industry towards inertial confinement fusion experiments, and hopefully other fusion concepts as well.

For example, following the LLNL announcement the US has committed over $600m towards the inertial fusion programme to build upon this result. This is in addition to the commitment to a “bold vision for commercial fusion energy” outlined earlier in 2022.

The achievement is a watershed moment, showing the public, governments and investors that despite this being an incredibly difficult science and engineering problem to solve, we are making real progress.

It took many years and a great deal of work to launch the first commercial airline flight after the Wright brothers first took to the skies. In much the same way, there is a path ahead for commercial fusion, but we have to put the resources and effort in to get there.

The Conversation
By Dr Nathan Garland

Dr Nathan Garland is a lecturer in applied mathematics and physics at Griffith University in Brisbane, Australia.

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