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Exploring the future of nuclear fusion

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Canto: So, with Christmas cookery and indulgence behind us, it’s time to focus on another topic we know little about, nuclear fusion – or I should say human-engineered nuclear fusion. Ignition has recently been achieved for the first time, so where do we go from here?

Jacinta: Well I listened to Dr Becky the astrophysicist on this and other topics, and she puts the ignition thing into perspective. So it occurred back on December 5 at the National Ignition Facility in California. As Dr Becky explains it, it involves ‘taking 4 atoms of hydrogen and forcing them together to make helium’, which is slightly lighter than the four hydrogens, and this mass difference can, and in this case has, produced energy according to special relativity. Of course fusion occurs in stars (not just involving hydrogen into helium) and it can potentially produce huge volumes of clean energy. But there’s a big but, and that’s about the high temperatures and densities needed for ignition. Those conditions are needed to overcome the forces that keep atoms apart. 

Canto: Yes they used high-powered lasers, which together focus on heavy hydrogen isotopes – deuterium and tritium – to produce helium. And this has been achieved before a number of times, but ignition specifically occurs when the energy output is greater than the input, potentially creating a self-sustaining cycle of fusion reactions. And the difficulties in getting to that output – that is, in creating the most effective input – have been astronomical, apparently. They’ve involved configuring the set of nearly 200 lasers in the right way, using ultra-complex computational analysis, recently guided by machine learning. And this has finally led to the recent breakthrough, in which an energy input of 2.05 megajoules produced an output of 3.15 megajoules…

Jacinta: 1.1 megajoules means ignition, though it’s nothing earth-shattering energy-wise. It’s apparently equivalent to about 0.3 kilowatt-hours (kWh) – enough energy for about two hours of TV watching according to Dr Becky. And also this was about the energy delivered to the particles to create the reaction, it didn’t include the amount of energy required to power the lasers themselves – approximately 300 megajoules. So, good proof-of-concept stuff, but scaling up will be a long and winding road, wethinks. 

Canto: Another favourite broadcaster of ours, theoretical physicist Sabine Hossenfelder, also covers this story, and provides much the same figures (400 megajoules for the lasers). She also points out that, though it’s a breakthrough, it’s hardly surprising given how close experimenters have been getting to ignition in recent attempts. And she is probably even more emphatic about the long road ahead – we need to ramp up the output more than a hundred-fold to achieve anything like nuclear fusion energy at economically viable levels. 

Jacinta: I’m interested in the further detail Dr Hossenfelder supplies. For example the NIF lasers were fired at a tiny golden cylinder of isotopes. There must be a good reason for the use of gold here. She also describes the isotopes as ‘a tiny coated pellet’. What’s the coating and why? She further explains ‘the lasers heat the pellet until it becomes a plasma, which in turn produces x-rays that attempt to escape in all directions’. This method of arriving at fusion is called ‘inertial confinement’. Another competing method is magnetic confinement, which uses tokamaks and stellarators. A tokamak – the word comes from a Russian acronym meaning ‘toroidal chamber with magnetic coils’ – uses magnetism to confine plasma in a torus – a doughnut shape. A stellarator…

Canto: Here’s the difference apparently:

In the tokamak, the rotational transform of a helical magnetic field is formed by a toroidal field generated by external coils together with a poloidal field generated by the plasma current. In the stellarator, the twisting field is produced entirely by external non-axisymmetric coils. 

Jacinta: Ah, right, we’ll get back to that shortly. The Joint European Torus (JET) holds the record for toroidal systems at 0.7, which presumably means they’re a little over two thirds of the way to ignition. 

Canto: A poloidal field (such as the geomagnetic field at the Earth’s surface) is a magnetic field with radial and tangential components. Radial fields are generated from a central point and weaken as they move outward.

Jacinta: PBS also reports this, citing precisely 192 lasers, and a 1mm pellet of deuterium and tritium fuel inside a gold cannister:

When the lasers hit the canister, they produce X-rays that heat and compress the fuel pellet to about 20 times the density of lead and to more than 5 million degrees Fahrenheit (3 million Celsius) – about 100 times hotter than the surface of the Sun. If you can maintain these conditions for a long enough time, the fuel will fuse and release energy.

Canto:  So the question is, does nuclear fusion have a realistic future as a fuel?

Jacinta: Well, did the internet have a realistic future 50 years ago? We’ve had a breakthrough recently, and the only way is up. 

Canto: Yeah the future looks interesting after I’m dead. Still, it’s worth following the progress. Back in February The Guardian reported that JET had smashed its own world record, producing ’59 megajoules of energy over five seconds (11 megawatts of power)’. Whatever that means, it wasn’t ignition – it might’ve been the .7 you mentioned earlier. Creating a mini-star for five seconds was what one experimenter called it, which I think was in some ways better than the current effort, in that it created more energy in absolutes terms, but less energy than the input. 

Jacinta: Perhaps, but what they call ‘gain’ is an important measure. This recent experiment created a gain of about 1.5 – remember just over 3 megajoules of energy was put out from just over 2 megajoules of input. It’s a start but a much bigger gain is required, and the cost and efficiency of the lasers – or alternative technologies – needs to be much reduced. 

Canto: Apparently deuterium and tritium are both needed for effective fusion, but tritium is quite rare, unlike deuterium, which abounds in ocean waters. Tritium is also a byproduct of the fusion process, so the hope is that it can be harvested along the way. 

Jacinta: Of course the costs are enormous, but the benefits could easily outweigh them – if only we could come together, like bonobos, and combine our wits and resources. Here’s an interesting quote from the International Atomic Energy Agency:

In theory, with just a few grams of these reactants [deuterium and tritium], it is possible to produce a terajoule of energy, which is approximately the energy one person in a developed country needs over sixty years.

Canto: Really? Who will be that lucky person? But you’re right – collaboration on a grand scale is what this kind of project requires, and that requires a thoroughly human bonoboism married to a fully bonoboesque humanism….

References

https://www.pbs.org/newshour/science/what-a-breakthrough-in-nuclear-fusion-technology-means-for-the-future-of-clean-energy

https://www.theguardian.com/commentisfree/2022/dec/13/carbon-free-energy-fusion-reaction-scientists

https://www.iaea.org/bulletin/what-is-fusion-and-why-is-it-so-difficult-to-achieve

https://www.bbc.com/news/science-environment-60312633

Written by stewart henderson

December 29, 2022 at 6:26 pm