Proton–boron fusion passes scientific milestone

09 Mar 2023

Team work: researchers in the control room of the Large Helical Device at Japan’s National Institute for Fusion Science in Toki. (Courtesy: TAE Technologies)

Physicists in the US and Japan have observed nuclear fusion between protons and boron-11 atoms in a magnetically confined plasma for the first time. They say that the result demonstrates the potential of proton–boron fusion as a plentiful, economical source of energy. But others caution that the scientific basis for such an energy source remains largely unproven and that huge technical hurdles stand in the way of commercial power plants.

All forms of fusion hold the promise of near limitless, clean, baseload energy without the problems of possible meltdown and long-lived waste that plague fission. But proton–boron (p11B) fusion brings a couple of additional virtues compared to the more mainstream reactions involving hydrogen isotopes deuterium and tritium.

Boron can be easily mined whereas tritium is rare on Earth and difficult to produce artificially. The proton–boron reactions also produce three helium atoms (alpha particles) – whose energy could in principle be directly converted into electricity – while generating no neutrons, and thereby substantially reducing radioactive contamination of reactor components.

However, those plus points come at a price. Deuterium–tritium fusion itself requires enormous temperatures to overcome the mutual repulsion of the nuclei – around 100 million kelvin. But proton–boron reactions need far more extreme conditions still – some 1.5 billion kelvin.

As the authors of the latest research explain in a paper published in Nature Communications, the higher a plasma’s temperature the more energy is usually radiated away in the form of synchrotron and bremsstrahlung radiation. This, they point out, makes it harder to generate more energy through fusion reactions than is needed to power a reactor – a major problem when a commercial plant is likely to need an energy gain of at least 50 to overcome inefficiencies in the power-generation process.

The new work was carried out by Richard Magee and colleagues at Californian fusion company TAE Technologies together with scientists at the National Institute for Fusion Science in Toki, Japan. The researchers did their experiments on the institute’s Large Helical Device (LHD), a stellarator with the necessary fusion fuel already in place – the protons being fired in as high-energy neutral beams while boron powder is injected into the plasma to help reduce impurities.

TAE provided the detector, which relied on a partially depleted silicon semiconductor generating a current when struck by alpha particles. It was made to avoid erroneously registering signals from X-rays and other plasma radiation by being angled away from the core plasma and having the charged alpha particles steered to it by the LHD’s large magnetic field.

The researchers performed several dozen experimental shots in February last year. They observed fusion reactions by comparing the signal on their detector before and after turning on the neutral beams as well as carrying out some shots without any boron powder. Only when they had both neutral beams and boron powder did they get a jump in output – the exact value of which told them that they were producing about 1012 fusion reactions per second, which agreed with computer simulations.

Challenges ahead

This is not the first demonstration of proton–boron fusion – scientists have previously observed it using particle accelerators and powerful lasers. But the US–Japanese collaboration argues it is important to study the reaction where it would ultimately be exploited – inside a magnetically confined, thermonuclear plasma. The researchers acknowledge that much more work needs to be done, but are confident that TAE will achieve energy gain in one of its devices.

Indeed, TAE claims to be well on the way to commercial fusion energy. The company has built a series of increasingly sophisticated reactors to explore field-reversed configuration fusion, which involves firing pulses of plasma into a chamber and holding them in place magnetically by rotating them. None of the devices to date have demonstrated proton–boron fusion – its current “Norman” reactor using a hydrogen plasma – but the firm says it intends to send electricity to the grid from a pilot proton–boron power plant by the early 2030s.READ MORE

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Peter Norreys, a plasma physicist at the University of Oxford in the UK, says the researchers have done “a fine job” in their experiments. But he argues that proton–boron fusion is still far from rivalling deuterium–tritium reactions. One potential complication, he says, is the need for relativistic descriptions of plasma dynamics at such high temperatures. He also thinks it likely that bremsstrahlung radiation could impair plasma confinement by eroding a reactor’s inner surfaces.

Scientists at the EUROfusion consortium in Garching, Germany, are also guarded. Tony Donné, Hartmut Zohm and Volker Naulin told Physics World that the observed reaction rate in the latest experiments is about ten orders of magnitude too small to be useful for fusion energy (taking into account proton–boron’s low power density).

They have “strong doubts” that it will ever be possible to achieve the gains needed for commercial power generation, and caution that bremsstrahlung radiation could in fact be so strong that it exceeds the power needed to heat and control the plasma – causing the plasma to collapse.

Edwin Cartlidge is a science writer based in Rome

FROM PHYSICSWORLD.COM   26/3/2023