A tabletop device has sparked a fusion reaction using boron plasma for the first time – and it was thousands of times more effective than attempts using solid targets. Although the method is still a way off from producing clean energy on a commercial scale, the device can double as an astrophysical lab, recreating the conditions needed to make elements in cosmic-ray crashes.
Mainstream fusion power schemes fuse hydrogen isotopes called deuterium and tritium to make helium nuclei, releasing large amounts of energy in the process. However, the reaction also produces high-energy neutrons, which would damage whatever vessel the fusion reactor is in and render anything around them radioactive.
The problem goes away with boron-based fusion, in which protons are fired at a boron target to create helium nuclei but no neutrons. Better still, boron is a lot more common on Earth than deuterium or tritium. The downside is that much more initial energy is needed to get the reactions going.
In 2005, scientists in Russia experimented with firing lasers at a solid boron target. They reported generating thousands of helium nuclei, but that is not nearly enough to make the reaction self-sustaining. Now Christine Labaune at the École Polytechnique in Palaiseau, France, and colleagues have used a high-energy infrared laser to first turn boron isotopes into a hot, charged gas, or plasma.
A fraction of a second later they fired another laser at an aluminium foil target. That generated a stream of fast-moving protons which scattered into the plasma, where some fused with boron. The reaction formed excited nuclei of carbon-12, with six protons and six neutrons each. In this high-energy state the carbon was unstable, and it broke apart into helium nuclei. Labaune and her team estimate their reaction made about 8 million of them.
“It seems like a remarkable achievement, a once-in-a-decade leap,” says François Waelbroeck, director of the Institute for Fusion Studies at the University of Texas in Austin. Still, the work is a far cry from making fusion power a reality.
As above, so below
In the meantime, astrophysicists could use the device to explore the ways chemical elements are born deep inside stars or during cosmic-ray collisions. For instance, the experiment has already yielded fresh data on cross-sections – the likelihood that speeding nuclei will hit each other and fuse into heavier elements – says Mordecai-Mark Mac Low, an astrophysicist at the American Museum of Natural History in New York.
Elements heavier than hydrogen and helium are forged in the nuclear furnaces of stars or during supernova explosions. We know approximately how much of certain atoms must be made in a given type of star, says Mac Low, but lab-based experiments could help nail down exact numbers under various conditions, such as changing temperatures or plasma density.
What’s more, the device could reveal more about the handful of lighter elements that are not made inside stars. Cosmic rays zooming through space sometimes crash into heavier atoms, knocking off protons and neutrons and creating elements such as beryllium and lithium. Since the experiment fires protons at boron plasma, it effectively mimics cosmic rays crashing into plasmas in space, which may aid studies of high-energy particle behaviour, says Mac Low. “For us to have more methods to do real experiments is always good,” he says.