Paul van Gerven
2 March

An experiment at a European testbed suggests that the Iter nuclear fusion reactor has a good chance of succeeding. Meanwhile, US scientists breathed new life into an entirely different approach to the almost perfect power source.

As Putin’s invasion of Ukraine pushes Europe towards faster decarbonization, nuclear fusion is unfortunately still far removed from being a viable alternative to fossil fuels. But two recent breakthroughs have reinforced scientists’ faith that the technology can one day provide abundant amounts of energy without generating much emissions or waste.

At the Joint European Torus (Jet) near Oxford, scientists generated 59 megajoules of sustained fusion energy, more than double the previous record. More importantly, that result was obtained with the same technology and fuel mixture as the famous Iter fusion reactor uses. Iter is scheduled to commence operations by 2025, with the overarching objective to extract more energy than was put in to start the nuclear reaction. The results at Jet inspire confidence that Iter will be able to achieve that goal.

NIF
Color-enhanced image of the inside of a NIF preamplifier support structure. Credit: Damien Jemison

A testbed for Iter, Jet is a tokamak as well. This donut-shaped reactor type employs magnetic fields to confine the extremely hot plasma in which hydrogen isotopes fuse into helium. Already in 1997, Jet produced a total of 22 megajoules of heat energy from fusion in 0.15 seconds, achieving a peak power of 16 megawatts. This wasn’t surpassed in the recent run, as the focus was on sustaining fusion power for longer periods of time: a total of 59 megajoules was produced over 5 seconds, averaging around 11 megawatts.

“If we can maintain fusion for five seconds, we can do it for five minutes and then five hours as we scale up our operations in future machines,” comments program manager Tony Donné of Eurofusion, the consortium of national fusion research institutes that runs Jet. “The operational experience we’ve gained under realistic conditions gives us great confidence for the next stage of experiments at Iter and Europe’s demonstration power plant EU Demo, which is being designed to put electricity on the grid.”

Tiniest imperfection

Using an entirely different approach called inertial confinement fusion, researchers of the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory achieved for the first time a state of burning plasma, in which nuclear reactions generate most of the heat needed to maintain those reactions. This brings them one step closer to ignition, where fusion of atomic nuclei releases more energy than goes in.

The Californians fired 192 high-power lasers into a roughly cylindrical chamber housing a tiny capsule holding 200 μg of nuclear fuel. The lasers aren’t focused on the capsule directly; instead, they strike the inside of the chamber, which turns them into X-rays. The powerful blast heats the outer layer of the capsule, making it expand rapidly and in turn heating and compressing its contents to the point that fusion reactions occur.

The challenge here is to make sure that all the energy is evenly delivered to the target. Any asymmetries can disrupt the formation of a burning plasma. For example, the tiniest imperfection in the spherical capsule can lead to a ‘leak,’ through which precious energy is lost by cooling. And due to thermal expansion of the reaction chamber, some of the lasers’ energy is directed away from the center of the capsule.

These kinds of effects have been showstoppers for decades, but through a series of careful adjustments to previous experimental designs, the NIF researchers managed to start a nuclear fusion reaction that was sustained by its own heat. In the best of four runs, 170 kilojoules of fusion energy was generated. The input laser energy was more than ten times that amount, but the point is that the result strengthens the case that the approach is capable of achieving ignition, opening up an alternative to tokamaks to harness the power source of the stars.