Fuel for world’s largest fusion reactor is set for test run

A pioneering reactor in Britain is gearing up to start pivotal tests of a fuel mix that will eventually power ITER — the world’s biggest nuclear-fusion experiment. Nuclear fusion is the phenomenon that powers the Sun and, if physicists can harness it on Earth, it would be a source of almost limitless energy, a report in Nature said.

In December, researchers at the Joint European Torus (JET) started conducting fusion experiments with tritium — a rare and radioactive isotope of hydrogen. The facility is a one-tenth-volume mock-up of the US$22-billion ITER project and has the same doughnut-shaped ‘tokomak’ design— the world’s most developed approach to fusion energy. It is the first time since 1997 that researchers have done experiments in a tokamak with any significant amount of tritium.

In June, JET will begin fusing even quantities of tritium and deuterium, another isotope of hydrogen. It is this fuel mix that ITER will use in its attempt to create more power from a fusion reaction than is put in — something that has never before been demonstrated. The reactor should heat and confine a plasma of deuterium and tritium such that the fusion of the isotopes into helium produces enough heat to sustain further fusion reactions.

“It’s very exciting now to, at last, get to the point where we can put into practice what we’ve been preparing all these years,” says Joelle Mailloux, who co-leads the scientific programme at JET. “We’re ready for it.”

Trial run

JET’s experiments will help scientists to predict how the plasma in the ITER tokamak will behave and to craft the mega-experiment’s operating settings. “It’s the closest we can get to achieving ITER conditions in present-day machines,” says Tim Luce, chief scientist at ITER, near Cadarache in France. The experiments are the culmination of around two decade’s work, says Luce. ITER will begin operations with low-power hydrogen reactions in 2025. But from 2035, it will run on a 50:50 mix of deuterium and tritium.

Both ITER and JET, based at the Culham Centre for Fusion Energy (CCFE) near Oxford, use extreme magnetic fields to confine plasma into a ring and heat it until fusion occurs. The temperatures in JET can reach 100 million degrees, many times hotter than the Sun’s core.

The world’s last tokamak fusion experiments with tritium also took place at JET. The goal then was to hit peak power, and the facility succeeded in achieving a record ratio of power out to power in (known as a Q value) of 0.67. That record still stands today; 1 would be break-even. But this year, the aim is to sustain a similar level of fusion power for 5 seconds or more, to eke out as much data from the experiments as possible and to understand the behaviour of longer-lasting plasmas, the report said.