Last year, scientists started up a new type of massive nuclear fusion reactor for the first time, known as a stellarator.

Researchers at the Max Planck Institute in Greifswald, Germany, injected a tiny amount of hydrogen and heated it until it became plasma, effectively mimicking conditions inside the sun.

But since then scientists have been asking whether the ambitious device — named Wendelstein 7-X (W7-X) — works as it is supposed to, producing the right magnetic fields.