Helium nuclei have an electric charge, so they stay in the tight hug of the magnetic fields.

ECONOMIST: How to get an energy rush without doughnuts

But the whole thing then breaks up into three helium nuclei, releasing a lot of energy at the same time.

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Helium nuclei are positively charged and thus easier to rein in using an electric field, in order to strip them of their energy.

ECONOMIST: A novel form of fusion power

But in Dr Rostoker's reactor, the helium nuclei created by the process of light fission would be speedy enough to escape from the ends of the ellipsoid, where the magnetic field is weakest.

ECONOMIST: How to get an energy rush without doughnuts