My PhD supervisor told me how in 2001. We have 4 turns of a 10cm radius Ti plated helix, into which we pass hydrogen gas – produced via the electrolysis of regular water.
As this circulates around the helix, we get H on H nucleus interaction above 1 W: We adjust the pressure to give this – I think it is above ½ an atmosphere. So we get nuclear fusion
We get the formation of a gas plasma, which gives us neutron formation – as the nuclei interact with free electrons.
This can burn some of the he back to H – converting some of the H into heat.
We run a steam cycle, and produce energy. We have a settling reservoir, where the H rises to the top – and we can extract He from the base.
We have also produced O gas, which has high commercial value. Helium has a higher value weight for weight, than gold.