The world's biggest experimental nuclear fusion reactor in operation was inaugurated in Japan on Friday, a technology in its infancy but billed by some as the answer to humanity's future energy needs.
Fusion differs from fission, the technique currently used in nuclear power plants, by fusing two atomic nuclei instead of splitting one.
The goal of the JT-60SA reactor is to investigate the feasibility of fusion as a safe, large-scale and carbon-free source of net energy — with more energy generated than is put into producing it.
The six-storey-high machine, in a hangar in Naka north of Tokyo, comprises a donut-shaped "tokamak" vessel set to contain swirling plasma heated up 200 million degrees Celsius.
It is a joint project between the European Union and Japan, and is the forerunner for its big brother in France, the under-construction International Thermonuclear Experimental Reactor.
The ultimate aim of both projects is to coax hydrogen nuclei inside to fuse into one heavier element, helium, releasing energy in the form of light and heat, and mimicking the process that takes place inside the Sun.
Unlike fission, fusion carries no risk of catastrophic nuclear accidents — like that seen in Fukushima in Japan in 2011 — and produces far less radioactive waste than current power plants, its exponents say.